Description: Shallow marine benthic communities around Antarctica show high levels of endemism, gigantism, slow growth, longevity and late maturity, as well as adaptive radiations that have generated considerable biodiversity in some taxa1. The deeper parts of the Southern Ocean exhibit some unique environmental features, including a very deep continental shelf2 and a weakly stratified water column, and are the source for much of the deep water in the world ocean. These features suggest that deep-sea faunas around the Antarctic may be related both to adjacent shelf communities and to those in other oceans. Unlike shallow-water Antarctic benthic communities, however, little is known about life in this vast deep-sea region2, 3. Here, we report new data from recent sampling expeditions in the deep Weddell Sea and adjacent areas (748-6,348 m water depth) that reveal high levels of new biodiversity; for example, 674 isopods species, of which 585 were new to science. Bathymetric and biogeographic trends varied between taxa. In groups such as the isopods and polychaetes, slope assemblages included species that have invaded from the shelf. In other taxa, the shelf and slope assemblages were more distinct. Abyssal faunas tended to have stronger links to other oceans, particularly the Atlantic, but mainly in taxa with good dispersal capabilities, such as the Foraminifera. The isopods, ostracods and nematodes, which are poor dispersers, include many species currently known only from the Southern Ocean. Our findings challenge suggestions that deep-sea diversity is depressed in the Southern Ocean and provide a basis for exploring the evolutionary significance of the varied biogeographic patterns observed in this remote environment.

Description: The benthic fauna was investigated during the expedition ANT-XXIV/2 (2007/08) in relation to oceanographic features, biogeochemical properties and sediment characteristics, as well as the benthic, pelagic and air-breathing fauna. The results document that Maud Rise (MR) differs distinctly from surrounding deep-sea basins investigated during previous Southern Ocean expeditions (ANDEEP 2002, 2005). Considering all taxa, the overall similarity between MR and adjacent stations was low (~20% Bray-Curtis-Similarity), and analyses of single taxa show obvious differences in species composition, abundances and densities. The composition and diversity of bivalves of MR are characterised by extremely high abundances of three species, especially the small sized Vesicomya spp. Exceptionally high gastropod abundance at MR is due to the single species Onoba subantarctica wilkesiana, a small brooder that may prey upon abundant benthic foraminiferas. The abundance and diversity of isopods also show that one family, Haplomunnidae, occurs with a surprisingly high number of individuals at MR while this family was not found at any of the 40 bathyal and abyssal ANDEEP stations. Similarly, polychaetes, especially the tube-dwelling, suspension-feeder fraction, are represented by species not found at the comparison stations. Sponges comprise almost exclusively small specimens in relatively high numbers, especially a few species of Polymastiidae. Water-column sampling from the surface to the seafloor, including observations of top predators, indicate the existence of a prospering pelagic food web. Local concentrations of top predators and zooplankton are associated with a rich ice-edge bloom located over the northern slope of MR. There the sea ice melts, which is probably accelerated by the advection of warm water at intermediate depth. Over the southern slope, high concentrations of Antarctic krill (Euphausia superba) occur under dense sea ice and attract Antarctic Minke Whales (Balaenoptera bonaerensis) and several seabird species. These findings suggest that biological prosperity over MR is related to both oceanographic and sea-ice processes. Downward transport of the organic matter produced in the pelagic realm may be more constant than elsewhere due to low lateral drift over MR.

Description: Sponges (Porifera) host diverse and species-specific communities of microbial symbionts with which they maintain tight interactions. However, knowledge on the microbiomes of sponges from deep waters and remote polar areas is still scarce, especially for the sponge class Hexactinellida. Therefore, our aim was to describe the community composition, richness and density of microbial symbionts of Antarctic deep-water sponges, including several species of hexactinellids, and relate the findings to host ultrastructure and histology. During the Antarctic expeditions PS96 (RV Polarstern, 2015/16, eastern Weddell Sea) and JR17003a (RRS James Clark Ross, 2018, western Weddell Sea), 28 sponge specimens, bottom water and sediment were sampled for molecular analysis of microbial communities. The sponges were collected from deep habitats of 290-845 m by Bottom Trawl or Agassiz Trawl and comprised 19 hexactinellids and 9 demosponges. Bottom water was collected with a CTD rosette sampler which also measured environmental data (temperature, salinity, oxygen) close to the start or end point of the trawls and at four additional stations. Sediment was collected from the Agassiz trawl together with sponge samples during JR17003a. The molecular microbiome analysis targeted bacteria and was based on 16S rRNA gene sequencing of the V3-V4 variable regions. Sequences were processed using the QIIME2 environment. Amplicon sequence variants (ASVs) were generated with the DADA2 algorithm and classified based on the Silva 132 99% OTUs 16S database. Eight sponge specimens collected during JR17003a were further investigated microscopically for microbial symbionts, sponge histology and ultrastructure. Histological sections of 7-30 µm were stained in either Masson's trichrome or Hematoxylin/Eosin and images captured on a Zeiss Axioskop 2 plus with a QiCam camera using Northern Eclipse software. Ultrastructural sections of 60 nm were stained in uranyl acetate and lead citrate and then viewed and photographed with a Philips Morgagni transmission electron microscope equipped with a Gatan CCD camera. This Data Collection includes sampling information, environmental data, NCBI accession numbers and photographs of the analyzed sponges, data on the microbial symbiont communities (amplicon sequence variants and microbial phyla) of sponges, seawater and sediment, as well as micrographs of sponge histology and ultrastructure.