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With ongoing climate change, research into the biological changes occurring in particularly vulnerable ecosystems, such as Antarctica, is critical. The Totten Glacier region, Sabrina Coast, is currently experiencing some of the highest rates of thinning across all East Antarctica. An assessment of the microscopic organisms supporting the ecosystem of the marginal sea‐ice zone over the continental rise is important, yet there is a lack of knowledge about the diversity and distribution of these organisms throughout the water column, and their occurrence and/or preservation in the underlying sediments. Here, we provide a taxonomic overview of the modern and ancient marine bacterial and eukaryotic communities of the Totten Glacier region, using a combination of 16S and 18S rRNA amplicon sequencing (modern DNA) and shotgun metagenomics (sedimentary ancient DNA, sedaDNA). Our data show considerable differences between eukaryote and bacterial signals in the water column versus the sediments. Proteobacteria and diatoms dominate the bacterial and eukaryote composition in the upper water column, while diatoms, dinoflagellates, and haptophytes notably decrease in relative abundance with increasing water depth. Little diatom sedaDNA is preserved in the sediments, which are instead dominated by Proteobacteria and Retaria. We compare the diatom microfossil and sedaDNA record and link the weak preservation of diatom sedaDNA to DNA degradation while sinking through the water column to the seafloor. This study provides the first assessment of DNA transfer from ocean waters to sediments and an overview of the microscopic communities occurring in the climatically important Totten Glacier region. Plain Language Summary: Antarctica is highly vulnerable to climate change and research into how marine organisms around the continent will respond to warming conditions is important. Microscopic organism, such as bacteria and phytoplankton, are key in the marine ecosystems because they support the entire marine food web. In this study, we investigated the composition of these microorganisms in the climatically important East Antarctic Totten Glacier region. We profiled compositional changes throughout the water column and into underlying subseafloor sediments, which provided information on both actively living and long dead organisms, and thus modern and past environmental conditions, respectively. We used genetic techniques to compare these marine communities, focusing on the taxonomically informative small subunit ribosomal ribonucleic acid (SSU rRNA) gene. Our study provides an overview about the microorganisms that live in the modern ocean in the Totten Glacier region, and how well their genetic signatures preserve in the underlying sediments. This knowledge is important both for assessments of present‐day marine ecosystem health and functioning, and when reconstructing past ocean environments as analogs to ongoing climate change. Key Points: Genetic signals of marine prokaryotes and eukaryotes change considerably from surface to deep waters and into the sedimentsDNA of surface‐dwelling organisms in a deep water column degrades while sinking with little DNA preserved in the sedimentsOrganisms that are well represented in deeper waters appear to have a higher likelihood of becoming preserved in the sediments [ABSTRACT FROM AUTHOR]

Radiolarians (holoplanktonic protozoa) preserved in marine sediments are commonly used as palaeoclimate proxies for reconstructing past Southern Ocean environments. Generating reconstructions of past climate based on microfossil abundances, such as radiolarians, requires a spatially and environmentally comprehensive reference dataset of modern census counts. The Southern Ocean Radiolarian (SO-RAD) dataset includes census counts for 238 radiolarian taxa from 228 surface sediment samples located in the Atlantic, Indian, and southwest Pacific sectors of the Southern Ocean. This compilation is the largest radiolarian census dataset derived from surface sediment samples in the Southern Ocean. The SO-RAD dataset may be used as a reference dataset for palaeoceanographic reconstructions, or for studying modern radiolarian biogeography and species diversity. As well as describing the data collection and collation, we include recommendations and guidelines for cleaning and subsetting the data for users unfamiliar with the procedures typically used by the radiolarian community. The SO-RAD dataset is available to download from 10.1594/PANGAEA.929903 (Lawler et al., 2021). [ABSTRACT FROM AUTHOR]

Some aspects of the life cycle of the Southern Ocean diatom Fragilariopsis kerguelensis have been investigated previously, but many of its details have not been surveyed in nature. We investigated material from a two-year sediment trap time series by high-throughput imaging and image analysis, looking for morphometric signals of life cycle stages. Valve length distributions appeared close to unimodal but positively (right-) skewed. Size cohorts resulting from synchronized sexual reproduction events were not clearly distinguishable. Nevertheless, based on changes in valve length distributions, we found three general seasonal phases. These corresponded to periods of proliferation (with higher proportions of smaller cells during late spring/early summer), cessation of growth (relative loss of smaller cells during late summer/early autumn), and overwintering (little change in size distributions, with an increased proportion of large cells). We discuss possible causes of these signals, and their relevance to growth, sexual activity and adaption to environmental conditions, such as grazing pressures and the need for an overwintering strategy. [ABSTRACT FROM AUTHOR]

Abstract: An “island mass effect” was observed from the sedimentary distribution of diatoms on the northeastern Kerguelen Plateau. Five new samples placed in context to species distributions previously reported from the Permanently Open Ocean Zone (POOZ) revealed a plateau community dominated by three species/taxa: Fragilariopsis kerguelensis, Thalassionema nitzschioides f. nitzschioides and Chaetoceros Hyalochaete resting spores. Intermediate abundances of Thalassiosira antarctica were unique to the plateau sediment signature as were increased abundances of Eucampia antarctica v. antarctica. The off-plateau sediment sample contained typical POOZ sediment distributions dominated by F. kerguelensis, Thalassiosira lentiginosa and Thalassiothrix antarctica. The role of deposition from the surface waters to the sediments was investigated by comparison to the diatom thanatocoenose observed in surface waters. These results suggest that the use of sediment abundances of C. Hyalochaete resting spores and F. kerguelensis to determine past productivity events (such as wind-sourced iron enrichment of the Southern Ocean at the Last Glacial Maximum) is not straightforward. The elevated abundance of T. nitzschioides f. nitzschioides on the plateau did not indicate an iron-deplete regime but the converse. For the evaluation of paleo-productivity hypotheses for the open Southern Ocean, we suggest caution in the use of Subantarctic and Kerguelen Plateau sediment signatures, and emphasize that there is a need to also study Antarctic coastal and especially sea-ice melt-water stimulated blooms. [Copyright &y& Elsevier]

Abstract: Analysis of the diatom assemblage during the recent KErguelen: compared study of Ocean and Plateau in Surface water (KEOPS) mission (January–February 2005), enabled a modern description of the summer bloom community over the Kerguelen Plateau in the context of the community in the surrounding high-nutrient, low-chlorophyll (HNLC) Southern Ocean waters. Net samples revealed biogeographic partitioning of certain species. Comparison of net samples with CTD-Niskin bottle samples revealed a considerable underestimation of large diatom species in the water samples. We analysed four plateau stations and one off-plateau HNLC station for individual species abundances and biomass contributions down to 150m. The stations can be divided into two groups based on species composition and total biomass contributions, equating to high (45.6–99.4μg CL−1) and low (2.5–25.7μg CL−1) biomass regimes. Individual species abundances were not related to the major species biomass contributions. Repeat analyses at the bloom station, A3, and the off-plateau HNLC station, C11, revealed evolution from a Chaetoceros subgenus Hyalochaete bloom to a remnant Eucampia antarctica assemblage. In contrast the HNLC station, C11, remained dominated by Fragilariopsis pseudonana and F. kerguelensis throughout the survey. Comparison to artificial iron experiments reveals differences in the responses of Pseudo-nitzschia spp. and F. kerguelensis, which may arise in part from differences in macro-nutrient supply, in particular silicic acid availability, as well as from seasonal succession. [Copyright &y& Elsevier]

Abstract: Based on the quantitative study of diatoms and radiolarians, summer sea-surface temperature (SSST) and sea ice distribution were estimated from 122 sediment core localities in the Atlantic, Indian and Pacific sectors of the Southern Ocean to reconstruct the last glacial environment at the EPILOG (19.5–16.0ka or 23000–19000cal yr. B.P.) time-slice. The statistical methods applied include the Imbrie and Kipp Method, the Modern Analog Technique and the General Additive Model. Summer SSTs reveal greater surface-water cooling than reconstructed by CLIMAP (Geol. Soc. Am. Map Chart. Ser. MC-36 (1981) 1), reaching a maximum (4–5°C) in the present Subantarctic Zone of the Atlantic and Indian sector. The reconstruction of maximum winter sea ice (WSI) extent is in accordance with CLIMAP, showing an expansion of the WSI field by around 100% compared to the present. Although only limited information is available, the data clearly show that CLIMAP strongly overestimated the glacial summer sea ice extent. As a result of the northward expansion of Antarctic cold waters by 5–10° in latitude and a relatively small displacement of the Subtropical Front, thermal gradients were steepened during the last glacial in the northern zone of the Southern Ocean. Such reconstruction may, however, be inapposite for the Pacific sector. The few data available indicate reduced cooling in the southern Pacific and give suggestion for a non-uniform cooling of the glacial Southern Ocean. This study is part of MARGO, a multiproxy approach for the reconstruction of the glacial ocean surface. [Copyright &y& Elsevier]