Biogeochemistry of methane in the permanently ice-covered Lake Untersee, central Dronning Maud Land, East Antarctica

Abstract

We found unprecedentedly high abundances of microbially produced CH4 in the anoxic deep waters of Lake Untersee, an oligotrophic, perennially ice-covered Antarctic freshwater lake. The maximum CH4 concentration (approaching 21.8 ± 1.4 mmol L−1) is one of the highest observed so far in a natural aquatic ecosystem. Although surficial lake sediments are the predominant source of CH4 in Lake Untersee, methanogenesis occurs also within the anoxic waters. Radiocarbon labeling experiments show that H2/CO2 reduction is the predominant methanogenic pathway (90–100%) both in the sediments and the water column, whereas acetate is only a minor CH4 precursor. This result is consistent with the stable carbon isotope fractionation between coexisting CH4 and CO2. In the water column, CH4 is partly consumed by both aerobic and anaerobic microbial oxidation as evidenced by CH4 concentration patterns, stable isotope analyses (13C, 2H), and 14C-CH4 assays. Dissimilatory sulfate reduction also occurs and peaks at 84 m water depth (1.83 µmol SO4 L−1 d−1). Intense methanogenesis in surficial lake sediments, diffusion of CH4 from sediments to the water column, additional CH4 production in the water column, gross CH4 production higher than CH4 consumption, and lack of mixing because of the permanent ice cover cause the exceptionally high CH4 concentration in the lake. Our studies demonstrate that H2/CO2 reduction may sometimes be the major pathway of methanogenesis in low-sulfate freshwater environments even at low temperatures. This pathway is obviously more important in Antarctic lakes than hitherto assumed.