# Sailing together: A review of the pelagic Sargassum microbiome

D D Cox, A K Stiffler, Vincent Hervé, M Léger-Pigout, L C Powers, P J

Lopez, O A Sosa, R J Parsons

► To cite this version:

D D Cox, A K Stiffler, Vincent Hervé, M Léger-Pigout, L C Powers, et al.. Sailing together: A review of the pelagic Sargassum microbiome. Harmful Algae, 2025, 148, pp.102904. 10.1016/j.hal.2025.102904. hal-05137213

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# Sailing together: A review of the pelagic *Sargassum* microbiome

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D.D. Cox<sup>a,b,*</sup>, A.K. Stiffler<sup>b</sup>, V. Hervé<sup>c</sup>, M. Léger-Pigout<sup>d</sup>, L.C. Powers<sup>e</sup>, P.J. Lopez<sup>f</sup>, O.A. Sosa<sup>g</sup>, R.J. Parsons<sup>h</sup>

## ARTICLE INFO

### Keywords:

* Pelagic *Sargassum*

* Seaweed holobionts

* Marine microbial communities

* Host-microbe dynamics

* Macronutrient cycling

* Organic matter remineralization

## ABSTRACT

Pelagic *Sargassum*, denoting the two species *S. natans* and *S. fluitans*, has become a focal point of both scientific and public interest due to recent superbloom events. As a result, investigation into the dynamics of *Sargassum*-associated microbiota, which was previously quite limited, has regained momentum in the past five years. This review collates the current state of knowledge relating to the *Sargassum* microbiome as well as synthesizes relevant biogeochemical findings which may aid future efforts in this field. We explore microbiome composition and functional roles within this holobiont, specifically in terms of taxonomic diversity, macronutrient cycling, and the fate of *Sargassum*-derived organic matter. Despite progress, significant gaps remain in our understanding of the dynamic interactions within these microbial communities and their broader ecological significance. Future research directions are identified alongside recommendations for standardized protocols, emphasizing the need for more in-depth studies utilizing advanced molecular and biogeochemical techniques to unravel the full complexity of the *Sargassum* microbiome.

## 1. Introduction

Pelagic *Sargassum*, a term denoting the two macroalgal species *S. natans* and *S. fluitans* (herein *Sargassum*), are endemic to the sub-tropical and equatorial Atlantic Ocean (Gower & King, 2020; Wang et al., 2019). *Sargassum* was historically contained to the Sargasso Sea, with biomass standing stocks of up to 10 million metric tons (Parr, 1939). Rafts formed by these seaweeds serve as biological hotspots for macrofauna in the pelagic surface ocean (Butler et al., 1983; Dooley, 1972; Hervé et al., 2021; Wells & Rooker, 2004), facilitate carbon sequestration (Gouvèa et al., 2020; Hu et al., 2021; Johnson & Richardson, 1977), and are valorized for key industrial products (Amador-Castro et al., 2021; Catarino et al., 2023; López Miranda et al., 2021; Rossignolo et al., 2022; Thompson, et al., 2020).

However, the escape of this seaweed in 2009 led to the formation of the Great Atlantic *Sargassum* Belt (GASB), which has reported standing

stocks up to 24.2 million metric tons dispersed across the equatorial Atlantic Ocean ranging from West Africa to the Gulf of Mexico (University of South Florida Optical Oceanography Lab, 2022; Wang, M. et al., 2019). The related superblooms, which have become increasingly common (Marsh et al., 2023; Wang et al., 2019), lead to significant environmental and economic obstacles (Devault et al., 2021; Hervé et al., 2024; Resiere et al., 2019; Robledo et al., 2021; van Tussenbroek et al., 2017). As such, a comprehensive understanding of the mechanisms supporting *Sargassum* proliferation are of rising importance from both ecological and economical perspectives.

When considering environmental adaptation and reproductive success of macrophytes, it is important to consider all members of the holobiont, a term referring to all associated biota with the host including the microbial community and epibionts (Simon et al., 2019). While there is a long history of *Sargassum* research, focus on the associated microbial community has only recently gained momentum (Fig. S1; Table S1) despite their important roles in other seaweed holobiont systems

<footnote><sup>a</sup> Earth Science Department, University of California Santa Barbara, Santa Barbara, CA, USA</footnote>

<footnote><sup>b</sup> Department of Biology, University of Miami, Coral Gables, FL, USA</footnote>

<footnote><sup>c</sup> Université Paris-Saclay, INRAE, AgroParisTech, UMR SayFood, 91120 Palaiseau, France</footnote>

<footnote><sup>d</sup> Aix Marseille Univ, Université de Toulon, Centre national de la recherche scientifique, Institut de recherche pour le développement, Institut Méditerranéen d'Océanologie, Marseille, France</footnote>

<footnote><sup>e</sup> State University of New York College of Environmental Science and Forestry, Department of Chemistry, Syracuse, New York, United States</footnote>

<footnote><sup>f</sup> UMR BOREA, Muséum national d'histoire naturelle, CNRS-8067, Sorbonne Université, IRD, Université de Caen Normandie, Université des Antilles, Paris, France</footnote>

<footnote><sup>g</sup> University of Puget Sound, Department of Biology, Tacoma, Washington, USA</footnote>

<footnote><sup>h</sup> Bermuda Institute of Ocean Sciences, a unit of the Julie Ann Wrigley Global Futures Laboratory at Arizona State University, St. George's, Bermuda</footnote>

<footnote>* Corresponding author.
E-mail address: dani@danicox.com (D.D. Cox).</footnote>
Acronyms

Sargassum pelagic Sargassum species S. natans and S. fluitans

GASB Great Atlantic Sargassum Belt

MAGs metagenome-assembled genomes

qPCR quantitative Polymerase Chain Reaction

C carbon

N nitrogen

P phosphorus

Fe iron

N₂ atmospheric nitrogen

NH₃ ammonia

Pᵢ inorganic phosphorus (orthophosphate)

As arsenic

P<sub>org</sub> organic phosphorus

APase alkaline phosphatase

C-P lyase carbon-phosphorus lyase

DOM dissolved organic matter

MPn methylphosphonate

DOC dissolved organic carbon

DIC dissolved inorganic carbon

(Dittami et al., 2016; Egan et al., 2013; Goecke et al., 2010; Lamarche et al., 2008; Meng et al., 2020). This review covers the current state of knowledge related to the *Sargassum* microbiome, which we define here as all microbiota living on or in this macroalgae, as well as relevant interactions with adjacent seawater microbial communities. A call to action for the advancement of this research niche is then presented with suggestions for future investigations, analytical techniques, and standard protocols.

## 2. Overview of the *Sargassum* holobiont

### 2.1. Microeukaryotic members of the *Sargassum* holobiont

*Sargassum* species host a diverse array of microeukaryotes (Hervé et al., 2021, Fig. 1). Photosynthetic protists such as diatoms (*Stramenopiles*, *Bacillariophyceae*) have been repeatedly observed on pelagic *Sargassum* (Carpenter, 1970; López-Fuerte et al., 2022; Maples, 1984) and a molecular survey recently reported hundreds of diatom species corresponding to about 2.4 % of the total abundance of pelagic *Sargassum*-associated eukaryotes (Hervé et al., 2021). Within *Stramenopiles*, the phagotrophic *Sagenista* represent another important group that needs to be further investigated (Fig. 1). Among the other protists, *Alveolata* are also abundant and diverse, especially among the clades *Dinophyceae* (Bomber et al., 1988; Okolodkov et al., 2007), *Oligohymenophorea*, *Spirotrichea*, *Phyllopharyngea* and *Syndiniales* (Hervé et al., 2021). Members of *Rhizaria* were also identified as a rich clade,

<figure><img src="image_4.png" /><figcaption>**Fig. 1.** Phylogenetic tree inferred from 4,127 microeukaryotic OTUs (99 % similarity) of 18S rRNA gene amplicon sequences extracted from pelagic and stranded *Sargassum* samples (Hervé et al., 2021). Outer circles indicate OTU origin. Branches of the richest protist and fungal clades are highlighted in colors. The classification was based in PR2 v5.0 (Guillou et al., 2013). Sequences were aligned with DECIPHER v3.0 (Wright, 2020), the tree was built with FastTree v2.1.11 (Price et al., 2010) and visualized with Iroki (Moore et al., 2020).</figcaption></figure>
especially in the phagotrophic group *Endomyxa* (*Cercozoa*) which are mainly represented by *Filoretidae* and *Vampyrellida* in *Sargassum* host (Hervé et al., 2021).

The presence of fungi inhabiting *Sargassum* was reported decades ago based on microscopic observations (Kohlmeyer, 1971; Kohlmeyer, 1972). A metabarcoding study also described the fungal diversity associated with stranded *Sargassum* waste, meaning *Sargassum* collected post-beaching on the coast of Barbados and identified *Basidiomycota* as the most dominant fungal phylum, followed by *Ascomycota*, *Zygomycota* and *Chytridiomycota* (Mohapatra, 2023). Another metabarcoding study revealed that stranded *Sargassum's* most abundant fungal phylum was *Ascomycota* (85.5 % of the total fungi abundance), while it was *Ascomycota* (35.1 %) followed by *Chytridiomycota* (9.3 %) in pelagic *Sargassum* (Hervé et al., 2021). Both studies found that a large portion of unclassifiable fungal species. This data suggests that fungal diversity associated with *Sargassum* remains understudied and that further investigation is needed to better understand the roles of fungi on *Sargassum* growth and degradation, with potential implications for industrial applications (Landeta-Salgado et al., 2021; Martorell et al., 2021; Patyshakuliyeva et al., 2020). More generally, the different ecological functions and trophic modes (phototroph versus phagotroph) among the *Sargassum* eukaryome need to be considered in further studies to start deciphering the network of interactions of this holobiont.

Besides differences between *Sargassum* species (Alleyne et al., 2023) or between *Sargassum* structures (Carral-Murrieta et al., 2024), various abiotic factors could hypothetically shape the *Sargassum* eukaryome, such as sampling period and location or surrounding physicochemical variables (e.g., pH, temperature, oxygen concentration, redox potential). Similarly, biotic factors could influence the *Sargassum* eukaryome since the community structures of prokaryotes and eukaryotes have been shown to be correlated (Hervé et al., 2021). Yet, dedicated experiments are needed to test these hypotheses. So far, it appears clear that the *Sargassum* eukaryome significantly differs between *Sargassum* rafts floating in the sea and decaying samples in terrestrial ecosystems (e.g., beaches, storage sites, digesters).

## 2.2. Archaeal and bacterial diversity of the *Sargassum* holobiont

The difference between *Sargassum* collected in pelagic and stranded sites was also noticeable for prokaryotes. Based on a presence-absence survey of nine studies where we extracted taxonomic information from figures and relative abundance data, we obtained 13 pelagic and four stranded datasets (Fig. 2; Table S2). A total of 50 phyla were identified, encompassing six archaeal and 44 bacterial groups, showing the extent of prokaryotic diversity of the *Sargassum* holobiont. Despite being based on presence-absence data, we identified recurring phyla across studies, including *Pseudomonadota*, *Bacteroidota*, *Cyanobacteriota*, *Actinomycetota*, *Planctomycetota*, *Chloroflexota*, *Bacillota*, *Bdellovibrio*, *Brionota*, *Verrucomicrobiota*, and *Desulfobacterota*.

Relative abundance data were obtained from supplementary datasets or the study's figures to represent the prokaryotic phyla associated with the *Sargassum* microbiome quantitatively. When datasets were unavailable, bar plot data were extracted using WebPlotDigitizer (Rohatgi, 2024) to generate the top seven ranked phyla per study (Table S3). This allowed for comparing seven of the original nine studies, yielding 14 datasets. The most abundant phyla were assigned a rank of one (Fig. 3; Table S4). *Pseudomonadota* ranked first in 79 % of the datasets, with the other six phyla in order of decreasing median rank as follows: *Bacteroidota*, *Cyanobacteriota* and *Actinomycetota* (equally ranked), *Planctomycetota*, *Cholorflexota*, and *Verrucomicrobiota*. This synthesis indicates that these seven phyla are core members of the *Sargassum* microbiome. Among *Pseudomonadota*, *Vibrionaceae* (*Gammaproteobacteria*), *Alteromonadaceae* (*Gammaproteobacteria*), and *Rhodobacteraceae* (*Alphaproteobacteria*) were frequently reported as dominant families (Michotey et al., 2020; Stiffler et al., 2024; Theirlynck et al., 2023; Torralba et al., 2017). Researchers have repeatedly detected members of *Flavobacteriaceae* (*Bacteroidota*) and proposed that they play a role in macroalgal polysaccharide degradation (Hervé et al., 2021; Mohapatra, 2023; Stiffler et al., 2024). Within the photosynthetic *Cyanobacteriota* phylum, members of the families *Cyanobiaceae*, *Phormidesmiaceae*, and *Nostocaceae* have also been regularly found associated with *Sargassum* (Hervé et al., 2021; Mendonça et al., 2024; Michotey et al., 2020; Stiffler et al., 2024; Theirlynck et al., 2023; van Tussenbroek et al. 2024). Besides molecular surveys, epiphytic *Cyanobacteriota* have

<figure><img src="image_5.png" /><figcaption>**Fig. 2.** Barplots showing the occurrence of prokaryotic phyla found in association with *Sargassum* samples from *n* = 13 pelagic and *n* = 4 stranded datasets extracted from 9 studies (Cox, 2024; Domínguez-Maldonado et al., 2024; Hervé et al., 2021; Mendonça et al., 2024; Michotey et al., 2020; Mohapatra, 2023; Stiffler et al., 2024; Theirlynck et al., 2023; Torralba et al., 2017). Taxonomy was standardized using the GTDB system (Parks et al., 2022).</figcaption></figure>
<figure><img src="image_6.png" /><figcaption>**Fig. 3.** Heatmap showing the top seven ranked prokaryotic phyla from *n* = 9 pelagic and *n* = 5 stranded datasets from seven studies based on relative abundance data (Domínguez-Maldonado et al., 2024; Hervé et al., 2021; Mendonça et al., 2024; Michotey et al., 2020; Mohapatra, 2023; Stiffler et al., 2024; Theirlynck et al., 2023). When abundance data was unavailable, relative abundance bar plots were extracted from the paper, uploaded into WebPlotDigitizer (Rohatgi, 2024), and calibrated as a barplot to obtain estimated relative abundances to generate ranks. Tables of the relative abundance values and ranks generated from this method are available in Table S3 and S4. Due to data availability and the number of phyla in the relative abundance figures, seven was chosen for the number of ranks displayed. A rank of one (dark red) represents the most abundant phyla, and a rank of seven represents the seventh most abundant phyla (cream). White demonstrates that the phyla was not in the top seven phyla for the associated study, it does not depict absence from the dataset. Blue squares and orange circles represent pelagic and stranded samples, respectively. *Sargassum* morphotypes are represented as triangles (grouped), stars (*S. natans* VIII), pentagons (*S. natans* I), and diamonds (*S. fluitans* III).</figcaption></figure>

been repeatedly isolated from pelagic *Sargassum* (Phlips et al., 1990; Johnson et al., 2023).

Several studies have compared *Sargassum* microbiome composition, diversity, and richness to that of the surrounding seawater (Hervé et al., 2021; Michotey et al., 2020; Stiffler et al., 2024; Torralba et al., 2017). All report that *Sargassum* and seawater microbiomes differ, with *Sargassum* having greater alpha or beta diversity, and a lower abundance of *Cyanobacteria* and *Pelagibacterales* (Hervé et al., 2021; Michotey et al., 2020; Stiffler et al., 2024). Archaea, including methanogens, have also been reported in various studies, especially for samples decaying in anaerobic conditions (Domínguez-Maldonado et al., 2024; Hervé et al., 2021; Salgado-Hernández et al., 2023). While seaweeds, including benthic *Sargassum* spp., are known to harbor endosymbionts (Hollants et al., 2013; Li et al., 2024; Saha et al., 2024), to our knowledge, no study has specifically investigated the existence and diversity of endosymbionts associated with pelagic *Sargassum*. Temporal, spatial, and environmental factors could explain the variations in prokaryotic diversity within the *Sargassum* prokaryome, with further studies needed to decipher the respective roles of these drivers.

### 2.2.1. Opportunistic Vibrio lineages associated with Sargassum

As *Sargassum* approaches and strands on the coast, members of the genus *Vibrio*—a bacterial group with opportunistic pathogens—become more prominent in its microbiome (Hervé et al., 2021; Stiffler et al., 2024; Theirlynck et al., 2023). Stranding and decomposition trigger dysbiosis, decreasing microbiome diversity as the community shifts to *Vibrio* dominance (Mendonça et al., 2024). It remains unclear whether *Sargassum* introduces novel *Vibrio* spp. to coastal areas or if algae exudates fuel the growth of *Vibrio* spp. present in coastal seawater. *Vibrio* bacteria are naturally found in marine, brackish, and freshwater environments (Baker-Austin et al., 2018), with temperature, salinity, and nutrient availability driving their proliferation (Coutinho et al., 2019;

Liang et al., 2019; Thickman & Gobler, 2017). Due to this, rising sea temperatures and storm frequency under climate change are expected to increase *Vibrio* abundances and infections (Engelthaler et al., 2005; Sodders, 2023; Vezzulli et al., 2016).

Three *V. alginolyticus* isolates and one *V. parahaemolyticus* isolate capable of pathogenicity were found on *Sargassum* (Mincer et al., 2023). *V. alginolyticus* causes ear and wound infections via seawater contact and is treatable with antibiotics (Schets et al., 2006). *V. parahaemolyticus* causes gastroenteritis through seafood ingestion or seawater exposure and, in rare cases, leads to necrotizing fasciitis in immunocompromised individuals (Baker-Austin et al., 2010; Tena et al., 2010). Notably, only a small subset of over 100 described *Vibrio* spp. are human pathogens (Baker-Austin et al., 2018; Grimes, 2020); several, like *V. fischeri* and *V. halictocoli*, form mutualistic relationships with squid and abalone (Grimes, 2020; Sawabe, 2006; Stabb, 2006). Metagenomic and qPCR data suggest *Vibrio* spp. associated with *Sargassum* may also participate in nitrogen fixation and organic phosphorus utilization, indicating potential for beneficial symbioses and biogeochemical cycling (Cox, 2024; Léger-Pigout et al., 2024). Further studies using selective media, whole genome sequencing, and phenotypic assays across *Sargassum*'s range are needed to better define *Vibrio* abundances and roles during bloom progression.

### 2.3. Viruses of the Sargasso sea and Sargassum holobiont

There are an estimated 10³¹ bacteriophages, viruses that infect prokaryotes, on Earth (Güemes et al., 2016), and in the oceans, 10²³ planktonic organisms are infected by viruses each second (Suttle, 2007). Through lytic infections, bacteriophages alter plankonic photosynthesis and nutrient cycling (Weinbauer, 2004; Wilhelm & Suttle, 1999), with lytic infections recycling an estimated 145 Gt C annually in the ocean (Lara et al., 2017). Alternatively, during lysogeny, bacteriophages
integrate into host genomes as a prophage, potentially altering phenotypes and increasing fitness through lysogenic conversion (Paul 2008; Middelboe et al., 2025). Both infection modes can modify host metabolism and ecological interactions (Breitbart et al., 2018; Lu & Henning, 1994; Rohwer & Thurber, 2009), making viruses key players in shaping microbial community structure.

Though little is known about *Sargassum*’s virome, the seawater of the Sargasso Sea has been well studied. In winter, viral abundance and community structure are uniform across depths of the Sargasso Sea (Goldsmith et al., 2015; Parsons et al., 2012), but in summer, community composition diverges, with abundance peaking between 60–100 m (Parsons et al., 2012). Viral abundance increases northeastward across the Sargasso Sea (Rowe et al., 2008), correlating positively with *Prochlorococcus* and *Rhodobacteraceae*, and negatively with SAR11 (Parsons et al., 2012). Metagenomics revealed that 54 % of Sargasso Sea viruses are endemic or insufficiently abundant for detection in other environments (Warwick-Dugdale et al., 2024). Unexpectedly, of phages whose host could be assigned, *Prochlorococcus* and SAR11 were underrepresented, while those infecting copiotrophic bacteria like *Flavobacterium* and *Pseudoalteromonas*—abundant in *Sargassum* microbiomes (Hervé et al., 2021; Michotey et al., 2020; Stiffler et al., 2024)—were prevalent (Warwick-Dugdale et al., 2024).

A metagenomic study of coastal *Sargassum* showed distinct bacteriophage communities compared to surrounding seawater (Stiffler et al., 2024). The assembly of bacterial metagenome-assembled genomes (MAGs) showed that *Sargassum*-associated bacteria harbored more prophages and that prophage induction, removal from the host genome, and subsequent host lysis decreased biofilm formation (Stiffler et al., 2024). Further, *Sargassum*-associated bacteriophages encoded genes linked to bacterial-macroalgal symbiosis, such as biofilm formation and nutrient metabolisms (Stiffler et al., 2024). A small subset of bacteriophages carried genes from *pho* and *ars* operons which manage inorganic phosphorus and arsenic, respectively (Stiffler et al., 2024). This data suggests that bacteriophages influence which bacteria colonize *Sargassum* from the surrounding environment and potentially impact bacterial-algal symbiosis. Similar phage-mediated bacterial niche expansion has been observed in a *Sulfitobacter* sp. isolated from phytoplankton blooms and *Vibrio anguillarum*, commonly isolated from aquaculture (Basso et al., 2020; Tan et al., 2020). As current *Sargassum* studies focus mainly on double-stranded DNA bacteriophages, future research should explore single-stranded DNA, single-stranded RNA, double-stranded RNA bacteriophages, and eukaryotic viruses associated with *Sargassum* as well as their functional roles, particularly as they relate to nutrient metabolism and macroalgal growth.

## 3. Biogeochemical roles of the *Sargassum*-associated microbiota

The carbon (C), nitrogen (N), and phosphorous (P) contents of *Sargassum* have been thoroughly recorded since the 1970s (Hanson, 1977; Lapointe et al., 2021; Wang et al., 2018) and trace metals have more recently been measured as well (Dassié et al., 2022; Devault et al., 2021; Gobert et al., 2022; Hernández-Almaraz et al., 2016; McGillicuddy et al., 2023). Cox (2024) included iron (Fe) additions as a variable in incubation experiments of *Sargassum* with and without excess methylphosphonate availability. While this data is based on a limited number of samples, the increases in relative abundance of *Rhodobacteraceae* and phosphonate degradation activity provide preliminary evidence for the function of Fe in the *Sargassum* microbiome. Outside this publication, the microbial role in trace metal biogeochemical cycles as it relates specifically to the *Sargassum* holobiont has, to our knowledge, yet to be assessed. Thus, this section centers around the major macronutrient (C, N, P) cycling components.

### 3.1. Nitrogen fixation

#### 3.1.1. Contribution of nitrogen-fixing *Sargassum* epiphytes

Nitrogen (N₂) fixation is the process by which prokaryotic microorganisms known as diazotrophs fix N₂ (available in almost unlimited quantities) into ammonia (NH₃), increasing the bioavailability of N, which often limits primary productivity in the ocean (Gruber, 2008; Hellriegel & Wilfarth, 1888; Moore et al., 2013). The process of N₂ fixation and the presence of diazotrophs have been detected on the surface of *Sargassum* across its main geographical areas: the Sargasso Sea, the Florida Straits, the Gulf Stream, and the GASB (Baker et al., 2018; Carpenter, 1972; Hanson, 1977; Johnson et al., 2023; Lapointe et al., 2021; Léger-Pigout et al., 2024; McGillicuddy et al., 2023; Philips et al., 1986; Philips & Zeman, 1990; Rooker et al., 2006). Measurement of δ¹⁵N using isotope ratio mass spectrometry has been used to determine the origin of N in *Sargassum*, with a low δ¹⁵N value synonymous with a predominantly atmospheric N source, acquired either through N₂ fixation activity by diazotrophic organisms (Montoya, 2008; Zerkle et al., 2008) or atmospheric deposition (Kendall et al., 2007). It has been shown that the δ¹⁵N of *Sargassum* tissue is notably low (close to 0‰) in the Sargasso Sea, GASB, Caribbean, and Gulf Stream (Baker et al., 2018; Lapointe et al., 2021; McGillicuddy et al., 2023; Léger-Pigout et al., 2024) (Fig. 4A; McGillicuddy et al., 2023). In contrast, in the Gulf of Mexico, δ¹⁵N values are above 2.5‰ (Lapointe et al., 2021; Rooker et al., 2006; Vázquez-Delfín et al., 2021) and are associated with urban wastewater discharges. In *Sargassum* biofilms from the GASB, a study quantified the relative abundance of diazotrophs through 16S rRNA gene-normalized abundance of the *nifH* gene, which encodes for a subunit of the nitrogenase enzyme involved in N₂ fixation (Léger-Pigout et al., 2024). This study found that the relative abundance of diazotrophs decreased in more nutrient-rich coastal waters in favor of denitrifiers and suggests that this could be correlated to increasing δ¹⁵N values as samples were collected closer to the coast (Fig. 4B; Léger-Pigout et al., 2024). This is in line with the hypothesized change in *Sargassum*'s primary N source from diazotrophic N₂ fixation in oligotrophic waters to abiotic sources in coastal regions which are more available due to upwelling and terrestrial input (Lapointe et al., 2021; McGillicuddy et al., 2023).

To measure N₂ fixation activity, the acetylene reduction method, in which ethylene production is measured in acetylene-amended samples (Dilworth, 1966), is commonly used. This practice is based on the principle that the nitrogenase enzyme responsible for N₂ fixation reduces acetylene to ethylene in a process similar to the reduction of N₂ to NH₃. Significant geographic variability in N₂ fixation rates has been reported within *Sargassum* rafts, with results indicating this activity accounts for 0 - 146 % of the *Sargassum* N demand (Carpenter, 1972; Hanson, 1977; Johnson et al., 2023; Philips et al., 1986; Philips & Zeman, 1990). Diazotrophic N₂ fixation activity in samples of *S. natans* and *S. fluitans* collected on the western edge of the Gulf Stream was estimated to fix on average, ~18-71 µg N g⁻¹ h⁻¹ (Hanson, 1977; Johnson et al., 2023) while fixation of 0.01 - 0.03 µg N m⁻² h⁻¹ was reported in the western Sargasso Sea (Carpenter, 1972). However, Carpenter et al. (1972) measured only the N₂ fixation rates of *Dichothrix* sp. by removing the Cyanobacteria from the *Sargassum* prior to incubation. Consequently, these rates are likely underestimated when considering the N₂ fixation potential of the *Sargassum* holobiont, given the diversity of its epiphytic diazotroph community. Along coastal Florida, comparably low fixation rates were measured, with ~0.01 µg N m⁻² h⁻¹ in Miami and 0.06 - 0.08 µg N m⁻² h⁻¹ in St. Augustine (Philips et al., 1986). Environmental variables such as terrestrial N input (Lapointe et al., 1995, 2021; Wang et al., 2019), atmospheric deposition (Kendall et al., 2007), and temperature have been proposed to explain the differences in fixation rates between locations. For example, diazotrophy on the surface of *Sargassum* has been observed to reach optimal activity at around 30°C (Philips et al., 1986).

Variations in N₂ fixation rates related to the intrinsic characteristics
<figure><img src="image_7.png" /><figcaption>**Fig. 4.** (A) δ¹⁵N measured in *Sargassum* tissues collected during two oceanographic campaigns (GO-SHIP lines A20 and A22). *S. fluitans* III is shown to the left of transects A20 and A22, and *S. natans* I to the right. Stations, where no *Sargassum* was found, are shown as circles, and stations where *Sargassum* sampling was not possible are shown as Xs. Blue lines are the 2000 m and 3000 m isobaths (McGillicuddy et al., 2023). (B) Log₁₀ evolution of the ratio of nifH, nosZ, and amoA gene abundances to 16S rRNA gene abundance in the *Sargassum* biofilm. Each grey point corresponds to one data item. The 95 % confidence intervals of the mean are shown (Léger-Pigout et al., 2024). GASB: Great Atlantic *Sargassum* Belt; SS: Sargasso Sea; MAR: Martinique; GUA: Guadeloupe.</figcaption></figure>

of *Sargassum* rafts have also been investigated, resulting in mixed findings. Fluctuations in δ¹⁵N within the different morphotypes of *Sargassum* stranded on Caribbean coasts have been observed, with lower isotopic signatures in *S. natans* I compared to *S. natans* VIII and *S. fluitans* III at the same location (Changeux et al., 2023), though three other studies using either δ¹⁵N signatures or acetylene reduction assays did not find a relationship between morphotype and N₂ fixation activity (Léger-Pigout et al., 2024; McGillicuddy et al., 2023; Philips et al., 1986) (Fig. 4A; McGillicuddy et al., 2023). Philips et al. (1986) also investigated the rates of N₂ fixation as a function of the frond age, finding that the oldest parts underwent 2.2 to 4.1 times more N₂ fixation than new growth zones. However, Léger-Pigout et al. (2024) found no isotopic evidence to support a difference in fixation rates amongst blades, stipes, and pneumatocysts in *Sargassum* samples from new growth sections within the GASB. However, no study has been carried out to compare isotopic signatures between these morphological structures in sections of old growth. With the studies currently available, this lack of consensus suggests that intrinsic characteristics of *Sargassum* are likely not driving variables in N₂ fixation activity but may play auxiliary roles.

### 3.1.2. Players in N₂ fixation among *Sargassum* epiphytes

Since the discovery of diazotrophs on the surface of *Sargassum*, studies have attempted to describe the diversity of these organisms. Early studies utilizing optical microscopes allowed for the identification of diazotrophs and provided initial insights into their abundance and taxonomy. For many years, the known diversity of diazotrophs associated with *Sargassum* was identified primarily as autotrophic Cyanobacteria, notably the genus *Dichothrix* (Carpenter, 1972), *Calothrix* (Philips et al., 1986) and *Oscillatoria* (Philips & Zeman, 1990). Hanson (1977) similarly found that blue-green algae were visible microscopically. The development of advanced quantification methods, such as flow cytometry and qPCR, along with modern sequencing techniques, has enabled a more precise taxonomic identification, particularly at the genus and species level, leading to a better consensus on the global diazotrophic community structure and diversity. Quantification of the nifH gene indicated that diazotrophs account for approximately 2 % of the bacteria that compose the *Sargassum* microbiome (Léger-Pigout et al., 2024). The

low abundance of diazotrophs relative to the previously mentioned high contribution of nitrogen fixed by these microbiota reinforces the concept that even less abundant groups in microbial communities can perform significant ecosystem services. The historically accepted concept that most diazotrophs are autotrophic has also been challenged, with metabarcoding analysis of the nifH gene showing that 75 % of diazotroph communities on the surface of *Sargassum* were heterotrophs, namely *Vibrio* spp. and *Filomicrobium* spp. (46 % and 33 %, respectively; Léger-Pigout et al., 2024). As heterotrophic diazotrophs need external sources of organic carbon, such as macroalgal exudate, for energy and growth, this finding implicates a mutualistic relationship where each member facilitates the proliferation of the other. Léger-Pigout et al. (2024) further demonstrated that pelagic *Sargassum* had lower δ¹⁵N signatures and increased diazotroph abundance than benthic *Sargassum*, pointing to a potential mutualism that is specific between pelagic *Sargassum* and its microbiome.

### 3.2. Phosphorus limitation

Seaweeds, like all organisms, require phosphorus in the inorganic form orthophosphate (PO₄³⁻ or P₁) to grow and for the synthesis of nucleic acids, phospholipids, sugar phosphates, and nucleotides such as adenosine triphosphate (Douglas et al., 2014). Surface P₁ concentration in the pelagic regions inhabited by *Sargassum* can be up to two orders of magnitude lower than other oceanic basins and varies significantly (Global Ocean Biogeochemistry Analysis and Forecast, 2021), ranging from 0 - 300 nM depending on determination method (modeling or *in situ* measurements), season, and location (Cardona et al., 2016; Conkright et al., 2000; European Union Copernicus Marine Service, 2024; Lomas et al., 2010; Macovei et al., 2019; Wu et al., 2000). The relative environmental depletion of P₁ is imprinted in the elemental composition of *Sargassum*, which has a C:N:P ratio of 778 - 886 C: 19 - 28 N: 1 P (Lapointe et al., 2014; Lapointe et al., 2021; Wang, et al., 2018). Additionally, the C:N:P ratios were higher in P-deplete regions (Bermuda) than P-replete sites (coastal Caribbean and Florida), correlating elevated ratios to low P₁ availability (Lapointe et al., 2014). High arsenic (As) content and As:P ratios in *Sargassum* tissue have been implicated as
another tool for determining P-limited growth, with 0.01-0.02 % As and 0.2-0.6 As:P reported for the Sargasso Sea compared to <0.01 % As and <0.2 As:P in other *Sargassum*-endemic regions (McGillicuddy et al., 2023). Thus, evidence for *Sargassum* being characterized by P-limited growth has been well-established. Investigating the phosphorus acquisition mechanisms employed by *Sargassum* and other brown algae could shed light on how macroalgal-holobiont communities adapt to marine biomes characterized by extreme nutrient depletion. A summary of the confirmed and proposed P cycling mechanisms within the *Sargassum* holobiont are depicted in Fig. 5 and explained further below.

### 3.2.1. Cycling of organic phosphorus in Sargassum communities

In low Pᵢ regions, remineralization of organic phosphorus (P<sub>org</sub>) is a strategy commonly utilized by marine microbes to meet their nutrient demands, supporting up to 25 % of annual primary production in the subtropical North Atlantic ocean (Lomas et al., 2010). Marine microbiota are able to utilize the P<sub>org</sub> pool through the deployment of extra- and intracellular alkaline phosphatase (APase) enzymes (Björkman & Karl, 2003; Hernández et al., 2002; Hoppe, 2003; Luo et al., 2009, 2011; Martínez et al., 2013), with bacterial APase activity ranging from 0.1 - 550 nM Pᵢ g⁻¹ dry weight hr⁻¹ in various marine environments (Hoppe, 2003). In comparison, marine seaweeds are known to only access the P<sub>org</sub> pool via extracellular APase enzymes (Walther & Fries, 1976; Weich & Granéli, 1989). Bacterial remineralization of the P<sub>org</sub> pool, primarily that which is excreted by associated fish and zooplankton, has been proposed to explain elevated Pᵢ concentrations within *Sargassum* patches relative to surrounding waters in the Sargasso Sea (Lapointe, 1995).

Lapointe (1995) measured an APase activity of 10.7 - 28.6 µM Pᵢ g⁻¹ dry weight hr⁻¹ for *Sargassum*, with increased rates in oceanic Sargasso Sea samples relative to those collected along the continental shelf of various Caribbean locations. However, either associated bacteria, *Sargassum* itself, or a combination of the two may account for these rates since the source of APase within the holobiont was not distinguished.

*Sargassum* found *in situ* has a P content of 0.1 % (Lapointe et al., 2021; Wang et al., 2018) and a mean growth rate of 0.04 doublings per day (range = 0.008 - 0.09 doublings d⁻¹; Corbin & Oxenford, 2023; Lapointe et al., 2014; Magaña-Gallegos et al., 2023). Using the standing stock mass of 34.2 million metric tons (Parr, 1939; Wang, et al., 2019), we thus estimate a phosphorus demand of 0.95 Gg P d⁻¹ (range = 0.237 - 2.13 Gg P d⁻¹) for the *Sargassum* population at peak season. This equates to a local availability need of 895 nmol P g⁻¹ *Sargassum* (dry weight) d⁻¹ (range = 224 - 2,017 nmol P g⁻¹ *Sargassum* (dry weight) d⁻¹) in the surrounding seawater. Based on this calculation, as little as 37.4 nmol P g⁻¹ *Sargassum* (dry weight) hr⁻¹ liberated via APase activity could support the entirety of *Sargassum's* P demand. This highlights the importance of P<sub>org</sub> remineralization, whether driven solely by bacteria or in combination with macroalgal-excreted enzymes, alongside Pᵢ uptake to sustain current *Sargassum* populations.

The carbon-phosphorus lyase (C-P lyase) pathway is another important microbial adaptation to low Pᵢ availability present among prominent members of the *Sargassum* microbiome, including *Vibrioales*, *Rhodobacterales*, *Alteromonadales*, *Pseudomonadales*, and *Oceanospirillales*, as well as some *Cyanobacteriia* and *Planctomycetota* (Cox et al., 2024). This feature of the *Sargassum* microbiome is consistent with

<figure><img src="image_8.png" /><figcaption>**Fig. 5.** Conceptual summary of P cycling within *Sargassum* mats, differentiated by macroalgal (orange) and microbial (blue) components. P<sub>org</sub> represents all organic phosphorus species (particulate and dissolved), Pᵢ represents all inorganic phosphorus species, C-P lyase stands for the carbon-phosphorus lyase, and APase stands for alkaline phosphatase. Solid arrows indicate confirmed transport mechanisms, while dashed arrows indicate pathways which have been hypothesized or have limited evidence for support, highlighting the need for further investigation.</figcaption></figure>
the strong selection pressure on Pᵢ-depleted Sargasso Sea bacterial communities to acquire C-P lyase (Sosa et al., 2019), which allows for utilization of phosphonate compounds in the P org pool (Sosa et al., 2017). In incubations of *Sargassum* from the Sargasso Sea supplemented with methylphosphonate (MPn), an abundant phosphonate in dissolved organic matter (DOM), Cox et al. (2023) demonstrated an active sub-strate degradation by the bacterial members of the *Sargassum* holobiont of 4.7 nmol P g⁻¹ *Sargassum* (wet weight) d⁻¹, though the authors note this may be an underestimation due to experimental design. Because C-P lyase is not a genetic capability of marine macroalgae, this observation indicates phosphonates could contribute to satisfying the P requirement of the microbiome, reducing competition with their host for this scarce nutrient.

### 3.2.2. Phosphorus-based Sargassum microbiome dynamics

Given the range of Pᵢ concentrations experienced by *Sargassum* over its life cycle, including prolonged periods of exposure to Pᵢ depletion, Pᵢ-regulated responses may be fundamental to the organization of the *Sargassum* microbiome and partially explain the variable community compositions reported (see Section 2.2). Changes in phosphorus avail-ability and substrate type within the microbiome may spur dynamics based on competition and succession as well as shape the genetic ad-aptations of its members. Genomic sequencing of the *Sargassum* micro-biome collected in the Gulf of Mexico and Sargasso Sea, incubated either with or without MPn amendments, revealed trends in community tax-onomy and C-P lyase capacity associated with substrate addition (Cox, 2024). 16S rRNA gene-based sequencing indicated decreases in the relative abundance of *Flavobacteriaceae*, with compensation by other bacterial groups differentiated by collection site. Metagenomic sequencing of the same samples showed increases in species-level di-versity amongst *phnJ*-carriers and the number of *phnJ* copies per MAG as MPn amendment concentration increased. Furthermore, the relative abundance of MAGs classified within *Rhodobacteraceae* were shown to increase with increasing MPn (Cox, 2024), which aligns with work identifying members of *Rhodobacteraceae* as prominent bacteria with C-P lyase genes in the Sargasso Sea (Sosa et al., 2019; Venter et al., 2004). However, Cox (2024) emphasized that the substantial influence of external variables on shaping the *Sargassum* microbiome structure could not be independently distinguished due to the small sample size. Thus, the role of changing ecological relationships between these bact-eria and their host, both in terms of P cycling and other drivers, has on community recruitment still remains to be determined.

## 3.3. Cycling of Sargassum biomass

### 3.3.1. Sargassum exudate composition

The composition of brown algae has been studied extensively, as many compounds are biologically active (Remya et al., 2022), and because the extraction of alginate, a cell wall polysaccharide, is economically viable (Gheorghita Puscaselu et al., 2020). *Sargassum*--derived DOM contains metabolites that can contribute to an aliphatic DOM pool (Powers et al., 2019), such as carotenoids and their oxidation products, lipids, peptides, and carbohydrates (Davis et al., 2021; Dem-bitsky et al., 1990; Haugan and Liaen-Jensen, 1994; Miyashita et al., 2013; Tonon et al., 2022).

High resolution techniques have also revealed that *Sargassum*-derived DOM contains features that are highly oxygenated and unsatu-rated, consistent with the composition of polyphenols and contains more aromatic DOM than that found in natural water samples (Powers et al., 2019). The foundational study by Sieburth and Jensen (1969) found that brown algae consisted of 500 - 800 mg phenolic kg⁻¹ (dry weight) and suggested that polyphenols react with proteinaceous and carbohydrate material to form chromophoric DOM, the light absorbing fraction of the DOM pool. Total phenolic content in *Sargassum* samples were highest during periods with the highest water temperatures and solar radiation (Fagundo-Mollineda et al., 2023), where *Sargassum* senescence is more

likely. Phenolic content in *Sargassum* may largely depend on the extraction method and phenolic assay used (Parys et al., 2007), with reported phenolic content ranging from 1 – 60 % dry weight (Davis et al., 2021; Fagundo-Mollineda et al., 2023; Machado et al., 2022; Nielsen et al., 2021; Saldarriaga-Hernandez et al., 2020). This under-standing of *Sargassum*-derived DOM composition may inform bacterial recruitment driven by substrate preference and metabolic capability.

### 3.3.2. Sargassum exudation and remineralization rates

In order to understand the microbial response to *Sargassum*-derived DOM, it is important to understand the flux of material from *Sargassum*. Thus, a few studies have reported dissolved organic carbon (DOC) exudation rates, with highly variable findings highlighting the impact of experimental methods and extraneous variables. Hanson (1977) incu-bated *Sargassum* in polyethylene bags under full sunlight around 30 – 33 °N, and found dissolved organic carbon (DOC) exudation rates of 46 µg C g⁻¹ h⁻¹ (dry weight) in April and 287 µg C g⁻¹ h⁻¹ in June), with a mean of 25 % of its fixed carbon as DOC. Light was also shown to enhance DOC exudation (5.9 µg C g⁻¹ h⁻¹ in the light and 2.6 µg C g⁻¹ h⁻¹ in the dark), yet only released 2.5 % and 6.9 % of its fixed carbon as DOC, respec-tively (Brylinsky, 1977). Powers et al. (2019) built upon this to compare the impact of type of light, finding DOC released under visible light was not significantly different from that released in the dark (~7 µg C g⁻¹ wet weight h⁻¹) yet significantly increased with sunlight exposure (23 µg C g⁻¹ wet weight h⁻¹), suggesting ultraviolet radiation as an important factor. Another study that incubated *Sargassum* in the dark for 11 days reported low exudation rates of 0.6 µg C g⁻¹ wet weight h⁻¹ (Perry et al., 2018), further emphasizing the variability of exudation rate from one study to another. Other variables assessed include *Sargassum* health and temperature, with higher DOC exudation observed with a greater degree of senescence or at higher temperatures (Powers et al., 2019; Shank et al., 2010a), suggesting that *Sargassum* under stress-conditions, such as during inundation events in coastal areas, may increase release of biomass as DOM. Furthermore, recent work determined that *Sargas-um*-derived DOM photoproduces dissolved inorganic carbon (DIC), carbon monoxide, and methane more efficiently than marine DOM (Zhang et al., 2024). Therefore, when *Sargassum* is present, localized areas can become hotspots of greenhouse gas production and likely emission.

Chromophoric DOM release rates similarly varied depending on collection site, light source, and *Sargassum* health (English et al., 2025, Hulatt et al., 2009; Perry et al., 2018; Powers et al., 2020; Shank et al., 2010a; Wada et al., 2007). Based on ¹³C nuclear magnetic resonance estimates of *Sargassum*-derived DOM phenolic content, *Sargassum* may release up to ~200 Gg C yr⁻¹ as polyphenols (Powers et al., 2019), making *Sargassum* the largest source of open ocean polyphenols identi-fied to date. Interestingly, phlorotannins, a class of polyphenols that are found exclusively in brown algae, have antimicrobial properties (Eom et al., 2012; Le Lann et al., 2016) which may inhibit bacterial con-sumption. English et al. (2025) found that heterotrophic bacteria remineralized 62 – 99 % of *Sargassum*-derived DOM within 30 days, yet this decreased to 14 – 47 % when phenols were pre-concentrated before supplementation (English et al., 2025). This study also demonstrated that *Sargassum*-derived DOM with high phenolic content was rapidly photooxidized to dissolved inorganic carbon upon exposure to light, with 66 % of the total phenolic content removed within 48 hours. Thus, both biological and photochemical processes (English et al., 2025, Shank et al., 2010a; 2010b; Zhang et al., 2024) are important in the rapid degradation of *Sargassum*-derived DOM in the surface ocean.

### 3.3.3. Potential microbial communities involved

Early work suggested that *Sargassum*-derived DOM was readily bio-degraded in the surface ocean (Hanson, 1977), estimating 42 – 74 % of this exudate respired in ¹⁴C-glucose incubations of water collected near *Sargassum*. Remineralization in the surface ocean by the heterotrophic microbial community may also be regulated by nutrient pulses (Cox,
2024). In this work, 8-day incubations of *Sargassum*-derived DOM amended with MPn observed significant total organic carbon (TOC) concentration decrease correlated with proliferation of *Rhodobacterales* and *Pelagibacterales* cells, implicating chemotrophic bacterioplankton, particularly members of Alphaproteobacteria, and their access to P<sub>org</sub> as key factors in DOC turnover under high phosphonate availability regimes (Cox, 2024). Proliferation of *Vibrionales* in *Sargassum* samples under stranding simulations (Mendonça et al., 2024) and high relative abundance of *Enterobacterales* in *Sargassum* samples collected from tidal locations in the Caribbean Basin (Hervé et al., 2021) provide some support for *Vibrio*-related bacteria also playing a notable role in *Sargassum*-derived organic matter consumption and *Sargassum* degradation.

In conical reactor incubations emulating degradation of *Sargassum* on beaches and at confinement sites, 16S rRNA gene sequencing of the microbial community associated with *Sargassum* indicated that leachate reduction coincided with proliferation of members of *Cyanobacteriota* and *Pseudomonadota* after 30 days, primarily *Fischerella* spp., *Hydrogenophilus* spp., and *Dolicospermum* spp. (Domínguez-Maldonado et al., 2024). A similar study which additionally incubated *Sargassum* with municipal organic solid waste found members of *Bacteroidota* (69.9 % total community) and *Bacillota* (13.3 % total community) composing the top two phyla, respectively (Salgado-Hernández et al., 2023). A follow-up study by Salgado-Hernández et al. (2024) found that these two phyla remained the most represented amongst microbial responders despite undergoing periods of anaerobic incubation.

These studies highlight the differences between aquatic, terrestrial, and reactor-based degradation of *Sargassum* biomass in regard to selected microbial communities, yet all agree that members of *Pseudomonadota* and *Bacteroidota* are commonly abundant. Interestingly, *Desulfobacterota* was also seen throughout multiple studies and its presence was associated with *Sargassum* senescence and degradation (Cox, 2024; Domínguez-Maldonado et al., 2024; Hervé et al., 2021; 2024; Salgado-Hernández et al., 2024). Notably, all these studies collected the associated microbiome from *Sargassum* which was either enclosed in a gas-tight containers (sealed glass bottle, conical reactors) or were in a dense, confined space (containment sites, beached rafts) which may have allowed for anoxia to occur. Without correlating oxygen measures from the collection sites to the data in these studies, it is still unclear as to whether members of *Desulfobacterota* are involved in initial or late-stage organic matter cycling. Moreover, it is unclear whether this group participates in *Sargassum*-derived DOM remineralization under typical surface ocean conditions.

# 4. Future directions

## 4.1. Molecular diversity of the *Sargassum* holobiont

Several factors can obfuscate our vision of the *Sargassum* microbiome. Differing *Sargassum* microbiome isolation protocols, sample vortexing and sonication (Hervé et al., 2021; Michotey et al., 2020; Stiffler et al., 2024; Torralba et al., 2017) compared to use of whole-tissue crushed samples (Mendonça et al., 2024; Salgado-Hernández et al., 2023; Theirlynck et al., 2023) can hamper direct comparison between studies. To establish standard procedures, we recommend physical detachment-based extraction methods to ensure epibiont data is not obscured by endophytic organisms and to reduce the macroalgal DNA signal. Incorporation of peptide nucleic acid clamps can further limit the amplification of *Sargassum* plastid and mitochondria (Hervé et al., 2021). To study the endophytic community, we recommend adoption of protocols applied to other marine algae (Kjer et al., 2010) which includes sterilization of surfaces prior to DNA extraction.

Variable rRNA gene copy numbers between phylogenetically distinct taxa can bias our view of microbial abundances for prokaryotes (Louca et al., 2018; Stoddard et al., 2015; Věrovský & Baldrian, 2013) and eukaryotes (Gong & Marchetti, 2019). A comprehensive study found

that *Vibrionaceae* bacteria had 5 - 15, *Pseudomonadaceae* 4 - 7, *Flavobacteriaceae* 1 - 6, and *Rhodobacteraceae* 1 - 4 16S rRNA gene copies (Věrovský & Baldrian, 2013). Additionally, long-read sequencing technologies for both eukaryotic (Overgaard et al., 2024) and prokaryotic (Martijn et al., 2019) metabarcoding as well as genome-centric analyses using MAGs (Stiffler et al., 2024; Tadrent et al., 2023) are preferential to short-read amplicons for metabarcoding studies, which limits taxonomic assignment resolution (Martijn et al., 2019). Finally, studies need to provide easily accessible metadata, raw reads (e.g., fastq files), and processed (Amplicon Sequence Variant or Operational Taxonomic Unit table) data, the lack of which can prevent the reuse of the microbial community sequencing data (Jurburg et al., 2020) and inhibit our community-wide compilation efforts.

## 4.2. Nitrogen cycling within *Sargassum* holobionts

Currently, there is a need to pair quantification of fixed nitrogen by *Sargassum* with presence and activity of associated diazotrophs. To strengthen our ability to quantify nitrogen fixation, we recommend combining the acetylene reduction method, which is a measure of gross nitrogen fixation, with incubations of *Sargassum* in ¹⁵N₂ enriched environments and mass spectrometry analysis to estimate net nitrogen fixation. Contextualization of these data to changes in biomass isotope signature, growth rates, and elemental stoichiometry of the macroalgal tissue can then be implemented to investigate whether nitrogen is a driver for *Sargassum* proliferation. In particular, sampling efforts in open waters and in areas of high growth would be of significant value toward answering these questions.

An analysis of *nif* gene abundance extracted from RNA would also be particularly relevant for linking presence and activity. Given no species or strain possessing only the *nifH* gene has demonstrated diazotrophic activity (Mise et al., 2021), it is crucial that we focus not only on *nifH* but also consider the entire *nif* operon. While current research often emphasizes N₂ fixation, a critical need remains for expanded studies on other key processes in the *Sargassum* nitrogen cycle such as nitrification, denitrification, and the organisms driving them. Understanding the full cycle will remain incomplete without deeper investigation into these understudied components.

## 4.3. *Phosphorus* cycling within *Sargassum* holobionts

The *Sargassum* microbiome presents a unique testbed to study the role of P stress and metabolism in structuring macroalgal-microbiome symbiotic relationships. Must *Sargassum* compete for P with its microbiome, or are cooperative strategies to alleviate P stress employed? Can bacterial epibionts transfer remineralized P to host cells? How does Pᵢ availability modulate the composition and genetic capabilities of the *Sargassum* microbiome across its range? How can we disentangle microbiome changes driven by the changing chemical seascapes the holobiont experiences from variables such as host health?

Methods involving sequential filtration (Hoppe, 2003) could help distinguish the fraction of measured APase activity within rafts attributed to *Sargassum*-associated bacteria from macroalgal-derived enzymes to investigate modalities of the resultant P incorporation. Isotopic tracers can help examine if P liberated from P<sub>org</sub> compounds is retained by the microbial community or transferred to their macroalgal host while genomic and transcriptomic evidence can shed light on the organisms and mechanisms driving Pᵢ acquisition in *Sargassum* communities. Integrating these approaches with manipulative incubation experiments and physicochemical field measurements are recommended avenues to construct ecological models relevant to how *Sargassum* holobiont communities thrive in these extreme marine niches.
## 4.4. Carbon cycling within Sargassum holobionts

Much work is still needed to quantify the turnover of Sargassum-derived DOM in the open ocean and in coastal areas during inundation events. Higher resolution studies on the changes in Sargassum-derived DOM composition over time should help in this regard. While Sargassum-derived DOM is both photo- and biolabile, more information is needed on how these degradation mechanisms compete in the environment. Moreover, how the coupled photochemical production of biolabile products for Sargassum-derived DOM affects microbial community structure has yet to be studied and identification of the microbial groups involved in Sargassum-derived DOM turnover has just begun.

Key needs in this area relate to determining the microbial population directly involved in facilitating exudation and organic matter diagenesis, as well as any indirectly related groups providing key substrates supporting this remineralization activity. In particular, data from open ocean regions or under experimental conditions reflective of *in situ* conditions are needed for accurate characterization of active dynamics in the holobiont. Finally, resolving fluctuations in carbon release and remineralization rates as well as the microbial response with regional and temporal variables is necessary to relate this work to the ever-growing interest in using Sargassum for carbon dioxide removal efforts.

## 4.5. Sargassum-associated microbial roles in macroalgal services and other biogeochemical cycles

The limited knowledge on Sargassum microbiome dynamics included in this review evidences the vast opportunity for investigation outside community composition and major macronutrient cycling. Other seaweed holobiont studies investigated microbial functions in morphological development, antifouling, quorum sensing, oxidative burst mechanisms, salinity tolerance, and virulence (Dittami et al., 2016; Egan et al., 2013; Goecke et al., 2010; Lamarche et al., 2008; Meng et al., 2020), which may serve as inspiration for the Sargassum microbiome research community. Additionally, assessment of silicon, sulfur, and trace metal interactions stand out as particularly understudied biogeochemical aspects of Sargassum microbiome roles and functioning. This knowledge would be crucial for a holistic understanding of the ecological success of Sargassum as well as for informing predictive and mitigative response efforts to mass inundation events.

### CRediT authorship contribution statement

D.D. Cox: Conceptualization, Data curation, Formal analysis, Funding acquisition, Project administration, Validation, Visualization, Writing – original draft, Writing – review & editing. A.K. Stiffler: Conceptualization, Data curation, Formal analysis, Visualization, Writing – review & editing, Writing – original draft. V. Hervé: Conceptualization, Data curation, Formal analysis, Visualization, Writing – review & editing, Writing – original draft. M. Léger-Pigout: Writing – review & editing, Writing – original draft, Visualization, Conceptualization. L.C. Powers: Conceptualization, Data curation, Writing – review & editing, Writing – original draft. P.J. Lopez: Writing – review & editing, Writing – original draft, Visualization, Data curation, Conceptualization. O.A. Sosa: Writing – review & editing, Writing – original draft, Visualization, Data curation, Conceptualization. R.J. Parsons: Writing – review & editing, Writing – original draft, Conceptualization.

### Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

### Acknowledgements

We would like to acknowledge all cited authors for their foundational work which made this review possible. AS was funded by the National Science Foundation's Graduate Research Fellowship Program (2023349872). VH and PJL were supported by the Labex DRIIHM, the French "Investissements d'Avenir" program (ANR-11-LABX-0010), managed by the ANR and the Human-Environment Observatory Littoral Caraibe. OAS was supported by the National Science Foundation OCE-2241668.

### Supplementary materials

Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.hal.2025.102904.

### Data availability

All data presented in this article are included as supplementary files.

### References

Alleyne, K.S.T., Neat, F., Oxenford, H.A., 2023. A baseline assessment of the epiphytic community associated with pelagic Sargassum in the Tropical Atlantic. Aquat. Bot. 186, 103635. https://doi.org/10.1016/j.aquabot.2023.103635.

Amador-Castro, F., García-Cayuela, T., Alper, H.S., Rodriguez-Martinez, V., Carrillo-Nieves, D., 2021. Valorization of pelagic Sargassum biomass into sustainable applications: Current trends and challenges. J. Environ. Manag. 283, 112013. https://doi.org/10.1016/j.jenvman.2021.112013.

Baker, P., Minzlaff, U., Schoenle, A., Schwabe, E., Hohlfeld, M., Jeuck, A., Brenke, N., Prausse, D., Rothenbecl, M., Brix, S., Frutos, I., Jörger, K.M., Neusser, T.P., Koppemann, R., Devey, C., Brandt, A., Arndt, H., 2018. Potential contribution of surface-dwelling Sargassum algae to deep-sea ecosystems in the southern North Atlantic. Deep Sea Res. II: Top. Stud. Oceanogr. 148, 21–34. https://doi.org/10.1016/j.dsr2.2017.10.002.

Baker-Austin, C., Oliver, J.D., Alam, M., Ali, A., Waldor, M.K., Qadri, F., Martinez-Urtaza, J., 2018. Vibrio spp. Infections. Nat. Rev. Dis. Primers 4 (1), 1–19. https://doi.org/10.1038/s41572-018-00056-8.

Baker-Austin, C., Stockley, L., Rangdale, R., Martinez-Urtaza, J., 2010. Environmental occurrence and clinical impact of Vibrio vulnificus and Vibrio parahaemolyticus: A European perspective. Environ. Microbiol. Rep. 2 (1), 7–18. https://doi.org/10.1111/j.1758-2229.2009.00096.x.

Basso, J.T.R., Ankrah, N.Y.D., Tuttle, M.J., Grossman, A.S., Sandaa, R.-A., Buchan, A., 2020. Genetically similar temperate phages form coalitions with their shared host that lead to niche-specific fitness effects. ISME J. 14 (7), 1688–1700. https://doi.org/10.1038/s41396-020-0637-z.

Björkman, K.M., Karl, D.M., 2003. Bioavailability of dissolved organic phosphorus in the euphotic zone at Station ALOHA, North Pacific Subtropical Gyre. Limnol. Oceanogr. 48 (3), 1049–1057. https://doi.org/10.4319/lo.2003.48.3.1049.

Bomber, J.W., Morton, S.L., Babinchak, J.A., Norris, D.R., Morton, J.G., 1988. Epiphytic dinoflagellates of drift algae—Another toxigenic community in the ciguatera food chain. Bull. Mar. Sci. 43 (2), 204–214.

Breitbart, M., Bonnain, C., Malki, K., Sawaya, N.A., 2018. Phage puppet masters of the marine microbial realm. Nat. Microbiol. 3 (7), 754–766. https://doi.org/10.1038/s41564-018-0166-y.

Brylinsky, M., 1977. Release of dissolved organic matter by some marine macrophytes. Mar. Biol. 39, 213–220. https://doi.org/10.1007/BF00390995.

Butler, J.N., Morris, B.F., Cadwallader, J., Stoner, A.W., 1983. Studies of Sargassum and the Sargassum community. In: Bermuda biological station for research special publications, 22. Bermuda Biological Station.

Cardona, Y., Bracco, A., Villareal, T.A., Subramaniam, A., Weber, S.C., Montoya, J.P., 2016. Highly variable nutrient concentrations in the Northern Gulf of Mexico. Deep Sea Res. II: Top. Stud. Oceanogr. 129, 20–30. https://doi.org/10.1016/j.dsr2.2016.04.010.

Carpenter, E.J., 1970. Diatoms attached to floating Sargassum in the western Sargasso Sea. Phycology 9 (3–4), 269–274. https://doi.org/10.2216/i0031-8884-9-3-269.1.

Carpenter, E.J., 1972. Nitrogen fixation by a blue-green epiphyte on pelagic Sargassum. Science 178 (4066), 1207–1209. https://doi.org/10.1126/science.178.4066.1207.

Carral-Murrieta, C.O., Serviere-Zaragoza, E., Rivero, F.R.C., Marques, A.C., Mendoza-Becerril, M.A., 2024. Sargassum species as hydrozoans substrates: Key patterns of association or just availability? Aquat. Bot. 191, 103738. https://doi.org/10.1016/j.aquabot.2023.103738.

Catarino, M.D., Silva-Reis, R., Chouh, A., Silva, S., Braga, S.S., Silva, A.M., Cardoso, S.M., 2023. Applications of antioxidant secondary metabolites of Sargassum spp. Mar. Drugs 21 (3), 172. https://doi.org/10.3390/md21030172.

Changeux, T., Berline, L., Podlejski, W., Guillot, T., Stiger-Pouvreau, V., Connan, S., Thibaut, T., 2023. Variability in growth and tissue composition (CNP, natural isotopes) of the three morphotypes of holopelagic Sargassum. Aquat. Bot. 187, 103644. https://doi.org/10.1016/j.aquabot.2023.103644.
Conkright, M.E., Gregg, W.W., Levitus, S., 2000. Seasonal cycle of phosphate in the open ocean. Deep Sea Res. I: Oceanogr. Res. Pap. 47 (2), 159–175. https://doi.org/10.1016/S0967-0637(99)00042-4.

Corbin, M., Oxenford, H.A., 2023. Assessing growth of pelagic sargassum in the tropical Atlantic. Aquat. Bot. 187, 103654. https://doi.org/10.1016/j.aquabot.2023.103654.

Coutinho, F.H., Thompson, C.C., Cabral, A.S., Paranhos, R., Dutilh, B.E., Thompson, F.L., 2019. Modelling the influence of environmental parameters over marine planktonic microbial communities using artificial neural networks. Sci. total environ. 677, 205–214.

Cox, D., 2024. Microbial phosphonate utilization within macroalgal holobionts and oligotrophic surface water communities. (Order No. 31562883). Available from Dissertations & Theses @ University of California. ProQuest Dissertations & Theses A&I; ProQuest Dissertations & Theses Global; Publicly Available Content Database. https://www.proquest.com/dissertations-theses/microbial-phosphonate-utilization-in-within/docview/3163372323/se-2.

Cox, D.D., Parsons, R.J., Van Mooy, B.A.S., Valentine, D.L., 2023. Methylphosphonate is utilized by commensal microbiota of macroalgae in the oligotrophic Sargasso Sea. J. Geophys. Res.: Oceans 128, e2023JC020315. https://doi.org/10.1029/2023JC020315.

Dassié, E.P., Gourves, P.Y., Cipolloni, O., Pascal, P.Y., Baudrimont, M., 2022. First assessment of Atlantic open ocean Sargassum spp. metal and metalloid concentrations. Environ. Sci. Pollut. Res. 1–11. https://doi.org/10.1007/s11356-021-17047-8.

Davis, D., Simister, R., Campbell, S., Marston, M., Bose, S., McQueen-Mason, S.J., Gomez, L.D., Gallimore, W.A., Tonon, T., 2021. Biomass composition of the golden tide pelagic seaweeds *Sargassum fluitans* and S. natans (morphotypes I and VIII) to inform valorisation pathways. Sci. Total Environ. 762, 143134. https://doi.org/10.1016/j.scitotenv.2020.143134.

Dembitsky, V.P., Rozentsvet, O.A., Pechenkina, E.E., 1990. Glycolipids, phospholipids and fatty acids of brown algae species. Phytochemistry 29 (11), 3417–3421. https://doi.org/10.1016/0031-9422(90)85249-F.

Devault, D.A., Pierre, R., Marfaing, H., Dolique, F., Lopez, P.J., 2021. *Sargassum* contamination and consequences for downstream uses: a review. J. Appl. Phycol. 33, 567–602. https://doi.org/10.1007/s10811-020-02250-w.

Dilworth, M.J., 1966. Acetylene reduction by nitrogen-fixing preparations from *Clostridium pasteuriarum*. Biochim. Biophys. Acta (BBA)-Gen. Subj. 127 (2), 285–294. https://doi.org/10.1016/0304-4165(66)90383-7.

Dittami, S.M., Duboscq-Bidot, L., Perennou, M., Gobet, A., Corre, A., Boyen, C., Tonon, T., 2016. Host-microbe interactions as a driver of acclimation to salinity gradients in brown algal cultures. ISME j. 10 (1), 51–63. https://doi.org/10.1038/ismej.2015.104.

Domínguez-Maldonado, J.A., Solís-Pereira, S.E., Valle-Gough, R.E., Álvarez, A.A.M., Olguín-Maciel, E., Alzate-Gaviria, L., Tapia-Tussell, R., 2024. Microbial communities present in *Sargassum* spp. Leachates from the Mexican Caribbean which are involved in their degradation in the environment, a tool to tackle the problem. Environ. Sci. Pollut. Res. 31 (13), 19904–19916. https://doi.org/10.1007/s11356-024-32363-5.

Dooley, J.K., 1972. Fishes associated with the pelagic *Sargassum* complex, with a discussion of the *Sargassum* community. Contrib. Mar. Sci. 16, 1–32. Retrieved from. https://www.researchgate.net/publication/285697458_FishesAssociatedWithThePelagicSargassumcomplexwithadiscussionoftheSargassumcommunity.

Douglas, E.J., Haggitt, T.R., Rees, T.A.V., 2014. Supply-and demand-driven phosphate uptake and tissue phosphorus in temperate seaweeds. Aquat. Biol. 23 (1), 49–60. https://doi.org/10.3354/ab00601.

Egan, S., Harder, T., Burke, C., Steinberg, P., Kjelleberg, S., Thomas, G., 2013. The seaweed holobiont: understanding seaweed-bacteria interactions. FEMS Microbiol. Rev. 37 (3), 462–476. https://doi.org/10.1111/1574-6976.12011.

Engelthaler, D., Lewis, K., Anderson, S., Snow, S., Gladden, L., Hammond, R.M., Hutchinson, R. J., Ratard, R., Straif-Bourgeois, S., Sokol, T., Thomas, A., Mena, L., Parham, J., Hand, S., McNeill, M., Byers, P., Amy, B., Charnas, G., Rolling, J., Friedman, A., 2005. *Vibrio* Illnesses After Hurricane Katrina–Multiple States, August-September 2005. MMWR: Morb. Mortal. Wkly. Rep. 54 (37) Retrieved from https://www.cdc.gov/mmwr/preview/mmwrhtml/mm5437a5.htm.

English, C.J., Parsons, R., Yongblah, K., Opalk, K., Carlson, C.A., 2025. Photooxidation removes biologically recalcitrant dissolved organic carbon released by the macroalgal Sargassum natans. Limnology and Oceanography Letters.

Eom, S.H., Kim, Y.M., Kim, S.K., 2012. Antimicrobial effect of phlorotrichins from marine brown algae. Food Chem. Toxicol. 50 (9), 3251–3255. https://doi.org/10.1016/j.fct.2012.06.028.

European Union Copernicus Marine Service, 2024. Global ocean biogeochemistry analysis and forecast: 30 Sep 2021 to 26 Sep 2024. https://doi.org/10.48670/moi-00015 [Dataset].

Fagundo-Mollineda, A., Robledo, D., Vásquez-Elizondo, R.M., Freile-Pelegrín, Y., 2023. Antioxidant activities in holopelagic *Sargassum* species from the Mexican Caribbean: Temporal changes and intra-thallus variation. Algal Res. 76, 103289. https://doi.org/10.1016/j.algal.2023.103289.

Gheorghta Puscaselu, R., Lobiuc, A., Dimian, M., Covasa, M., 2020. Alginate: From food industry to biomedical applications and management of metabolic disorders. Polymers 12 (10), 2417. https://doi.org/10.3390/polym12102417.

Global Ocean Biogeochemistry Analysis and Forecast, 2021. E.U. Copernicus Marine Service Information [Dataset]. Accessed through Resource Watch, (date). https://www.resourcewatch.org.

Gobert, T., Gautier, A., Connan, S., Rouget, M.L., Thibaut, T., Stiger-Pouvreau, V., Waeles, M., 2022. Trace metal content from holopelagic Sargassum spp. sampled in the tropical North Atlantic Ocean: Emphasis on spatial variation of arsenic and phosphorus. Chemosphere 308, 136186. https://doi.org/10.1016/j.chemosphere.2022.136186.

Goecke, F., Labes, A., Wiese, J., Imhoff, J.F., 2010. Chemical interactions between marine macroalgae and bacteria. Mar. ecol. prog. ser. 409, 267–299. https://doi.org/10.3354/meps08607.

Goldsmith, D.B., Brum, J.R., Hopkins, M., Carlson, C.A., Breitbart, M., 2015. Water column stratification structures viral community composition in the Sargasso Sea. Aquat. Microb. Ecol. 76 (2), 85–94. https://doi.org/10.3354/ame01768.

Gong, W., Marchetti, A., 2019. Estimation of 18S Gene Copy Number in Marine Eukaryotic Plankton Using a Next-Generation Sequencing Approach. Front. Mar. Sci. 6. https://doi.org/10.3389/fmars.2019.00219.

Gouvêa, L.P., Assis, J., Gurgel, C.F., Serrão, E.A., Silveira, T.C., Santos, R., Duarte, C.M., Peres, L.M.C., Carvalho, V.F., Batista, M., Bastos, E., Sissini, M.N., Horta, P.A., 2020. Golden carbon of Sargassum forests revealed as an opportunity for climate change mitigation. Sci. Total Environ. 729, 138745. https://doi.org/10.1016/j.scitotenv.2020.138745.

Gower, J., King, S., 2020. The distribution of pelagic Sargassum observed with OLCI. Int. J. Remote Sens. 41 (15), 5669–5679. https://doi.org/10.1080/01431161.2019.1658240.

Grimes, D.J., 2020. The Vibrios: Scavengers, Symbionts, and Pathogens from the Sea. Microb. Ecol. 80 (3), 501–506. https://doi.org/10.1007/s00248-020-01524-7.

Gruber, N. (2008). The marine nitrogen cycle: overview and challenges. In D. G. Capone, D. A. Bronk, M. R. Mulholland, & E. J. Carpenter (Eds), *Nitrogen in the marine environment* (2nd ed., pp. 1-50). https://doi.org/10.1016/B978-0-12-372522-6.00001-3.

Güemes, A.G.C., Youle, M., Cantú, V.A., Felts, B., Nulton, J., Rohwer, F., 2016. Viruses as Winners in the Game of Life. Annu. Rev. Virol. 3 (Volume 3, 2016), 197–214. https://doi.org/10.1146/annurev-virology-100114-054952.

Guillou, L., Bachar, D., Audic, S., Bass, D., Berney, C., Bittner, L., Boutte, C., Burgaud, G., de Vargas, C., Decelle, J., del Campo, J., Dolan, J.R., Dunthorn, M., Edvardsen, B., Holzmann, M., Kooistra, W.H.C.F., Lara, E., Le Bescot, N., Logares, R., Mahé, F., Massana, R., Montresor, M., Morard, R., Not, F., Pawlowski, J., Probert, I., Sauvadet, A., Siano, R., Stoeck, T., Vaulot, D., Zimmermann, P., Christen, R., 2013. The Protist Ribosomal Reference database (PR2): A catalog of unicellular eukaryote Small Sub-Unit rRNA sequences with curated taxonomy. Nucleic Acids Res 41 (D1), D597–D604. https://doi.org/10.1093/nar/gks1160.

Hanson, R.B., 1977. Pelagic Sargassum community metabolism: Carbon and nitrogen. J. Exp. Mar. Biol. Ecol. 29 (2), 107–118. https://doi.org/10.1016/0022-0981(77)90042-9.

Hellriegel, H., & Wilfarth, H. (1888). Untersuchungen über die Stickstoffnahrung der Grämenine und Leguminosen. https://doi.org/10.5962/bhl.title.27102.

Haugan, J.A., Liaaen-Jensen, S., 1994. Algal carotenoids 54. Carotenoids of brown algae (Phaeophyceae). Biochem. Syst. Ecol. 22 (1), 31–41. https://doi.org/10.1016/0305-1978(94)90112-0.

Hernández, I., Niell, F.X., Whitton, B.A., 2002. Phosphatase activity of benthic marine algae. An overview. J. Appl. Phycol. 14, 475–487. https://doi.org/10.1023/A:1022370526665.

Hernández-Almaraz, P., Méndez-Rodríguez, L., Zenteno-Savín, T., et al., 2016. Concentrations of trace elements in sea urchins and carotenoids commonly present in Sargassum beds: implications for trophic transfer. Ecol Res 31, 785–798. https://doi.org/10.1007/s11284-016-1390-7.

Hervé, V., Lambourdière, J., René-Trouillefou, M., Devault, D.A., Lopez, P.J., 2021. *Sargassum* differentially shapes the microbiota composition and diversity at coastal tide sites and inland storage sites on Caribbean Islands. Front. Microbiol. 12, 701155. https://doi.org/10.3389/fmicb.2021.701155.

Hervé, V., Sabatier, P., Lambourdière, J., Raymond, R., Foulquier, E., Le Berre, I., Lopez, P.J., 2024. Pesticide upsurge, cross-contamination and biodiversity: case studies from the Caribbean Coast Human-Environment Observatory. C. R. Géosci. 356 (S3), 1–22. https://doi.org/10.5802/crgeosci.236.

Hollants, J., Leliaert, F., Verbruggen, H., De Clerck, O., Willems, A., 2013. Host specificity and coevolution of Flavobacteriaceae endosymbionts within the siphonous green seaweed Bryopsis. Mol. phylogenetics evol. 67 (3), 608–614. https://doi.org/10.1016/j.jmpev.2013.02.025.

Hoppe, H.G., 2003. Phosphatase activity in the sea. Hydrobiologia 493, 187–200. https://doi.org/10.1023/A:1025453918247, 2003.

Hu, C., Wang, M., Lapointe, B.E., Brewton, R.A., Hernandez, F.J., 2021. On the Atlantic pelagic *Sargassum*’s role in carbon fixation and sequestration. Sci. Total Environ. 781, 146801. https://doi.org/10.1016/j.scitotenv.2021.146801.

Hulatt, C.J., Thomas, D.N., Bowers, D.G., Norman, L., Zhang, C., 2009. Exudation and decomposition of chromophoric dissolved organic matter (CDOM) from some temperate macroalgae. Estuar. Coast. Shelf Sci. 84 (1), 147–153. https://doi.org/10.1016/j.ecss.2009.06.014.

Johnson, C., Dubbs, L.L., Piehler, M., 2023. Reframing the contribution of pelagic Sargassum epiphytic N2 fixation. Plos one 18 (8), e0289485. https://doi.org/10.1371/journal.pone.0289485.

Johnson, D.L., Richardson, P.L., 1977. On the wind-induced sinking of Sargassum. J. Exp. Mar. Biol. Ecol. 28 (3), 255–267. https://doi.org/10.1016/0022-0981(77)90095-8.

Jurburg, S.D., Konzack, M., Eisenhauer, N., Heintz-Buschart, A., 2020. The archives are half-empty: An assessment of the availability of microbial community sequencing data. Commun. Biol. 3 (1), 1–8. https://doi.org/10.1038/s42003-020-01204-9.

Kendall, C., Elliott, E.M., Wankel, S.D., 2007. Tracing anthropogenic inputs of nitrogen to ecosystems. In: Michener, R.H., Lajtha, K. (Eds.), Stable isotopes in ecology and environmental science, 2nd ed. Blackwell Publishing, pp. 375–449. https://doi.org/10.1002/9780470691854.ch12.

Kjer, J., Debbab, A., Aly, A.H., Proksch, P., 2010. Methods for isolation of marine-derived endophytic fungi and their bioactive secondary products. Nat. Protoc. 5 (3), 479–490.
Kohlmeyer, J., 1971. Fungi from the Sargasso Sea. Mar. Biol. 8 (4), 344-350. https://doi.org/10.1007/BF00348012.

Kohlmeyer, J., 1972. Parasitic Haloguignardia oceanica (Ascomycetes) and hyperparasitic Sphaceloma cecidii sp. Nov. (Deuteromycetes) in drift Sargassum in North Carolina. J. Elisha Mitchell Sci. Soc. 88 (4), 255-259. https://www.jstor.org/stable/24334598.

Lamarche, M.G., Wanner, B.L., Crépin, S., Harel, J., 2008. The phosphate regula and bacterial virulence: a regulatory network connecting phosphate homeostasis and pathogenesis. FEMS Microbiol. Rev. 32 (3), 461-473. https://doi.org/10.1111/j.1574-6976.2008.00101.x.

Landeta-Salgado, C., Cicatiello, P., Stanzione, I., Medina, D., Berlanga Mora, I., Gomez, C., Lienqueo, M.E., 2021. The growth of marine fungi on seaweed polysaccharides produces cerato-platanin and hydrophobin self-assembling proteins. Microbiol. Res. 251, 126835. https://doi.org/10.1016/j.micres.2021.126835.

Lapointe, B.E., 1995. A comparison of nutrient-limited productivity in Sargassum natans from neritic vs. oceanic waters of the western North Atlantic Ocean. Limnol. Oceanogr. 40 (3), 625-633. https://doi.org/10.4319/lo.1995.40.3.0625.

Lapointe, B.E., Brewton, R.A., Herren, L.W., Wang, M., Hu, C., McGillicuddy Jr., D.J., Lindell, S., Hernandez, F.J., Morton, P.L., 2021. Nutrient content and stoichiometry of pelagic Sargassum reflects increasing nitrogen availability in the Atlantic Basin. Nat. Commun. 12 (1), 3060. https://doi.org/10.1038/s41467-021-23135-7.

Lapointe, B.E., West, L.E., Sutton, T.T., Hu, C., 2014. Ryther revisited: nutrient excretions by fishes enhance productivity of pelagic Sargassum in the western North Atlantic Ocean. J. Exp. Mar. Biol. Ecol. 458, 46-56. https://doi.org/10.1016/j.jembe.2014.05.002.

Lara, E., Vaqué, D., Sá, E.L., Boras, J.A., Gomes, A., Borrull, E., Díez-Vives, C., Teira, E., Pernice, M.C., Garcia, F.C., Forn, I., Castillo, Y.M., Peiró, A., Salazar, G., Morán, X.A., Massana, R., Català, T.S., Luna, G.M., Agustí, S., Estrada, M., Gasol, J.M., Duarte, C.M., 2017. Unveiling the role and life strategies of viruses from the surface to the dark ocean. Sci. Adv. 3 (9), e1602565. https://doi.org/10.1126/sciadv.1602565.

Léger-Pigot, M., Navarro, E., Ménard, F., Ruitton, S., Le Loc'h, F., Guasco, S., Munaron, J.-M., Thibault, D., Changeux, T., Connan, S., Stiger-Pouvreau, V., Thibaut, T., Michotey, V., 2024. Predominant heterotrophic diazotrophic bacteria are involved in *Sargassum* proliferation in the Great Atlantic *Sargassum* Belt. ISME J. 18 (1). https://doi.org/10.1093/ismejo/wrad026 wrad026.

Le Lann, K., Surget, G., Couteau, C., Coiffard, L., Cérantola, S., Gaillard, F., Larnicol, M., Zubia, M., Guérard, F., Poupart, N., Stiger-Pouvreau, V., 2016. Sunscreen, antioxidant, and bactericidal capacities of phlorotannins from the brown macroalgal Halidrys siliquosa. J. Appl. Phycol. 28 (6), 3547-3559. https://doi.org/10.1007/s10811-016-0853-0.

Li, Y., Wang, J., Sun, T., Yu, X., Yang, Z., Zhao, Y., Tang, X., Xiao, H., 2024. Community structure of endophytic bacteria of *Sargassum tubergii* in the intertidal zone of Qingdao in China. AMB Express 14 (1), 35. https://doi.org/10.1186/s13568-024-01688-2.

Liang, J., Liu, J., Wang, X., Lin, H., Liu, J., Zhou, S., Sun, H., Zhang, X.H., 2019. Spatiotemporal dynamics of free-living and particle-associated *Vibrio* communities in the northern Chinese marginal seas. Appl. environ. microbiol. 85 (9), e00217-19. https://doi.org/10.1128/AEM.00217-19.

Lomas, M.W., Burke, A.L., Lomas, D.A., Bell, D.W., Shen, C., Dyhrman, S.T., Ammerman, J.W., 2010. Sargassum Sea phosphorus biogeochemistry: an important role for dissolved organic phosphorus (DOP). Biogeosciences 7 (2), 695-710. https://doi.org/10.5194/bg-7-695-2010.

López Miranda, J.L., Celis, E., Estévez, M., Chávez, V., van Tussenbroek, B.I., Uribe-Martínez, A., Cuevas, E., Rosillo Pantoja, I., Masia, L., Cauich-Kantun, C., Silva, R., 2021. Commercial potential of pelagic Sargassum spp. in Mexico. Front. Mar. Sci. 8, 768470. https://doi.org/10.3389/fmar.2021.768470.

López-Fuerte, F.O., Siqueiros Beltrones, D.A., Martínez, Y.J., Altamirano-Cerecedo, M., del, C., 2022. Floristics and Biogeochemical Affinity of Diatoms Attached to *Sargassum fluitans* (Børgesen) Børgesen and *Sargassum natans* (Linnaeus) Gaillon Arriving on Mexico's Caribbean Coasts. Diversity 14 (9). https://doi.org/10.3390/d14090758. Article 9.

Louca, S., Doebeli, M., Parfrey, L.W., 2018. Correcting for 16S rRNA gene copy numbers in microbiome surveys remains an unsolved problem. Microbiome 6 (1), 41. https://doi.org/10.1186/s40168-018-0420-9.

Lu, M.-J., Henning, U., 1994. Superinfection exclusion by T-even-type coliphages. Trends Microbiol. 2 (4), 137-139. https://doi.org/10.1016/0966-842X(94)90601-7.

Luo, H., Benner, R., Long, R.A., Hu, J., 2009. Subcellular localization of marine bacterial alkaline phosphatases. Proc. Natl. Acad. Sci. 106 (50), 21219-21223. https://doi.org/10.1073/pnas.0907586106.

Luo, H., Zhang, H., Long, R.A., Benner, R., 2011. Depth distributions of alkaline phosphatase and phosphonate utilization genes in the North Pacific Subtropical Gyre. Aquat. Microb. Ecol. 62 (1), 61-69. https://doi.org/10.3354/ame01458.

Machado, C.B., Maddix, G.M., Francis, P., Thomas, S.L., Burton, J.A., Langer, S., Larson, T.R., Marsh, R., Webber, M., Tonon, T., 2022. Pelagic *Sargassum* events in Jamaica: Provenance, morphotype abundance, and influence of sample processing on biochemical composition of the biomass. Sci. Total Environ. 817, 152761. https://doi.org/10.1016/j.scitotenv.2021.152761.

Macovei, V.A., Torres-Valdés, S., Hartman, S.E., Schuster, U., Moore, C.M., Brown, P.J., Hydes, D.V., Sanders, R.J., 2019. Temporal variability in the nutrient biogeochemistry of the surface North Atlantic: 15 years of ship of opportunity data. Glob. Biogeochem. Cycles 33 (12), 1674-1692. https://doi.org/10.1029/2018GB006132.

Magaña-Gallegos, E., García-Sánchez, M., Graham, C., Olivos-Ortíz, A., Siuda, A.N., van Tussenbroek, B.I., 2023. Growth rates of pelagic Sargassum species in the Mexican

Caribbean. Aquat. Bot. 185, 103614. https://doi.org/10.1016/j.aquabot.2022.103614.

Maples, R., 1984. The epiphytic diatom flora of two *Sargassum* species. Gulf Caribb. Res. 7 (4), 373-375. https://doi.org/10.18785/grr.0704.09.

Marsh, R., Skiris, L., Tompkins, E.L., Dash, J., Dominguez Almela, V., Tonon, T., Oxenford, H.A., Webber, M., 2023. Climate-*Sargassum* interactions across scales in the tropical Atlantic. PLOS Clim 2 (7), e0000253. https://doi.org/10.1371/journal.pclm.0000253.

Martijn, J., Lind, A.E., Schön, M.E., Spiertz, I., Juzokaite, L., Bunikis, I., Pettersson, O.V., Ettema, T.J.G., 2019. Confident phylogenetic identification of uncultured prokaryotes through long read amplicon sequencing of the 16S-ITS-23S rRNA operon. Environ. Microbiol. 21 (7), 2485-2498. https://doi.org/10.1111/1462-2920.14636.

Martínez, A., Ventouras, L.A., Wilson, S.T., Karl, D.M., DeLong, E.F., 2013. Metatranscriptomic and functional metagenomic analysis of methylphosphonate utilization by marine bacteria. Front. Microbiol. 4, 1-18. https://doi.org/10.3389/fmicb.2013.00340.

Martorell, M.M., Lannert, M., Matula, C.V., Quartino, M.L., de Figueroa, L.I.C., Cormack, W.M., Ruberto, L.A.M., 2021. Studies toward the comprehension of fungal-macrolife interactions in cold marine regions from a biotechnological perspective. Fungal Biol. 125 (3), 218-230. https://doi.org/10.1016/j.funbio.2020.11.003.

McGillicuddy Jr., D.J., Morton, P.L., Brewton, R.A., Hu, C., Kelly, T.B., Solow, A.R., Lapointe, B.E., 2023. Nutrient and arsenic biogeochemistry of Sargassum in the western Atlantic. Nat. Commun. 14 (1), 6205. https://doi.org/10.1038/s41467-023-41904-4.

Mendonça, I.R.W., Theirlynck, T., Zettler, E.R., Amaral-Zettler, L.A., Oliveira, M.C., 2024. Microbiome changes in a stranding simulation of the helopelagic macroalgae *Sargassum natans* and *Sargassum fluitans*. Ocean Coast. Res. 72, e24037. https://doi.org/10.1590/2675-2824072.23111.

Meng, X., Ahator, S.D., Zhang, L.H., 2020. Molecular mechanisms of phosphate stress activation of *Pseudomonas aeruginosa* quorum sensing systems. mSphere 5 (2). https://doi.org/10.1128/mSphere.00119-20 e00119-20.

Michotey, V., Blanfuné, A., Chevalier, C., Garel, M., Diaz, F., Berline, L., Le Grand, L., Armougoum, F., Guasco, S., Ruitton, S., Changeux, T., Belloni, B., Blanchot, J., Ménard, F., Thibaut, T., 2020. In situ observations and modelling revealed environmental factors favouring occurrence of *Vibrio* in microbiome of the pelagic Sargassum responsible for strandings. Sci. Total Environ. 748, 141216. https://doi.org/10.1016/j.scitotenv.2020.141216.

Middelboe, M., Traving, S.J., Castillo, D., Kalatzis, P.G., Glud, R.N., 2025. Prophage-encoded chitinase gene supports growth of its bacterial host isolated from deep-sea sediments. The ISME Journal 19 (1), wrafo004.

Mincer, T.J., Bos, R.P., Zettler, E.R., Zhao, S., Asub, A.A., Orsi, W.D., Guzzetta, V.S., Amaral-Zettler, L.A., 2023. Sargasso Sea *Vibrio* bacteria: Underexplored potential pathovars in a perturbed habitat. Water Res. 242, 120033. https://doi.org/10.1016/j.watres.2023.120033.

Mise, K., Masuda, Y., Senoo, K., Itoh, H., 2021. Undervalued pseudo-nifH sequences in public databases distort metagenomic insights into biological nitrogen fixers. M sphere 6 (6). https://doi.org/10.1128/msphere.00785-21 e00785-21.

Miyashita, K., Mikami, N., Hosokawa, M., 2013. Chemical and nutritional characteristics of brown seaweed lipids: A review. J. Funct. Foods 5 (4), 1507-1517. https://doi.org/10.1016/j.jff.2013.09.019.

Mohapatra, B.R., 2023. Phylogenetic and functional characterization of the microbiome of *Sargassum* seaweed waste. Appl. Phycol. 4 (1), 87-98. https://doi.org/10.1080/26388081.2023.2220384.

Montoya, J.P., 2008. Nitrogen stable isotopes in marine environments. In: Capone, D.G., Bronk, D.A., Mulholland, M.R., Carpenter, E.J. (Eds.), Nitrogen in the marine environment, 2nd ed., pp. 1277-1302. https://doi.org/10.1016/B978-0-12-372522-6.00029-3.

Moore, R.M., Harrison, A.O., McAllister, S.M., Polson, S.W., Wommack, K.E., 2020. Iroki: Automatic customization and visualization of phylogenetic trees. PeerJ 8, e8584. https://doi.org/10.7717/peerj.8584.

Moore, C.M., Mills, M.M., Arrigo, K.R., Berman-Frank, I., Bopp, L., Boyd, P.W., Galbraith, E.D., Geider, R.J., Guieu, C., Jaccard, S.L., Jickells, T.D., La Roche, J., Lenton, T.M., Mahowald, N.M., Marañón, E., Marinov, I., Moore, J.K., Nakatsuka, O., Oschlies, A., Saito, M.A., Thingstad, T.F., Tsuda, A., Ulloa, O., 2013. Processes and patterns of oceanic nutrient limitation. Nat. geosci. 6 (9), 701-710. https://doi.org/10.1038/ngeo1765.

Nielsen, B.V., Milledge, J.J., Hertler, H., Maneein, S., Al Farid, M.M., Bartlett, D., 2021. Chemical characterisation of *Sargassum* inundation from the turks and caicos: seasonal and post stranding changes. Phycology 1 (2), 143-162. https://doi.org/10.3390/phycoly1020011.

Okolodkov, Y.B., Campos-Bautista, G., Gárate-Lizárraga, I., González-González, J., Hoppenrath, M., Arenas, V., 2007. Seasonal changes of benthic and epiphytic dinoflagellates in the Veracruz reef zone, Gulf of Mexico. Aquat. Microb. Ecol. 47, 223-237. https://doi.org/10.3354/ame047223.

Overgaard, C.K., Jamy, M., Radutoiu, S., Burki, F., Dueholm, M.K.D., 2024. Benchmarking long-read sequencing strategies for obtaining ASV-resolved rRNA operons from environmental microeukaryotes. Mol. Ecol. Resour. 24 (7), e13991. https://doi.org/10.1111/17559-0998.13991.

Parr, A.E., 1939. Quantitative observations on the pelagic Sargassum vegetation of the western North Atlantic. *Bull. Bingham ocean. Coll.* 6, 1-94. Retrieved from. https://cir.nii.ac.jp/crid/1573105974476221824.

Parsons, R.J., Breitbart, M., Lomas, M.W., Carlson, C.A., 2012. Ocean time-series reveals recurring seasonal patterns of viroplankton dynamics in the northwestern Sargasso Sea. ISME J. 6 (2), 273-284. https://doi.org/10.1038/ismej.2011.101.
Parys, S., Rosenbaum, A., Kehraus, S., Reher, G., Glombitza, K.W., König, G.M., 2007. Evaluation of quantitative methods for the determination of polyphenols in algal extracts. J. nat. prod. 70 (12), 1865-1870. https://doi.org/10.1021/np070302f.

Patyshakuliyeva, A., Falkoski, D.L., Wiebenga, A., Timmermans, K., de Vries, R.P., 2020. Macroalgae Derived Fungi Have High Abilities to Degrade Algal Polymers. Microorganisms 8 (1). https://doi.org/10.3390/microorganisms8010052 Article 1.

Paul, J.H., 2008. Prophages in marine bacteria: dangerous time molecular time bombs or the key to survival in the seas? ISME J. 2 (6), 579-589. https://doi.org/10.1038/ ismej.2008.35.

Perry, R.A., Vaudrey, J.M., Dierssen, H.M., 2018. Long range transport and carbon and nitrogen dynamics of floating seagrass wracks in Greater Florida Bay. Estuar. Coast. Shelf Sci. 209, 9-17. https://doi.org/10.1016/j.eess.2018.05.006.

Philips, E.J., Willis, M., Verchick, A., 1986. Aspects of nitrogen fixation in Sargassum communities off the coast of Florida. J. Exp. Mar. Biol. Ecol. 102 (2-3), 99-119. https://doi.org/10.1016/0022-0981(86)90170-X.

Philips, E., Zeman, C., 1990. Photosynthesis, growth and nitrogen fixation by epiphytic forms of filamentous cyanobacteria from pelagic *Sargassum*. Bull. mar. sci. 47 (3), 613-621. Retrieved from. https://www.ingentaconnect.com/content/umrsmas/ bullmar/1990/00000047/00000003/art00003.

Powers, L.C., Hertkorn, N., McDonald, N., Schmitt-Kopplin, P., Del Vecchio, R., Blough, N.V., Gonsior, M., 2019. *Sargassum* sp. act as a large regional source of marine dissolved organic carbon and polyphenols. Glob. Biogeochem. Cycles 33 (11), 1423-1439. https://doi.org/10.1029/2019GB006225.

Powers, L.C., Del Vecchio, R., Blough, N.V., McDonald, N., Schmitt-Kopplin, P., Gonsior, M., 2020. Optical properties and photochemical transformation of the dissolved organic matter released by *Sargassum*. Front. Mar. Sci. 7, 588287. https://doi.org/10.3389/fmars.2020.588287.

Price, M.N., Dehal, P.S., Arkin, A.P., 2010. FastTree 2 – Approximately Maximum-Likelihood Trees for Large Alignments. PLOS ONE 5 (3), e9490. https://doi.org/10.1371/journal.pone.0009490.

Remya, R.R., Samrot, A.V., Kumar, S.S., Mohanavel, V., Karthick, A., Chinaiyan, V.K., Muhibbullah, M., 2022. Bioactive potential of brown algae. Adsorpt. Sci. Technol. 2022, 9104835. https://doi.org/10.1155/2022/9104835.

Resiere, D., Valentino, R., Nevière, R., Banydeen, R., Gueye, P., Florentin, J., Cabié, A., Lebrun, T., Mégarbane, B., Guerrier, G., Mehdaoui, H., 2019. *Sargassum* seaweed on Caribbean islands: an international public health concern. Lancet (Lond. Engl.) 392 (10165), 2691. https://doi.org/10.1016/S0140-6736(18)32777-6.

Robledo, D., Vázquez-Delfín, E., Freile-Pelegrín, Y., Vásquez-Elizondo, R.M., Qui- Minet, Z.N., Salazar-Garibay, A., 2021. Challenges and opportunities in relation to *Sargassum* events along the Caribbean Sea. Front. Mar. Sci. 8, 699664. https://doi. org/10.3389/fmars.2021.699664.

Rohatgi, A., 2024. Webplotdigitizer: Version 5.2. Retrieved October 8, 2024 from. https://ps/automerisio/PlotDigitizer.

Rohwer, F., Thurber, R.V., 2009. Viruses manipulate the marine environment. Nature 459 (7244), 207-212. https://doi.org/10.1038/nature08060.

Rooker, J.R., Turner, J.P., Holt, S.A., 2006. Trophic ecology of *Sargassum*-associated fishes in the Gulf of Mexico determined from stable isotopes and fatty acids. Mar. Ecol. Prog. Ser. 313, 249-259. https://doi.org/10.3354/meps313249.

Rosignolo, J.A., Duran, A.J.F.P., Bueno, C., Martinelli Filho, J.E., Junior, H.S., Tonin, F. G., 2022. Algae application in civil construction: A review with focus on the potential uses of the pelagic *Sargassum* spp. biomass. J. Environ. Manag. 303, 114258. https://doi.org/10.1016/j.jenvmen.2021.114258.

Rowe, J.M., Saxton, M.A., Cottrell, M.T., DeBruyn, J.M., Berg, G.M., Kirchman, D.L., Hutchins, D.A., Wilhelm, S.W., 2008. Constraints on viral production in the Sargasso Sea and North Atlantic. Aquat. Microb. Ecol. 52 (3), 233-244. https://doi.org/10.13354/ame01231.

Saha, M., Dittami, S.M., Chan, C.X., Raina, J.B., Stock, W., Ghaderiardakani, F., Valathuparambil Baby John, A.M., Corr, S., Schleyer, G., Todd, J., Cardini, U., Bengtsson, M.M., Prado, S., Skillings, D., Sonnenschein, E.C., Engelen, A.H., Wang, G., Wichard, T., Brodie, J., Leblanc, C., Egan, S., 2024. Progress and future directions for seaweed holobiont research. New Phytol 244 (2), 364-376. https://doi.org/10.1111/nph.20018.

Saldarriaga-Hernandez, S., Hernandez-Vargas, G., Iqbal, H.M., Barceló, D., Parra- Saldívar, R., 2020. Bioremediation potential of *Sargassum* sp. biomass to tackle pollution in coastal ecosystems: Circular economy approach. Sci. Total Environ. 715, 136978. https://doi.org/10.1016/j.scitotenv.2020.136978.

Salgado-Hernández, E., Ortiz-Ceballos, Á.I., Alvarado-Lassman, A., Martínez- Hernández, S., Dorantes-Acosta, A.E., Rosas-Mendoza, E.S., 2024. Adaptation of a microbial consortium to pelagic *Sargassum* modifies its taxonomic and functional profile that improves biomethane potential. Environ. Sci. Pollut. Res. 31 (43), 55169-55186. https://doi.org/10.1007/s11356-024-34853-y.

Salgado-Hernández, E., Ortiz-Ceballos, Á.I., Alvarado-Lassman, A., Martínez- Hernández, S., Rosas-Mendoza, E.S., Velázquez-Fernández, J.B., Dorantes-Acosta, A. E., 2023. Energy-saving pretreatments affect pelagic *Sargassum* composition and DNA metabarcoding reveals the microbial community involved in methane yield. PLOS ONE 18 (8), e0289972. https://doi.org/10.1371/journal.pone.0289972.

Sawabe, T., 2006. The mutual partnership between Vibrio haliotifici and abalone. biol. vibrios 219-230.

Schets, F.M., Berg, H.H.J.L.van den, Demeulmeester, A.A., Dijk, E.van, Rutjes, S.A., Hooijdonk, H.J.P.van, Husman, A.M.de R, 2006. Vibrio alginolyticus infections in the Netherlands after swimming in the North Sea. Wkly. Releases (1997-2007) 11 (45), 3077. https://doi.org/10.2807/esw.11.45.03077-en.

Shank, G.C., Lee, R., Vähätalo, A., Zepp, R.G., Bartels, E., 2010a. Production of chromophoric dissolved organic matter from mangrove leaf litter and floating Sargassum colonies. Mar. Chem. 119 (1-4), 172-181. https://doi.org/10.1016/j. marchem.2010.02.002.

Shank, G.C., Zepp, R.G., Vähätalo, A., Lee, R., Bartels, E., 2010b. Photobleaching kinetics of chromophoric dissolved organic matter derived from mangrove leaf litter and floating Sargassum colonies. Mar. Chem. 119 (1-4), 162-171. https://doi.org/10.1016/j.marchem.2010.01.003.

Sieburth, J.M., Jensen, A., 1969. Studies on algal substances in the sea. II. The formation of Gelbstoff (humic material) by exudates of Phaeophyta. J. Exp. Mar. Biol. Ecol. 3 (3), 275-289. https://doi.org/10.1016/0022-0981(69)90051-3.

Simon, J.C., Marchesi, J.R., Mougel, C., Selosse, M.A., 2019. Host-microbiota interactions: from holobiont theory to analysis. Microbiome 7, 1-5. https://doi.org/10.1186/s40168-019-0619-4.

Sodders, N., 2023. Notes from the field: vibriosis cases associated with flood waters during and after Hurricane Ian—Florida, September–October 2022. MMWR. Morb. mortal. wkly. rep. 72. Retrieved from. https://www.cdc.gov/mmwr/volumes/72/wr/mm7218a5.htm.

Sosa, O.A., Repeta, D.J., Ferrón, S., Bryant, J.A., Mende, D.R., Karl, D.M., DeLong, E.F., 2017. Isolation and characterization of bacteria that degrade phosphonates in marine dissolved organic matter. Front. microbiol. 8, 1786. https://doi.org/10.3389/fmicb.2017.01786.

Sosa, O.A., Repeta, D.J., DeLong, E.F., Ashkezari, M.D., Karl, D.M., 2019. Phosphate- limited ocean regions select for bacterial populations enriched in the carbon–phosphorus lyase pathway for phosphonate degradation. Environ. microbiol. 21 (7), 2402–2414. https://doi.org/10.1111/1462-9200.14628.

Stabb, E.V., 2006. The *Vibrio fischeri-Europyrina scolopes* Light Organ Symbiosis. The Biology of Vibrios. John Wiley & Sons, Ltd., pp. 204–218. https://doi.org/10.1128/ 9781555815714.ch14.

Stifler, A.K., Hesketh-Best, P.J., Varona, N.S., Zagame, A., Wallace, B.A., Lapointe, B.E., Silveira, C.B., 2024. Genomic and induction evidence for bacteriophage contributions to *Sargassum*-bacteria symbioses. Microbiome 12 (1), 143. https://doi.org/10.1186/s40168-024-01860-7.

Stoddard, S.F., Smith, B.J., Hein, R., Roller, B.R., Schmidt, T.M., 2015. rnr DB: improved tools for interpreting rRNA gene abundance in bacteria and archaea and a new foundation for future research. Nucleic acids res. 43 (D1), D593–D598. https://doi.org/10.1093/nar/gku1201.

Suttle, C.A., 2007. Marine viruses—Major players in the global ecosystem. Nat. Rev. Microbiol. 5 (10), 801–812. https://doi.org/10.1038/nrmicro1750.

Tadrent, N., Dedeine, F., Hervé, V., 2023. SnakeMAGs: A simple, efficient, flexible and scalable workflow to reconstruct prokaryotic genomes from metagenomes. F1000Research 11, 1522. https://doi.org/10.12688/f1000research.1280912.

Tan, D., Hansen, M.F., de Carvalho, L.N., Roder, H.L., Burmolle, M., Middelboe, M., Lo Svenningsen, S., 2020. High cell densities favor lysogeny: Induction of an H2O2 prophage is repressed by quorum sensing and enhances biofilm formation in *Vibrio anguillarum*. ISME J. 14 (7), 1731–1742. https://doi.org/10.1038/s41396-020-0641-3.

Tena, D., Arias, M., Álvarez, B.T., Mauleón, C., Jiménez, M.P., Bisquert, J., 2010. Fulminant necrotizing fasciitis due to *Vibrio parahaemolyticus*. J. med. microbiol. 59 (2), 235-238.

Theirlynck, T., Mendonça, I.R.W., Engelen, A.H., Bolhuis, H., Collado-Vides, L., van Tussenbroek, B.I., García-Sánchez, M., Zettler, E., Muyzer, G., Amaral-Zettler, L., 2023. Diversity of the holopelagic *Sargassum* microbiome from the Great Atlantic *Sargassum* Belt to coastal stranding locations. Harmful Algae 122, 102369. https://doi.org/10.1016/j.hal.2022.102369.

Thickman, J.D., Gobler, C.J., 2017. The ability of algal organic matter and surface runoff to promote the abundance of pathogenic and non-pathogenic strains of *Vibrio parahaemolyticus* in Long Island Sound, USA. PLOS ONE 12 (10), e0185994. https://doi.org/10.1371/journal.pone.0185994.

Thompson, T.M., Young, B.R., Baroutian, S., 2020. Pelagic *Sargassum* for energy and fertiliser production in the Caribbean: A case study on Barbados. Renew. Sustain. Energy Rev. 118, 109564. https://doi.org/10.1016/j.rser.2019.109564.

Tonon, T., Machado, C.B., Webber, M., Webber, D., Smith, J., Pilsbury, A., Allen, M.J., 2022. Biochemical and elemental composition of pelagic *Sargassum* biomass harvested across the caribbean. Phycology 2 (1), 204–215. https://doi.org/10.3390/ phycology2010011.

Torralba, M.G., Franks, J.S., Gomez, A., Yooseph, S., Nelson, K.E., Grimes, D.J., 2017. Effect of Macondo Prospect 252 Oil on Microbial Community Associated with Pelagic *Sargassum* in the Northern Gulf of Mexico. Microb. Ecol. 73 (1), 91-100. https://doi.org/10.1007/s00248-017-0857-y.

University of South Florida Optical Oceanography Lab, 2022-2024. *Sargassum* Watch System for Jan 2022-March 2024 [Dataset]. https://optics.marine.usf.edu/projects/ saws.html.

van Tussenbroek, B.I., Arana, H.A.H., Rodríguez-Martínez, R.E., Espinoza-Avalos, J., Canizales-Flores, H.M., González-Godoy, C.E., Barba-Santos, M.G., Vega-Zepeda, A., Collado-Vides, L., 2017. Severe impacts of brown tides caused by *Sargassum* spp. on near-shore Caribbean seagrass communities. Mar. pollut. bull. 122 (1-2), 272-281. https://doi.org/10.1016/j.marpolbul.2017.06.057.

van Tussenbroek, B.I., Monroy-Velázquez, L.V., Rodríguez, D., Suescún-Bolívar, L.P., Thomé, P.E., Cerqueda-García, D., García-Maldonado, J.Q., Martínez-López, I.G., López-Portillo, J.A., Barba-Santos, M.G., Gómez-Reali, M.A., Escalante-Mancera, J. E., 2024. Monitoring drift and associated biodiversity of nearshore rafts of holopelagic *Sargassum* spp. in the Mexican Caribbean. Aquat. Bot. 195, 103792.

Vázquez-Delfín, E., Freile-Pelegrín, Y., Salazar-Garibay, A., Serviere-Zaragoza, E., Méndez-Rodríguez, J.C., Robledo, D., 2021. Species composition and chemical characterization of Sargassum influx at six different locations along the Mexican Caribbean coast. Sci. total Environ. 795, 148852. https://doi.org/10.1016/j. scitotenv.2021.148852.
Venter, J.C., Remington, K., Heidelberg, J.F., Halpern, A.L., Rusch, D., Eisen, J.A., et al., 2004. Environmental Genome Shotgun Sequencing of the Sargasso Sea. Science 304 (5667), 66–74. https://doi.org/10.1126/science.1093857.

Vetrovský, T., Baldrian, P., 2013. The Variability of the 16S rRNA Gene in Bacterial Genomes and Its Consequences for Bacterial Community Analyses. PLoS ONE 8 (2), e57923. https://doi.org/10.1371/journal.pone.0057923.

Vezzulli, L., Grande, C., Reid, P.C., Hélaouët, P., Edwards, M., Höfle, M.G., Brettar, I., Colwell, R.R., Pruzzo, C., 2016. Climate influence on Vibrio and associated human diseases during the past half-century in the coastal North Atlantic. Proc. Natl. Acad. Sci. 113 (34), E5062–E5071. https://doi.org/10.1073/pnas.1609157113.

Wada, S., Aoki, M.N., Tsuchiya, Y., Sato, T., Shinagawa, H., Hama, T., 2007. Quantitative and qualitative analyses of dissolved organic matter released from Ecklonia cava Kjellman, in Oura Bay, Shimoda, Izu Peninsula, Japan. J. Exp. Mar. Biol. Ecol. 349 (2), 344–358. https://doi.org/10.1016/j.jembe.2007.05.024.

Walther, K., Fries, L., 1976. Extracellular alkaline phosphatase in multicellular marine algae and their utilization of glycerophosphate. Physiol. Plant. 36 (2), 118–122. https://doi.org/10.1111/j.1399-3054.1976.tb03920.x.

Wang, M., Hu, C., Cannizzaro, J., English, D., Han, X., Naar, D., Lapointe, B., Brewton, R., Hernandez, F., 2018. Remote sensing of Sargassum biomass, nutrients, and pigments. Geophys. Res. Lett. 45 (22), 12–359. https://doi.org/10.1029/2018GL078858.

Wang, M., Hu, C., Barnes, B.B., Mitchum, G., Lapointe, B., Montoya, J.P., 2019. The great Atlantic *Sargassum* belt. Science 365 (6448), 83–87. https://doi.org/10.1126/science.aaw7912.

Warwick-Dugdale, J., Tian, F., Michelsen, M.L., Cronin, D.R., Moore, K., Farbos, A., Chittick, L., Bell, A., Zayed, A.A., Buchholz, H.H., Bolanos, L.M., Parsons, R.J., Allen, M.J., Sullivan, M.B., Temperton, B., 2024. Long-read powered viral

metagenomics in the oligotrophic Sargasso Sea. Nat. Commun. 15 (1), 4089. https://doi.org/10.1038/s41467-024-48300-6.

Weich, R.G., Granéli, E., 1989. Extracellular alkaline phosphatase activity in *Ulva lactuca* L. J. Exp. Mar. Biol. Ecol. 129 (1), 33–44. https://doi.org/10.1016/0022-0981(89)90061-0.

Weinbauer, M.G., 2004. Ecology of prokaryotic viruses. FEMS Microbiol. Rev. 28 (2), 127–181. https://doi.org/10.1016/j.femsre.2003.08.001.

Wells, R.J., Rooker, J.R., 2004. Spatial and temporal patterns of habitat use by fishes associated with Sargassum mats in the northwestern Gulf of Mexico. Bull. Mar. Sci. 74 (1), 81–99. Retrieved from. https://www.tamug.edu/wellsfisherieslab/pdf/publications/BMS2004.pdf.

Wilhelm, S.W., Suttle, C.A., 1999. Viruses and Nutrient Cycles in the Sea: Viruses play critical roles in the structure and function of aquatic food webs. BioScience 49 (10), 781–788. https://doi.org/10.2307/1313569.

Wright, E.S., 2020. RNAconTest: Comparing tools for noncoding RNA multiple sequence alignment based on structural consistency. RNA 26 (5), 531–540. https://doi.org/10.1261/rna.073015.119.

Wu, J., Sunda, W., Boyle, E.A., Karl, D.M., 2000. Phosphate depletion in the western North Atlantic Ocean. Sci. (N. Y. N.Y.) 289 (5480), 759–762. https://doi.org/10.1126/science.289.5480.759.

Zerkle, A.L., Junium, C.K., Canfield, D.E., House, C.H., 2008. Production of 15N-depleted biomass during cyanobacterial N2-fixation at high Fe concentrations. J. Geophys. Res.: Biogeosciences 113 (G3). https://doi.org/10.1029/2007JG000651.

Zhang, Y., Fang, K., Liu, M., Liu, J., Zhao, X., Zhai, W., Zhang, H., Wang, X., Xie, H., 2024. High photoreactivity of chromophoric dissolved organic matter derived from *Ulva prolifera* and *Sargassum*. Front. Mar. Sci. 11, 1397705. https://doi.org/10.3389/fmars.2024.1397705.