Biosignatures of in situ carbon cycling driven by physical isolation and sedimentary methanogenesis within the anoxic basin of perennially ice-covered Lake Untersee, Antarctica

Organic biomarker distribution and stable isotope composition was used to investigate biogeochemical carbon cycling in the anoxic basin of Lake Untersee, Antarctica. Phospholipid fatty acid (PLFA) concentrations indicative of microbial abundances were low in the oxic water column overlying the basin but rose in the suboxic transition zone and further increased within the underlying anoxic water, with the highest abundances near the sediment water interface. Archaeol (up to 24.8 mg/kg) and glycerol dialkyl glycerol tetraethers were only detected within the deepest water sample and sediment. High methane (CH4) concentrations (ca. 172 mg/L or 11 mmol/L) were observed in the deepest water samples and produced via hydrogenotrophy (CO2-reduction) based on methane isotopes and highly 13C-enriched dissolved inorganic carbon. Methane concentration slowly decreased away from the sediment, across the anoxic water column and then decreased rapidly at the oxic/anoxic interface (78–68 m). Here a ca. 10‰ increase in δ13CCH4 values combined with δ13CPLFA values that decreased as CH4 concentrations rapidly declined indicated an aerobic methanotrophy fueled microbial community. Findings suggest that upward methane diffusion drives microbial productivity within the suboxic/anoxic zones resulting in the observed high PLFA biomass. Subsequent sinking of detrital material from these communities supports heterotrophic microbes throughout the anoxic water column and potentially supplied nutrients to support phototrophy in the upper suboxic transition zone, itself contributing to sinking detrital material and accumulation of sedimentary organic material. Notably, while a clear biosignature of methane oxidation is present in suboxic zone PLFA, this signature is not recognizable within the sediments.