Organic matter amount and types affect sediment microbial community in the Larsen A embayment after ice shelf disintegration

Climate change can influence seafloor and subseafloor environments by changing the types and amounts of organic material input to sediments, thereby affecting the geochemical characteristics of the sediment porewater and the microbial communities. Global warming in the polar regions likely increases primary productivity on the sea surface and enhances organic matter input into sediments. The Larsen A Embayment in Eastern Antarctic Peninsula has experienced ice shelf disintegration during the past 200 years. The increase in organic matter from the water column after the ice shelf collapse could induce a rapid change in the sedimentation regime, redox status, and subseafloor microbial ecosystems. We collected sediment cores from five stations along a nearshore to offshore transect in the Larsen A embayment in 2012 and analyzed depositional lipid biomarkers and microbial DNA. We found that phytoplankton and bacterial lipids concentrations in the sediments were correlated with the increasing gradient of primary productivity from onshore to offshore of the embayment, and with the lateral transport of organic matter and bioturbation. By using redundancy analysis, we found that relative abundances of labile and phytoplankton originated organic matter positively correlated with relative abundances of chemoheterotrophic bacteria such as Alteromonas, Oceanospirillales, Rhodobacterales, Nitrosomonadales, and Flavobacteriales. The relative abundances of recalcitrant organic matter correlated with relative abundances of sulfate reducers and other chemoheterotrophic bacteria such as Desulfuromonadales, Desulfobacterales, Spirochaetaceae, Bacteroidales. Our results indicate that quality and quantity of organic matter could shape microbial organic matter in sediments. Marine sediment is one of the largest global reservoirs of living microorganisms and the largest sink of organic carbon on Earth. Nevertheless, the impacts of environmental changes on subseafloor microbial communities are not well understood. Understanding how microbes respond to rapid environmental changes such as the ice shelf collapse may provide insights into the vulnerability of sediment microbial communities and impact on subseafloor biogeochemical cycles.