On giant shoulders: how a seamount affects the microbial community composition of seawater and sponges
Authors: Busch Kathrin , Hanz Ulrike , Mienis Furu , Mueller Benjamin , Franke Andre , Roberts Emyr Martyn , Rapp Hans Tore , Hentschel Ute 
Published: July 2020

Abstract. Seamounts represent ideal systems to study the influence
and interdependency of environmental gradients at a single geographic
location. These topographic features represent a prominent habitat for
various forms of life, including microbiota and macrobiota, spanning benthic
as well as pelagic organisms. While it is known that seamounts are globally
abundant structures, it still remains unclear how and to which extent the
complexity of the sea floor is intertwined with the local oceanographic
mosaic, biogeochemistry, and microbiology of a seamount ecosystem. Along
these lines, the present study aimed to explore whether and to what extent
seamounts can have an imprint on the microbial community composition of
seawater and of sessile benthic invertebrates, sponges. For our
high-resolution sampling approach of microbial diversity (16S rRNA gene
amplicon sequencing) along with measurements of inorganic nutrients and
other biogeochemical parameters, we focused on the Schulz Bank seamount
ecosystem, a sponge ground ecosystem which is located on the Arctic
Mid-Ocean Ridge. Seawater samples were collected at two sampling depths
(mid-water, MW, and near-bed water, BW) from a total of 19 sampling sites.
With a clustering approach we defined microbial microhabitats within the
pelagic realm at Schulz Bank, which were mapped onto the seamount's
topography and related to various environmental parameters (such as
suspended particulate matter, SPM; dissolved inorganic carbon, DIC;
silicate, SiO4-; phosphate, PO43-; ammonia,
NH4+; nitrate, NO32-; nitrite, NO2-;
depth; and dissolved oxygen, O2). The results of our study reveal a
“seamount effect” (sensu stricto) on the microbial mid-water pelagic
community at least 200 m above the sea floor. Further, we observed a strong
spatial heterogeneity in the pelagic microbial landscape across the
seamount, with planktonic microbial communities reflecting oscillatory and
circulatory water movements, as well as processes of bentho-pelagic
coupling. Depth, NO32-, SiO4-, and O2
concentrations differed significantly between the determined pelagic
microbial clusters close to the sea floor (BW), suggesting that these
parameters were presumably linked to changes in microbial community
structures. Secondly, we assessed the associated microbial community
compositions of three sponge species along a depth gradient of the seamount.
While sponge-associated microbial communities were found to be mainly
species-specific, we also detected significant intra-specific differences
between individuals, depending on the pelagic near-bed cluster they
originated from. The variable microbial phyla (i.e. phyla which showed
significant differences across varying depth, NO32-,
SiO4-, O2 concentrations, and different from local
seawater communities) were distinct for every sponge species when
considering average abundances per species. Variable microbial phyla
included representatives of both those taxa traditionally counted for the
variable community fraction and taxa counted traditionally for the
core community fraction. Microbial co-occurrence patterns for the three
examined sponge species Geodia hentscheli, Lissodendoryx complicata, and Schaudinnia rosea were distinct from each other. Over all,
this study shows that topographic structures such as the Schulz Bank
seamount can have an imprint (seamount effect sensu lato) on both the
microbial community composition of seawater and sessile benthic
invertebrates such as sponges by an interplay between the geology, physical
oceanography, biogeochemistry, and microbiology of seamounts.