Modern Southern Ocean sea ice environments are reflected in eukaryotic DNA from the sea floor

In the Southern Ocean, reconstructions of past sea ice conditions remain challenging because of limited proxy and calibration approaches. Sedimentary DNA can complement the available proxies for reconstructing past sea ice environments and thereby provide additional and/or new insights into past regional and global climate evolution. However, its application requires an established relationship between seafloor surface sediment DNA composition and modern sea ice conditions. Here we show, using a metabarcoding approach (V9 18S rRNA) on 45 surface sediment samples from the Weddell Sea, that contemporary sea ice environments are reflected in the sedimentary DNA community composition. Our results further reveal that sea ice concentration and water depth have a significant effect on community variance. Most relevant for paleoenvironmental applications, our data indicates a dominance in relative abundance of Chaetocerotaceae diatom DNA in polynya-related regions, agreeing with previous diatom microfossil assemblage studies. These results establish this family as a robust molecular proxy for these productive environments. We also show that planktic taxa were more influenced by sea ice concentration than benthic taxa, indicating that focusing on planktic taxa for sea ice reconstructions in sedimentary ancient DNA may be more effective. We conclude that an environmental genomics approach can complement and extend existing proxy records, especially for organisms without a fossil record and areas with limited conventional proxy data.