Influence of past climate change on subglacial microbial communities and biogeochemical processes beneath Siple Coast ice streams, West Antarctica

Abstract:

The microbial ecosystems of West Antarctic subglacial lakes, Mercer Subglacial Lake (SLM) and Whillans Subglacial Lake (SLW), have been strongly influenced by ice sheet grounding line dynamics during the late Pleistocene and Holocene that were structured by climate and sea level change. Seawater may have inundated this region during the Holocene, suggesting that microbial species in the water columns and sediments of these subglacial lakes may have been replaced through turnover, or alternatively, have persisted and are tolerant of freshwater-estuarine transitions. The bacterial and archaeal community structures in sediment cores and water column samples collected from SLM and SLW were analyzed by small subunit rRNA gene sequence analysis and whole genome sequencing. The communities in the water columns of SLW, SLM, and the marine cavity downstream of the Whillans Ice Stream grounding zone (WGZ) are significantly different, with few species shared between the fresh- and sea-water ecosystems. Although the SLW and SLM sediment communities are highly similar (667 amplicon sequence variants; ASVs) and are significantly different from those at WGZ, the sediments at all locations shared 71 ASVs. Moreover, communities in the SLM sediments become increasingly dissimilar to those in SLW with depth and more similar to those observed at the WGZ. Microbial extracellular polymeric substances (EPS) produced within the surface sediments contains a carbohydrate component that comprises up to ~1% of total organic carbon in the sediments. δ¹³C values -22 to -26‰ of the EPS fall closely in line with acid insoluble organic material in both the SLM and WGZ cores. This implies that contemporary carbon cycling in the lake sediments relies in part on relict marine organic matter deposition. Higher community similarities with depth between lake and WGZ sediments suggests that marine incursions have exerted an historical influence on the contemporary microbial composition and carbon cycling in West Antarctic subglacial aquatic environments.