# Seasonal dynamics of cable bacteria in an estuarine intertidal mudflat subject to green tides: Implications for the foraminiferal community and test preservation

Maxime Daviray, Emmanuelle Geslin, Edouard Metzger, Sophie Quinchard, Bruno Deflandre, Céline Charbonnier, Pierre Anschutz

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## To cite this version:

Maxime Daviray, Emmanuelle Geslin, Edouard Metzger, Sophie Quinchard, Bruno Deflandre, et al. Seasonal dynamics of cable bacteria in an estuarine intertidal mudflat subject to green tides: Implications for the foraminiferal community and test preservation. Journal of Sea Research (JSR), 2026, 210, pp.102683. 10.1016/j.seares.2026.102683. hal-05507363

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# Seasonal dynamics of cable bacteria in an estuarine intertidal mudflat subject to green tides: Implications for the foraminiferal community and test preservation

Maxime Daviray<sup>a,*</sup>, Emmanuelle Geslin<sup>a</sup>, Edouard Metzger<sup>a</sup>, Sophie Quinchard<sup>a</sup>, Bruno Deflandre<sup>b</sup>, Céline Charbonnier<sup>b</sup>, Pierre Anschutz<sup>b</sup>

## ARTICLE INFO

### Keywords:

Benthic biogeochemistry

Benthic foraminifera

Sediment record

Sediment acidification

Dissolution process

## ABSTRACT

As many coastal areas draining intensive agricultural activities, Ledano estuary mudflats (French Brittany) experience *Ulva* proliferation, causing green tides. We studied the seasonal dynamics of sulphoxidizing cable bacteria from April 2019 to July 2020 using microsensors (O₂, pH, H₂S). The activity of these filamentous bacteria, called electrogenic sulphur oxidation (eSOx), results in strong acidification and in pore-water CO₃²⁻ depletion in the first few centimeters of sediment (within the suboxic zone). Living and dead benthic foraminiferal assemblages were studied in July 2020 to observe the effects of eSOx on the calcareous meiofauna and their shell preservation in the sediment. eSOx was patchy on the mudflat but persistent throughout the year. It contributed up to 45% of oxygen consumption during the algal mat decay, and exceeded 100% during the flooding period suggesting stimulation by nitrate inputs. The corrosive effect was maximal in July 2020 (ΔpH ~ 1.7, [CO₃²⁻]ᵣate < 10 µM). The living foraminiferal community was sparse and nearly monospecific, dominated by the calcareous species *Haynesina germanica*, probably due to green tides coupled with eSOx-driven acidification. However, living specimens of *H. germanica* showed no signs of advanced dissolution of their shell suggesting a biological capacity to survive in such environmental acidification, potentially linked to their photosynthetic capability. In contrast, the dead assemblages displayed greater diversity despite a loss of about 20% of the calcareous shell recording with depth due to the synergetic effect of low salinity and eSOx. Overall, this study shows that green tides strongly influence cable bacteria activity and then, sedimentary biogeochemical processes in eutrophic coastal environments.

## 1. Introduction

A significant proportion of coastal ecosystems in Brittany, including beaches and mudflats, have been affected by the seasonal proliferation of *Ulva* resulting in the formation of green tides since the 70's (Gravier et al., 2012; Louis et al., 2023; Piriou et al., 1991). The occurrence of a sudden and significant enrichment of organic matter in low-permeability environments, such as intertidal mudflats, leads to significant biomineralization processes. As aerobic respiration rapidly exhausts the oxygen available in the surface sediment, the whole diagenetic sequence of anaerobic reactions follows (Froelich et al., 1979). In such organic-rich sediments, sulphate reduction is a major pathway leading to large sulphide accumulations in the pore water of the first few centimeters of sediment (Jørgensen, 1982). Sulphide have a

strong structuring effect on the benthic community decreasing diversity and density both in macrofauna (Vaquer-Sunyer and Duarte, 2010) and meiofauna (Zeppilli et al., 2018). Microorganisms use sulphide as electron donors for their metabolic activity by coupling with oxygen and/or nitrate reduction such as cable bacteria.

This new group of filamentous bacteria (discovered by Pfeffer et al., 2012) from the *Desulfobulbaceae* family lives in marine and freshwater sediments worldwide (Burdorf et al., 2017). They can combine the oxidation of sulphide in subsurface sediments (e.g., H₂S + 4H₂O → SO₄²⁻ + 8e⁻ + 10H⁺) with the reduction of oxygen and/or nitrate at the surface (e.g., O₂ + 4e⁻ + 4H⁺ → 2H₂O; Nielsen et al., 2010; Marzocchi et al., 2014). This spatial decoupling produces a maximum pH peak in the oxic zone associated with the cathodic proton consumption, followed by a strong acidification of the suboxic zone linked to the anodic

<footnote><sup>a</sup> Univ Angers, Nantes Université, Le Mans Université, CNRS, Laboratoire de Planétologie et Géosciences, LPG UMR 6112, 49000 Angers, France</footnote>

<footnote><sup>b</sup> Univ. Bordeaux, CNRS, Bordeaux INP, EPOC, UMR 5805, F-33600 Pessac, France</footnote>

<footnote>* Corresponding author.</footnote>
proton production. These vertical O₂-pH-H₂S profiles are the geochemical fingerprint of cable bacteria activity (Meysman et al., 2015; Nielsen et al., 2010). Cable bacteria generate a vertical flow of electrons through the sediment over several millimeters (up to ~2 cm). The resulting electrogenic sulphide oxidation (eSOx) produces an electric field that can have a significant effect on sedimentary geochemistry. The electric field promotes an ion migration towards the sediment, and suboxic acidification induces a dissolution process of both carbonates and iron sulphide (Malkin et al., 2017; Rao et al., 2016; Risgaard-Petersen et al., 2012). Richirt et al. (2022), Fouet et al. (2024) and Daviray et al. (2024) suggested that the dissolution process induced by cable bacteria may have an impact on low-motility calcareous benthic meiofauna such as foraminifera.

Benthic foraminifera are protists capable of building a shell (called a test) that can be either agglutinated (cemented grains), hyaline (calcium carbonate) or porcelaneous (magnesium-enriched calcium carbonate; Haynes, 1981). They can be very abundant in estuarine environments where they are subject to strong variability in physical and geochemical parameters (i.e., temperature, salinity, pH; Buzas, 1969). Understanding benthic foraminiferal communities at a systemic level is crucial as they can be used as bioindicators of marine water quality (Schönfeld et al., 2012) or proxies for the functioning of past environments (Martin, 2000). Therefore, it is necessary to understand the processes that can disturb a community in equilibrium or alter the preservation of tests within sedimentary archives. Daviray et al. (2024) showed that calcareous foraminifera have been affected by cable bacteria activity, as evidenced by the test dissolution of living specimens and the degradation of preserved tests in sediments as previously suggested by Richirt et al. (2022) and Fouet et al. (2024). This could result in a loss of environmental signal carried by dead foraminiferal assemblages that no longer reflect past living communities.

In order to further understand how the dynamics of cable bacteria activity would affect the carbonate system and thus impact benthic foraminiferal ecology, which depend on their ability to build and maintain calcite shells, two stations from a temperate intertidal mudflat were seasonally monitored regarding their chemistry, and benthic foraminifera were studied in summer, when they are presumed to be most abundant. The dead shells were also examined at this period to determine their preservation and disentangle the signal these “fossils” might record.

## 2. Methodology

### 2.1. Studied area

The Trieux river (Fig. 1) has a mean annual input discharge of 10 m³ s⁻¹ and flows into the English Channel. It drains a catchment area of 875 km². The Trieux estuary is subject to a megatidal regime with a tide range that can reach 13 m at the mouth during spring tides. Saltwater flows upstream over 18 km from the mouth of the estuary which is tide-dominated (952 m wide; OFB and HydroPortail). Ledano cove is located about 8 km from the sea between the narrow bed of the Trieux river to the south and a rocky cleft to the north (Gerla, 2006). This rocky embankment induces strong currents in the riverbed. The estuarine intertidal mudflat covers an area of 120 ha. The sandy-muddy plateau that makes up this intertidal mudflat rises between 5 and 7 m above mean sea level. The north-eastern and south-western parts bear seagrass meadows. The less elevated central part of the mudflat is crossed by a network of intertidal channels meandering from the mouths of two minor rivers (Traou Du and Kerbiguet; Gerla, 2006). Based on data collected between 2007 and 2020 (Louis et al., 2023) and our observations in 2019 and 2020, summer green algae (Ulva genus) stranding on the mudflat decay and create anoxic conditions at the sediment-water interface over 80% of the area.

Stations B and D were seasonally monitored at low tide in April 2019, October 2019, February 2020, and July 2020 in the Ledano mudflat (Fig. 1). Their characteristics for each sampling campaign are presented in Table 1.

### 2.2. Sediment sampling and processing

At each station, four cores were collected during low tide just after emersion with Plexiglas® tubes (96 mm inner diameter, 30 cm length). Overlying seawater was collected on site during the previous or next high tide. Cores were transported in a cool box to the field laboratory. Three cores were sliced less than 3 h after core collection using a core pusher. Slice thickness was 0.5 cm for the first 2 cm depth, 1 cm up to 4 cm depth, 2 cm up to 16 cm depth and then 4 cm until 24 cm depth. For each layer, a sub-sample was sealed in a pre-weighted vial and frozen for the determination of porosity, median grain size and chemical analyses of the solid fraction. Another sub-sample was centrifuged for 12 min at 4000 rpm to extract pore water that was filtered through acetate

<figure><img src="image_3.png" /><figcaption>**Fig. 1.** (a) Location of the Ledano intertidal mudflat in the Trieux estuary on the English Channel coast (France). (b) Locations of the sampling stations (B and D) in the mudflat. The image (b) is from Google Earth (09–2017) satellite capture.</figcaption></figure>
<table><caption>**Table 1**

Characteristics of the stations at each sampling time.</caption><thead><tr><th>Station</th><th>GPS point</th><th>Date</th><th>T (°C)</th><th>Water Salinity</th><th>Observations</th></tr></thead><tbody><tr><td rowspan="5">B</td><td rowspan="5">48.7687°<br>N,<br>-3.1077°<br>W</td><td>Apr-2019<br>(spring)</td><td>12.4</td><td>33.1</td><td>Ulva mat, some<br>Hydrobia and burrows</td></tr><tr><td>Oct-2019<br>(autumn)</td><td>14.4</td><td>34.5</td><td>a few Ulva</td></tr><tr><td>Feb-2020<br>(winter)</td><td>10.5</td><td>22.0</td><td>no algae; some<br>Hydrobia</td></tr><tr><td>Jul-2020<br>(summer)</td><td>22.4</td><td>33.0</td><td>patches of old Ulva</td></tr><tr><td>Apr-2019<br>(spring)</td><td>12.4</td><td>33.1</td><td>a few Ulva and<br>Hydrobia</td></tr><tr><td rowspan="5">D</td><td rowspan="5">48.7694°<br>N,<br>-3.1036°<br>W</td><td>Oct-2019<br>(autumn)</td><td>14.4</td><td>34.5</td><td>Ulva mat (1 cm)</td></tr><tr><td>Feb-2020<br>(winter)</td><td>8.6</td><td>22.7</td><td>some small eelgrass<br>leaves</td></tr><tr><td>Jul-2020<br>(summer)</td><td>22.4</td><td>33.0</td><td>thick Ulva mat (1–3<br>cm)</td></tr><tr><td>Apr-2019<br>(spring)</td><td>12.4</td><td>33.1</td><td>a few Ulva and<br>Hydrobia</td></tr><tr><td>Oct-2019<br>(autumn)</td><td>14.4</td><td>34.5</td><td>Ulva mat (1 cm)</td></tr></tbody></table>

cellulose membrane (0.2 µm). A first aliquot of filtered interstitial water was immediately acidified with HCl to prevent precipitation of reduced dissolved metals and dissolved inorganic phosphorus. A second aliquot was frozen for other nutrient analyses. A third aliquot was stored in glass vials for dissolved inorganic carbon determination. A last aliquot served for salinity measurement. The frozen solid sub-sample was freeze-dried and porosity was calculated and corrected from salinity (Anschutz and Charbonnier, 2021). The fourth core was submerged in aerated seawater maintained in the dark at in situ temperature for microsensor profiling. Microprofiling started at least 2 h after core installation in order to allow thermic equilibrium.

In July 2020, one supplementary core (82 mm inner diameter) was sampled at each station to study the foraminiferal community. They were sliced in 0.2 cm layers until 2 cm depth and 0.5 cm layers until 5 cm depth. Living foraminifera were labelled using the vital probe Cell Tracker Green (Bernhard, 2000; Bernhard et al., 2006), as described in Daviray et al. (2024).

## 2.3. Microsensor profiling

Microsensors were connected on the Field Microsensor Multimeter equipped with a motorized micromanipulator (Unisense A/S) controlled by a computer. Oxygen Clark-type microsensors with 50-µm and 100-µm tips (OX50 and OX100; Revsbech, 1989), pH sensors with a 200-µm tip diameter (PH200, Unisense; Thomas, 1978) and sulphide sensors with a 200-µm tip diameter (SUL200; Jeroschewski et al., 1996) were used. The increment was 100 µm within the first centimeter depth for oxygen and 200 to 250 µm for pH and H₂S, and it was adapted in real time to 250 or 500 µm depending on the profile evolution until three to five centimeters depth. For each core, a dozen profiles were carried out for O₂, while up to seven were done for pH and sulphide. ΔpH was calculated as the difference between maximum and minimum pH values in a profile.

Oxygen microsensors were calibrated using the O₂ concentration of the overlying water (100% saturation) measured with a SDOT300 (NKE instrumentation) equipped with an Aanderaa O₂ optode (model 4330) and the zero oxygen in the anoxic part of the sediment. pH microsensors were calibrated using 3 NIST buffers (4.001, 6.901, 9.25; WTW). For sulphide microsensors, a two-point standard curve (0 and 100 µM) was made using the Sulphide Sensor Calibration Kit by Unisense A/S. Total hydrogen sulphide (ΣH₂S = H₂S + HS⁻) was calculated based on the same depth H₂S and pH measurements according to salinity and temperature using the R package 'AquaEnv' (Hofmann et al., 2010). S²⁻ was overlooked given the pH values obtained.

## 2.4. Dissolved oxygen uptake

Dissolved oxygen uptakes (DOU) were calculated from oxygen depth profiles using a modified version of the PRO2FLUX software (Deflandre and Duchène, 2010) and the PROFILE software (Berg et al., 1998) to calculate the depth-integrated consumption rate. The diffusion coefficient in sediment (D₅) was corrected for the porosity as D₅ = D₀/(1 + 3(1-Φ)) (Rasmussen and Jorgensen, 1992) where Φ is the sediment porosity in the oxic zone, and D₀ is the diffusion coefficient in seawater.

## 2.5. Alkalinity and carbonate concentration calculations

Dissolved inorganic carbon (DIC) was measured by conductimetry by continuous flow injection analysis according to the method proposed by Hall and Aller (1992). Salinity was measured with a refractometer (Hanna HI96822). From salinity, temperature and DIC measurements, and pH averaged at the same 5-mm resolution from microsensor profiles, total alkalinity (TA) and carbonate ion concentration were calculated using the CO2Sys software v.3.0 (Lewis et al., 1998; Pierrot et al., 2021) with equilibrium constants of Cai and Wang (1998), and pH NBS scale. Calcite saturation (Ωₚ) was also determined for surface seawater using the salinity (S) value to assess dissolved calcium concentrations according to Dittmar ratio Ca²⁺ (mmol.kg⁻¹): S = 0.2937 (Dittmar, 1884), expressed in psu.

## 2.6. Current density calculation

The electrical current density (Jₑ, expressed in mA.m⁻²) generated by eSOx was estimated using the alkalinity balance method (Malkin et al., 2017; Malkin et al., 2014; Nielsen et al., 2010). This method considers the cathodic proton consumption (CPC expressed in mmol.m⁻².d⁻¹) from eSOx (i.e., O₂ + 4e⁻ + 4H⁺ → 2H₂O) as the sum of the upward and downward alkalinity fluxes above and below the pH maximum within the oxic zone (Eq. 1).

$$CPC = J_{TA}^{down} - J_{TA}^{up} \quad (1)$$

The electrical current density was calculated from the CPC using the factor of one electron per proton (1 A = 1.036 × 10⁻⁵ mol e·s⁻¹). Alkalinity fluxes were calculated using the PROFILE software (Berg et al., 1998) and the bicarbonate diffusion coefficient as described in the section 2.4. Then, we assumed the bicarbonate ion to be the main alkalinity factor, the carbonate system the most important pH buffer in the sediment and no carbonate precipitation (Nielsen et al., 2010). We also assumed an entire electron transfer to oxygen neglecting nitrate consumption by cable bacteria (Marzocchi et al., 2014). These hypotheses will be discussed in the Discussion section.

The cathodic oxygen uptake (COU; eSOx contribution to respiration) was calculated from the stoichiometry of the cathodic proton consumption as proposed by Malkin et al. (2014):

$$COU = 25 * \frac{CPC}{DOU} \quad (2)$$

This model does not assume either consumption or production of alkalinity by other biogeochemical processes in the oxic zone. However, nitrification, ferrous iron oxidation and calcium carbonate precipitation can occur and consume alkalinity (Risgaard-Petersen et al., 2012). The COU determined here therefore underestimated the actual proportion of oxygen consumed by eSOx.

## 2.7. Foraminiferal analyses

In the laboratory, all sediment layers for foraminiferal analyses were sieved using 315, 150 and 125-µm meshes. Living foraminifera were all picked in all sieved fractions (>125 µm) using a fine brush under an epifluorescent binocular (Olympus SZX12 with a light source CoolLED pE-100, emission wavelength λ = 470 nm). The shell dissolution stages
of living foraminifera from three sediment layers (0–2 / 6–8 / 40–50 mm depth), according to main pH features, were characterized with the dissolution scale of Daviray et al. (2024) using a scanning electronic microscope (DEBEN Hitachi TM4000; samples not metallized, 15 kV, wd = 6.5 mm, partial vacuum – 60 Pa). Based on calcareous foraminiferal test observations, Daviray et al. (2024) defined six gradual stages of dissolution (DS) from intact test (DS-0) to fully dissolved test (DS-5). Total living foraminiferal densities were expressed per 50 cm² of sediment and foraminiferal densities for sediment layers per 10 cm³ volume.

Dead foraminiferal assemblages were also observed in all sieved fractions (>125 µm) in three sediment layers (0–2 / 6–8 / 45–50 mm depth). The relative abundances of the species were based on minimum 300 specimens per fraction size per sample (Buzas, 1990; Fatela and Taborda, 2002). Specimens with fully dissolved test exposing the inner organic lining (DS5) were also estimated.

The loss of calcareous foraminifera in the living community and dead assemblages was estimated using the ratio of calcareous foraminifera in the total foraminiferal community/assemblage (C/T ratio), and the intensity of dissolution suffered by the ratio of fully dissolved calcareous specimen in the total calcareous foraminifera (DS-5/C ratio) as defined by Daviray et al. (2024).

Taxonomy of foraminiferal species was based on reference publications on estuarine species (Jorissen et al., 2023; Murray, 1979), and on the World Register of Marine Species (WoRMS Editorial Board, 2024). Minority foraminiferal species were grouped at genus level.

## 2.8. Statistical procedure

Geochemical parameters were statistically compared across stations through time with the non-parametric Kruskal-Walis' test as they did not follow a normal trend according to the Shapiro's test. The *post-hoc* test was using the criterium Fisher's least significant difference criterium and carried out when the Kruskal-Wallis' test was significant. The significance level for all tests was set to 0.05. Statistics were carried out using the R software using the “agricolae” packages (de Mendiburu, 2023).

# 3. Results

## 3.1. Microsensor profiles and cable bacteria activity estimation

At both stations B and D, oxygen depth penetration (OPD) did not exceed 3-mm depth (Fig. 2; Table 2; Appendix 1). In winter, OPD was maximal at both stations (2.1 ± 0.6 and 2.2 ± 0.2 mm respectively B and D), and DOU was minimal (17.5 ± 10.2 and 11.6 ± 1.5 mmol.m⁻².d⁻¹ respectively). On the contrary in summer, OPD was minimal (1.0 ± 0.2 and 1.5 ± 0.3 mm) and DOU maximal (25.4 ± 4.8 and 21.6 ± 3.6 mmol. m⁻².d⁻¹) at both stations. These seasonal variations are significant (Kruskal-Wallis' test, *p* < 0.05).

The free sulphide front (occurrence depth of sulphide in pore water) moved significantly deeper into the sediment from summer to winter (from 20 mm to more than 35 mm, *p* < 0.05; Fig. 2; Table 2).

At station B, a maximum pH peak was observed in the oxic sediment in autumn, winter and summer with a maximum value of 8.4 ± 0.5 (Fig. 2; Table 2; Appendix 1). At station D, the maximum pH peak at the sediment surface was observed in spring and autumn. At both stations, this peak within the surface sediment was always followed by strong sediment acidification in the suboxic zone, with pH as low as 6.4. At station B, the ΔpH in the upper 3 cm of the sediment was the lowest in spring compared to the other seasons (0.5 ± 0.1 and > 1.4 respectively). At station D, the ΔpH was the strongest in autumn compared to the other seasons (1.7 ± 0.2 and < 1.0 respectively). At each station, the difference of pH between the oxic and suboxic zones (ΔpH) was significant over time (*p* < 0.05).

At station B, current density ranged significantly from 14.48 ± 5.22 mA.m⁻² in summer to 39.7 ± 8.6 mA.m⁻² in autumn and 102 ± 112 mA.m⁻² in winter (*p* < 0.05; Table 2). Then, cathodic oxygen uptake

<figure><img src="image_4.png" /><figcaption>**Fig. 2.** Representative vertical microprofiles of O₂, pH, and H₂S measured in all four seasons at two stations on the Ledano mudflat. From the different replicates, one profile per chemistry has been selected as being representative of eSOx process measured. Zoom at the sediment-water interface available in Appendix 1 and the diversity of profiles in Appendix 2.</figcaption></figure>

(COU) followed the same significance and varied from 12.7 ± 4.6% to 46.8 ± 10.2% and 131 ± 143% respectively. At station D, current density ranged significantly from 14.0 ± 0.1 mA.m⁻² in spring to 35.5 ± 7.4 mA.m⁻² in autumn, and COU from 19.2 ± 0.2 to 35.9 ± 7.5%.

## 3.2. Species related to the carbonate system

Salinity varied seasonally on the Ledano mudflat in overlying water with a minimum value of 23.4 in winter (Fig. 3) while it remained above 33.0 in other seasons. In winter at both stations, pore-water salinity within the upper centimeters of sediment was lower than the rest of the year (30.0 and 26.0 respectively) while it remained stable deeper down (between 33.0 and 36.0).

Dissolved inorganic carbon (DIC) in overlying water varied from 2 mM in winter to 3 mM in autumn (Fig. 3). The pore-water DIC showed the greatest accumulation in the sediments at both stations in autumn. At station B, DIC increased from 4.4 ± 0.4 to 10.3 ± 2.6 mM, and at station D, from 5.1 ± 1.4 to 9.8 ± 1.3 mM. In winter, pore-water DIC was at its lowest concentration, with concentrations not exceeding 4.5 ± 0.6 mM at station B, and 3.7 ± 0.1 mM at station D at 3.5-cm depth. In spring and autumn, the pore-water DIC increased from 3.8 ± 0.6 to 6.8 ± 1.5 mM and 3.1 ± 0.4 to 8.1 ± 0.4 mM respectively at station B while at station D, pore-water DIC increased slightly over the first centimeter of sediment before stabilizing, rising from 4.2 ± 0.5 to 4.8 ± 0.6 mM and from 3.3 ± 1.0 to 5.0 ± 0.9 mM respectively.

Calculated carbonate ion concentrations ([CO₃²⁻]ᵣcalc) in overlying seawater varied seasonally (Fig. 3). Maxima were observed in summer and autumn around 175 µM, and minima during winter around 70 µM. Calcite saturation (ΩCa) followed the same trend, remaining always oversaturated (ΩCa > 1). At station B, except in spring, pore-water [CO₃²⁻]ᵣcalc within the suboxic zone was under 35 µM, and it was minimal in summer and winter with values under 10 µM. At station D, pore-water [CO₃²⁻]ᵣcalc was low within the suboxic zone in spring, autumn and winter (< 40 µM). In summer, it was around 60 ± 2 µM through depth. The large error bars associated to pore-water [CO₃²⁻]ᵣcalc in the 0–0.5 cm sediment layer can be explained by the strong pH variation observed at high resolution (Fig. 2).
<caption>**Table 2**</caption>

Electrogenic sulphur oxidation parameters at both stations for each sampling time. Abbreviations: OPD, oxygen penetration depth; DOU, diffusive oxygen uptake at the sediment-water interface; FSF, free sulphide front; eSOx, electrogenic sulphide oxidation; ΔpH, pH variation between the oxic and suboxic zones; J<sub>e</sub>, electrical current density; TA, total alkalinity; COU, cathodic oxygen uptake. Values indicate mean ± sd of between a dozen profiles for O<sub>2</sub> and two to seven for pH and H<sub>2</sub>S (see examples in Appendix 2). * indicates less than three replicates.

<table><thead><tr><th rowspan="2">Date</th><th rowspan="2">site</th><th rowspan="2">OPD (mm)</th><th rowspan="2">DOU (mmol.m<sup>-2</sup>.d<sup>-1</sup>)</th><th rowspan="2">FSF (mm)</th><th colspan="3">eSOx pH signature</th><th rowspan="2">J<sub>e</sub> (mA.m<sup>-2</sup>)<br>TA balance method</th><th rowspan="2">COU (%)</th></tr><tr><th>pH maximum</th><th>pH minimum</th><th>ΔpH</th></tr></thead><tbody><tr><td rowspan="2">Spring Apr-19</td><td>St B</td><td>1.7 ± 0.3</td><td>16.4 ± 3.7</td><td>16.8 ± 6.0*</td><td>8.0 ± 0.0*</td><td>7.4 ± 0.1*</td><td>0.5 ± 0.1*</td><td>/</td><td>/</td></tr><tr><td>St D</td><td>1.5 ± 0.0</td><td>16.3 ± 2.8</td><td>23.5 ± 3.5*</td><td>7.9 ± 0.1*</td><td>7.0 ± 0.1*</td><td>1.0 ± 0.1*</td><td>14.0 ± 0.1*</td><td>19.2 ± 0.2*</td></tr><tr><td rowspan="2">Autumn Oct-19</td><td>St B</td><td>1.6 ± 0.5</td><td>19.7 ± 3.6</td><td>30.2 ± 6.0</td><td>8.2 ± 0.1</td><td>6.9 ± 0.2</td><td>1.4 ± 0.3</td><td>39.7 ± 8.6*</td><td>46.8 ± 10.2*</td></tr><tr><td>St D</td><td>1.3 ± 0.3</td><td>23.8 ± 5.1</td><td>29.7 ± 8.5</td><td>8.2 ± 0.1</td><td>6.5 ± 0.2</td><td>1.7 ± 0.2</td><td>35.5 ± 7.4</td><td>35.9 ± 7.5</td></tr><tr><td rowspan="2">Winter Feb-20</td><td>St B</td><td>2.1 ± 0.6</td><td>17.5 ± 10.2</td><td>34.8 ± 3.1</td><td>8.4 ± 0.5</td><td>6.6 ± 0.1</td><td>1.8 ± 0.6</td><td>102 ± 112</td><td>131 ± 143</td></tr><tr><td>St D</td><td>2.2 ± 0.2</td><td>11.6 ± 1.5</td><td>&lt; 45</td><td>8.0 ± 0.0*</td><td>7.3 ± 0.1*</td><td>0.7 ± 0.1*</td><td>/</td><td>/</td></tr><tr><td rowspan="2">Summer Jul-20</td><td>St B</td><td>1.0 ± 0.2</td><td>25.4 ± 4.8</td><td>17.5 ± 3.6</td><td>8.2 ± 0.0</td><td>6.4 ± 0.0</td><td>1.8 ± 0.1</td><td>14.5 ± 5.2</td><td>12.7 ± 4.6</td></tr><tr><td>St D</td><td>1.6 ± 0.5</td><td>21.6 ± 3.6</td><td>17.2 ± 3.9</td><td>8.2 ± 0.0</td><td>7.4 ± 0.1</td><td>0.8 ± 0.1</td><td>/</td><td>/</td></tr></tbody></table>

<figure><img src="image_5.png" /><figcaption>**Fig. 3.** Vertical profiles of salinity, DIC, pH and [CO<sub>3</sub><sup>2-</sup>]<sub>calc</sub> for the four seasons at two stations on the Ledano mudflat. [CO<sub>3</sub><sup>2-</sup>]<sub>calc</sub> was calculated using the CO2Sys software. The mean is shown with error bars for standard deviation analyses for salinity and DIC. For [CO<sub>3</sub><sup>2-</sup>]<sub>calc</sub> and pH, the mean is the depth integration of all microprofiles at the same low-resolution depth as DIC with horizontal error bars for standard deviation. The 0 layer corresponded to bottom water measurements. The dash line represents the depth limit for [CO<sub>3</sub><sup>2-</sup>]<sub>calc</sub> and pH profiles; be mindful of the change in vertical scale.</figcaption></figure>
### 3.3. Living foraminiferal communities and dissolution stages

The abundance of living foraminiferal communities in July 2020 at the Ledano mudflat was 245 and 322 ind.50 cm⁻² (0–5 cm depth) at stations B and D, respectively. The species were those commonly found in estuarine mudflats (Debenay et al., 2000; Fouet et al., 2022; Jorissen et al., 2023). However, at both stations B and D, the communities were broadly dominated by one species with a calcareous test, *Haynesina germanica* (Ehrenberg, 1840) accounting for 92 and 87% of the communities respectively. The species diversity maximum at both stations was counted within the first few millimeters of depth (Fig. 4). *Ammonia* spp. (A. veneta (Schultze, 1854), A. aberdoveyensis Haynes, 1973, and A. confertitesta Zheng, 1978) was the second most abundant species of the communities representing only respectively 7 and 5%. At station D, *Quinqueloculina* spp. (Q. dimiadata Terquem, 1876 and Q. seminulum (Linnaeus, 1758)) accounted for 5% of the foraminiferal community. Agglutinated foraminifera were absent at station B and counted for around 1% of the community at station D (*Ammobaculites balkwilli* Haynes, 1973). Then, the ratio of calcareous foraminifera in the living foraminiferal community (C/T) was constant through depth and equal to 1 at both stations, except for the surface layer (0–2 mm depth) at station D (0.98).

At both stations B and D, the highest living foraminiferal density and diversity was observed in the upper millimeters of sediment (74 and 217 ind.10 cm⁻³ respectively; Fig. 4) where dioxygen was available and pH maximal (Fig. 2). Then, foraminiferal density decreased to less than 5 ind.10 cm⁻³ below 8-mm depth following the pH trend, with exception at station B with a density peak at 18-mm depth (12 ind.10 cm⁻³) where pH started to rise again at the FSF.

After detailed SEM observations of the test dissolution stages of living foraminifera according to Daviray et al. (2024), very few DS-5 ("fully dissolved tests" exposing entirely the organic lining of the cell) specimens were observed in sediment at both stations (2 ind.50 cm⁻²). Except for the subsurface layer at station B, all the other layers at both stations showed specimens dissolution stages ranging from DS-0 ("intact test") to DS-2 ("peeled test"; Fig. 5). Given the small number of specimens observed in the subsurface and deep layers, even the proportion of DS-2 remained very low, with only a few individuals (around 10%). Then, DS-0 and DS-1 ("slight surface dissolved tests") represented at least 90% of dissolution stage of calcareous foraminifera. Within the subsurface layer at station B, DS-2 and DS-5 counted for 25% of dissolution stage, with one DS-5 specimen. According to the Fisher's test, there was no significant difference between stations (p > 0.27) or between layers for each station (p > 0.21 for station B and p = 1 for station D).

### 3.4. Foraminiferal thanatocoenosis

Calcareous species also dominated the foraminiferal thanatocoenosis at both stations on the Ledano mudflat (C/T ~ 0.9; Fig. 6). The ratio of fully dissolved calcareous specimens in the total calcareous foraminifera

<figure><img src="image_6.png" /><figcaption>**Fig. 4.** Vertical distributions of living-foraminiferal communities at stations B and D in July 2020 (summer; >125 µm fraction per 10 cm³ of sediment). FSF, free sulphide front.</figcaption></figure>

(DS-5/C) remained low in the sediment at station B (< 0.05). However, the DS-5/C ratio was well higher at station D, increasing from 0.20 in the surface layer to 0.39 just below, representing 46% of calcareous tests. In the deep layer, DS-5/C ratio decreased until 0.1.

The relative proportions of species in dead foraminiferal assemblages were different from the living one at both stations (Fig. 6). At station B, if *H. germanica* remained the most abundant calcareous species in the dead assemblages (> 45%) followed by *Ammonia* spp. about 16%, *Elphidium* spp. about 10%, *Quinqueloculina* spp. with 13% at the surface. Agglutinated foraminifera counted for about 5% in the surface assemblage and more than 10% deeper down. At station D, *H. germanica* was not the dominating species in dead assemblages even if it remained abundant (from 22% in the surface layer to 10% in the deep one; Fig. 6). The calcareous most abundant species was *Ammonia* spp. (from 29% in the surface to 44% in deep layer), then followed *Elphidium* spp. (from 3% to 16% with depth), and few specimens of *Quinqueloculina* spp. Dead agglutinated foraminifera represented 24% of the surface assemblage and 12% for the deep one. *Ammobaculites balkwilli* was the major agglutinated species in the surface dead assemblage (16%) while it was *Entzia macrescens* (Brady, 1870) in the deep one (7%).

## 4. Discussion

### 4.1. Seasonal variability of cable bacteria activity through the salinity gradient and impact on the carbonate system

The geochemical signature of electrogenic sulphur oxidation by the activity of cable bacteria (Nielsen et al., 2010; Meysman et al., 2015) could be observed throughout the duration of the seasonal monitoring on the Ledano mudflat. The significant intensification of eSOx was measured in autumn compared to spring and summer, with electron fluxes two to three times higher representing up to 47% of oxygen consumption in the sediment. This is in line with previous field observations in coastal sediments (Malkin et al., 2017; van de Velde et al., 2016). We suppose that the accumulation of *Uva* and their advanced stage of decomposition in the surface sediment in autumn may have led to the accumulation of refractory OM to the benefit of the spatial reactive decoupling of eSOx, while fresh deposits may have dominated organic carbon deposition in summer (Cividanes et al., 2002; Fernández et al., 2025; Li et al., 2019). A slight spatial disparity of eSOx was noticed in the mudflat between stations (Appendix 2). The reduction in organic inputs and temperatures during winter decreases the intensity of mineralization processes in sediments, particularly the reduction of sulfate leading to an overall decrease of pore-water free sulphide concentration (Al-Raei et al., 2009; Koretsky et al., 2003; Metzger et al., 2007). The lower salinities in autumn and winter within the first centimeter of sediment at station D could result in lower sulfate reduction by the microbial consortium than at station B (Louis et al., 2022). It might have led to the deepening of the FSF and therefore the shutdown of eSOx at station D. Such spatial variability could be related to a bloom-and-fade dynamics that would have affected cable bacteria activity during *Uva* decay. This was observed in laboratory cultures where cable bacteria developed over a few tens of days to reach a maximum eSOx intensity before dying out when the sulphide front became too deep (Malkin et al., 2014; Rao et al., 2016).

The most unexpected observation was the maximum activity of cable bacteria in winter at station B leading to the estimation of oxygen uptake by CB activity above 100%. If this surprising result is not lead by measurement uncertainty and error propagation during modelling, it could be explained by the use of nitrate in addition to oxygen by CB to oxidize sulphide. In winter, the higher discharge of freshwater enriched in terrigenous nitrate favors nitrate penetration within the sediment as well as denitrification processes in the Ledano mudflat (Louis et al., 2022). Indeed, nitrate concentration at the sediment surface was ten times higher during winter (> 180 µM) reaching comparable concentrations of pore-water oxygen at the sediment water interface. Cathodic
<figure><img src="image_7.png" /><figcaption>**Fig. 5.** Relative proportion of each dissolution stage in the calcareous foraminiferal communities (>125 µm fraction). Three depth layers were analysed: the surface (0–2 mm; oxic zone), the subsurface (6–8 mm; suboxic zone corresponding to pH minimum), and the deeper (40–50 mm; anoxic zone). The numbers on the lower right of the boxes are the total numbers of living specimens observed under SEM. The dissolution scale is proposed by Daviray et al. (2024). No DS-3 or DS-4 were observed at both stations.</figcaption></figure>

<figure><img src="image_8.png" /><figcaption>**Fig. 6.** Relative species proportions in dead foraminiferal assemblages (>125 µm fraction). Three depth layers were analysed: the surface (0–2 mm; oxic zone), the subsurface (6–8 mm; suboxic zone corresponding to pH minimum), and the deeper (45–50 mm; anoxic zone). A minimum of 300 specimens was counted for each fraction size (>315-, 150-, and 125 µm) per sample.</figcaption></figure>

reduction of $NO_3^-$ could be responsible for a non-negligible electron flux (Malkin et al., 2014; Marzocchi et al., 2022, 2014).

The marine influence is predominant throughout the year (S > 33), except for winter, when river flooding (S = 23) occurs in the Trieux river and the tidal channels (Fig. 3). The consequence is a reduction in both the pH and the saturation state of the calcite on the sediment surface. Subsequently, corrosion within the suboxic zone exhibits a variable intensity in relation to eSOx. The highest levels of acidification were observed during the summer-autumn period, in correlation with the algal deposit. Corrosion appeared to be sustained during winter by the lower buffering capacity of the overlying waters influenced by riverine discharge. Therefore, we hypothesize that in eutrophic environments, cable bacteria highly structure the carbonate system within the first centimeters of sediment. Such conditions would exert strong pressure on the low-motility carbonate benthic meiofauna, such as benthic foraminifera.

## 4.2. Living foraminiferal community under eSOx

The vertical distributions of living foraminifera in Ledano mudflat in July 2020 were very similar to those observed by Daviray et al. (2024) in the Auray estuary colonized by cable bacteria. Most of the community was concentrated in the superficial layers of the sediment where conditions were favorable (oxygen available, pH > 7.5). The few specimens observed at depth at station B were probably dragged down by bio-turbation (Bouchet et al., 2009; Maire et al., 2016; Thibault de Chanvalon et al., 2022; Thibault de Chanvalon et al., 2015). The low

abundances observed in summer compared to similar estuarine mudflats in the English Channel and the Atlantic French mudflats (Fouet et al., 2022; Francescangeli et al., 2020; Thibault de Chanvalon et al., 2022), suggest that the foraminiferal community was probably senescent. We assume that the summer green tide led to the collapsing of the community by promoting oxygen depletion, sulphide accumulation and eSOx resulting in corrosive pore waters. However, Papaspyrou et al. (2013) did not detect any effect of the macroalgal bloom (Ulva genus) in the Bay of Cádiz (SW Spain) on foraminiferal abundance or community. In this case, the bloom occurred in winter (from December to February) when low temperature (12 °C) may have slowed down microbial activity that probably limited anaerobic remineralization processes and their impact on foraminifera. On the other hand, Murray and Alve (2000) and Alve (2001) observed a decrease in foraminiferal density during the summers of 1994 and 1995 when Ulva covered the intertidal zone of the Hamble estuary (England) from late spring to autumn. Some benthic foraminiferal species are known to tolerate euxinic conditions (Langlet et al., 2014; Moodley et al., 1998; Richirt et al., 2020). At the Ledano mudflat, the almost only remaining species was *H. germanica*, albeit in low densities.

Living foraminiferal communities of almost monospecific densities of *H. germanica* were observed in other intertidal estuarine mudflats (downstream mudflat in the Auray estuary, Fouet et al., 2022; Authie and Somme estuaries, Francescangeli et al., 2020). The corrosive pore-water conditions recorded over several consecutive months in the first few centimeters of sediment on the Ledano mudflat, however, did not appear to induce severe dissolution of living *H. germanica* tests. Few
specimens were found in the acidic suboxic zone (12 and 4 individuals in the 6–8 mm depth layer at station B and D respectively), and advanced stages of dissolution represented only 15% of the community. These observations are in contradiction with observations made in the Auray river (Daviray et al., 2024) where CB activity is similar and dissolution processes are more severe. Hyaline foraminifera are affected by environmental acidification if the ΔpH surrounding the test is greater than the pH variation induced by foraminifera during biomineralization (0.6 ≤ ΔpH ≤ 1.1 for *Ammonia* spp.; François et al., 2025; Toyofuku et al., 2017), and if the exposure time is longer than calcification time (up to eight hours). In the present study, the ΔpH measured in the suboxic zone of the sediment due to eSOx were much greater than 1.0 and were maintained for longer time than the calcification process (days to months). Despite such unfavorable conditions for biomineralization and calcite integrity, the few tests of living *H. germanica* found within this zone were only slightly dissolved suggesting a specific resistance mechanism.

As the foraminiferal community was almost monospecific, we assume that the particular ability to perform photosynthesis developed by *H. germanica* through kleptoplasty (Lopez, 1979) is beneficial. *Haynesina germanica* is able to retain the functional chloroplasts of the diatoms it feeds on (Pillet et al., 2011) enabling it to carry out photosynthesis for several days (Jauffrais et al., 2016). During photosynthesis, the consumption of CO₂ leads to an increase in pH (Köhler-Rink and Kühl, 2001; Köhler-Rink and Kühl, 2000) and the release of O₂ creating a protective microhabitat against reduced chemical species. Photosynthetic capacity acquired by *H. germanica* (Jauffrais et al., 2016; Jesus et al., 2021) may give it an advantage over other species to ensure its maintenance in the corrosive conditions as previously suggested by Cesbron et al. (2017). The experiments conducted with *H. germanica* to test this hypothesis did not show a specific resistance to corrosion of *H. germanica* through photosynthesis under acidification (pH = 6.8) and low light (Daviray et al., 2025). More thorough investigation would be necessary to test a higher range of environmental conditions and especially more intense illumination. Another potential advantage of kleptoplasty could be, as suggested by Cesbron et al. (2017), related to the metabolism of ammonium (Grzymski et al., 2002; LeKieffre et al., 2022) that is abundant in porewaters in the Ledano mudflat. This mixotrophy may be the reason why almost only *H. germanica* can survive under green tide conditions.

### 4.3. Foraminiferal test record in sediment

As hypothesized by Richirt et al. (2022) and observed by Daviray et al. (2024), the dissolution process resulting from eSOx induced by cable bacteria activity had a significant impact on the dead foraminiferal assemblages through a loss of carbonate tests and an accumulation of organic linings (low C/T ratio associated with high DS-5/C ratio) in the sedimentary record. This is in line with the observations at station D in July 2020, where eSOx was detected in spring and autumn 2019. If calcareous foraminifera remained dominant in the assemblage (Fig. 6), the proportion of fully dissolved tests increased to 46% of dead calcareous foraminifera in the most corrosive subsurface layer (6–8 mm). At station B, the observations suggest something different. While eSOx was recorded in the seasons prior to the foraminiferal study (autumn 2019, winter 2019 and summer 2020), calcareous tests were the most abundant (C/T ≥ 0.16) and were well preserved with a low proportion of fully dissolved specimens (DS-5/C ≤ 0.05). The effect of the dissolution process associated with eSOx on the thanatocoenosis seemed to be heterogeneous in the Ledano mudflat. At station D, we observed lower salinity and pore-water [CO₃²⁻]calc within the upper centimeters of sediment over time than at station B (Fig. 3). Saraswat et al. (2015) and Charrieau et al. (2018) observed a coupled effect of low salinity/low pH (i.e., ranged as 20 < S < 35, 7.2 < pH < 8.0 and 14 < [CO₃²⁻]calc < 113 µM) on the dissolution of calcareous foraminiferal tests. The thanatocoenosis at station D had a longer exposure time to corrosive pore water

within the sediment column compared to station B, resulting from the coupled lower salinity/eSOx.

At both stations, the species found in the dead assemblages showed different proportions than in living community. The dead assemblage is a combining reflection of 1) the past foraminiferal community which has lived in the mudflat, 2) the transport of specimens from adjacent habitats and 3) specimens loss through dissolution or crunching processes (Alve and Murray, 1994; Berkeley et al., 2007; de Rijk and Troelstra, 1999; Murray and Alve, 1999). First, we can suggest that past living foraminiferal community was more abundant and more diverse than the one observed under a thick algal mat in summer 2020. As foraminiferal blooms occur mostly in spring, following microphytobenthos blooms (Alve and Murray, 1994; Papaspyrou et al., 2013; Schönfeld and Numberger, 2007), the living community in the Ledano mudflat in late winter and spring, in absence of the *Ulva* mat, was probably similar to those observed in other estuarine intertidal mudflats, for instance by Daché et al. (Abber estuary, Daché et al., 2024); Francescangeli et al. (Grand-Fort-Philippe, 2020), Fouet et al. (Auray and Vie estuaries, 2022) or Daviray et al. (Auray estuary, 2024). Secondly, a low proportion of the dead specimens may have been transported. Indeed, *Elphidium* spp. is preferentially living nearer the estuary mouth where the marine influence is stronger (Debenay et al., 2000; Fouet et al., 2022; Jorissen et al., 2023) whereas the agglutinated species (i.e., *A. balkwilli*, *E. macrescens*) may come from the upper part of the mudflat (Alve and Murray, 1994; Debenay et al., 2000; Fouet et al., 2022; Jorissen et al., 2023). Finally, the dead specimens are more sensitive to dissolution than the living ones, and the calcareous test of dead *H. germanica* seems to be more sensitive than the one test of *Ammonia* spp. (Daviray et al., 2025), which is almost twice as thick for the same number of calcareous layers (Choquel et al., 2023; Cisneros-Lazaro et al., 2022). The specific response to dissolution of different calcareous tests could explain the significant variation in species proportions between the dead assemblage and the living community.

### 5. Conclusion

The activity of cable bacteria was heterogeneous on the Ledano mudflat but persisted throughout the year. It contributed up to 45% of oxygen consumption in the sediment during green tide. eSOx appeared to benefit from winter nitrate inputs into the mudflat and from autumn decay of the algal mat. The low densities of living foraminifera observed in summer seemed to indicate a population in decline, probably due to summer green tides coupled with sediment acidification by eSOx. The monospecific community of *Haynesina germanica*, known to be kleptoplastidic, suggests specific metabolic mechanisms that ensure its maintenance in these corrosive conditions, which should be thoroughly studied. Dissolution on the thanatocoenosis is more important than in the living community and can be heterogeneous in the mudflat. Here, it appears to be driven by coupled low salinity/eSOx, which can cause more than 20% loss of calcareous tests within the acidic sediment layer. As a worldwide coastal phenomenon, the role of such cable bacteria dynamics requires more assessments to understand their effective temporal resolution on hard-shelled benthic meiofauna and their calcareous test record in the sediment.

### CRediT authorship contribution statement

**Maxime Daviray:** Writing – review & editing, Writing – original draft, Visualization, Validation, Methodology, Investigation, Formal analysis, Data curation. **Emmanuelle Geslin:** Writing – review & editing, Writing – original draft, Visualization, Validation, Supervision, Project administration, Investigation, Funding acquisition, Conceptualization. **Edouard Metzger:** Writing – review & editing, Writing – original draft, Visualization, Validation, Data curation. **Sophie Quinchard:** Data curation. **Bruno Deflandre:** Writing – review & editing, Validation, Formal analysis, Data curation. **Céline Charbonnier:** Writing –
review & editing, Formal analysis, Data curation. **Pierre Anschutz:**
Writing – review & editing, Visualization, Validation, Supervision,
Project administration, Funding acquisition, Formal analysis, Data
curation, Conceptualization.

Author contributions

Maxime DAVIRAY (foraminiferal and microprofile analysis, statistical inference, writing and editing), Emmanuelle GESLIN (head of CB-For project, foraminiferal analysis, writing and editing), Edouard METZGER (field work, writing and editing), Sophie QUINCHARD (foraminiferal picking), Bruno DEFLANDRE (field work, microprofiles acquisition and analysis, writing and editing), Céline CHARBONNIER (field work, geochemical acquisition and analysis), Pierre ANSCHUTZ (head of IMPRO project, field work, geochemical acquisition and analysis, writing and editing).

Funding

The authors received co-fundings from the Région Bretagne and the Agence de l'Eau Loire-Bretagne (project IMPRO), from the CNRS-INSU (LEFE-CYBER program, project CB-FOR), and from the Ministère de l'Enseignement Supérieur et de la Recherche.

Declaration of competing interest

The authors declare no conflicts of interest with funding or pub-
lishing bodies.

Acknowledgments

The authors thank Matéo TOUGNE for his help in identifying dead foraminifera, Julia COURTIAL for her help with statistics, and Nils RISGAARD-PETERSEN for his advice on bioelectric calculations.

<figure><img src="image_9.png" /><figcaption>Appendix 1. Zoom at the sediment-water interface (—2 and 5-mm depth) of the representative vertical microprofiles of O₂, pH, and H₂S from the Fig. 1</figcaption></figure>
<figure><img src="image_10.png" /><figcaption>Appendix 2. Heterogeneity of vertical microprofiles of O₂, pH, and H₂S for both stations B and D</figcaption></figure>

## Data availability

Data will be made available on request.

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