Diversity and function of sulfur cycling microorganisms in sediments from Subglacial Lake Whillans, Antarctica

There is a growing consensus that metabolically and phylogenetically diverse assemblages of microorganisms mediate subglacial nutrient and elemental cycling. Subglacial Lake Whillans (SLW), located under 801 m of glacial ice, was recently penetrated using environmentally clean protocols. SLW is a permanently dark, cold (-0.5 °C [degrees Celsius]), and shallow (~2.2 m) freshwater lake beneath the West Antarctic Ice Sheet. The presence and diversity of key functional genes involved in dissimilatory sulfur oxidation and reduction were examined at various depths in two sediment cores taken from SLW. Our data show a diversity of sulfur transformation genes throughout the top 34 cm of SLW sediments, which changes with depth. The surficial sediments appear dominated by genes related to known sulfur-oxidizing chemoautotrophs. Sequences encoding the adenosine-5’-phosphosulfate (APS) reductase gene, involved in both dissimilatory sulfate reduction and sulfur oxidation were present in all samples and clustered into 16 distinct Operational Taxonomic Units (OTUs). The majority (74%) of APS reductase sequences clustered with known chemoautotrophic sulfur oxidizers including the “Sideroxydans” and Thiobacillus genera, but members of the genera Thermodesulfovibrio, Desulfobacterium, and Desulfotomaculum were also observed. Detection of sequences encoding for reverse-acting dissimilatory sulfite reductase (rDSR) were consistent with 16S rRNA gene data which indicated phylotypes of “Sideroxydans” and Thiobacillus were abundant in the top 2 cm of SLW sediments. Low rates (1.4 pmol [picomoles] cm^-3 [cubed centimeter] d^-1 [day]) of biologically-mediated sulfate reduction occurred in samples from 0-8 cm in bulk anaerobic sediment incubations. To our knowledge, these are the first reported measurements of sulfate reduction from an Antarctic subglacial environment. Sequences encoding for the dissimilatory sulfite reductase gene (DSR) from known sulfate-reducing prokaryotes (SRP) indicates that the sediment microbial community has the genetic potential to reduce sulfate. The contribution of microbial activity to mineral weathering via sulfur driven chemosynthesis is relevant for understanding the ecology of subglacial lake ecosystems and their role as solute and nutrient sources to the Southern Ocean.