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Sustained release of methane (CH4) to the atmosphere from thawing Arctic permafrost may be a positive and significant feedback to climate warming. Atmospheric venting of CH4 from the East Siberian Arctic Shelf (ESAS) was recently reported to be on par with flux from the Arctic tundra; however, the future scale of these releases remains unclear. Here, based on results of our latest observations, we show that CH4 emissions from this shelf are likely to be determined by the state of subsea permafrost degradation. We observed CH4 emissions from two previously understudied areas of the ESAS: the outer shelf, where subsea permafrost is predicted to be discontinuous or mostly degraded due to long submergence by seawater, and the near shore area, where deep/open taliks presumably form due to combined heating effects of seawater, river run-off, geothermal flux and pre-existing thermokarst. CH4 emissions from these areas emerge from largely thawed sediments via strong flare-like ebullition, producing fluxes that are orders of magnitude greater than fluxes observed in background areas underlain by largely frozen sediments. We suggest that progression of subsea permafrost thawing and decrease in ice extent could result in a significant increase in CH4 emissions from the ESAS.

The permanent ice cover of Lake Vida (Antarctica) encapsulates an extreme cryogenic brine ecosystem (−13°C; salinity, 200). This aphotic ecosystem is anoxic and consists of a slightly acidic (pH 6.2) sodium chloride-dominated brine. Expeditions in 2005 and 2010 were conducted to investigate the biogeochemistry of Lake Vida's brine system. A phylogenetically diverse and metabolically active Bacteria dominated microbial assemblage was observed in the brine. These bacteria live under very high levels of reduced metals, ammonia, molecular hydrogen (H 2 ), and dissolved organic carbon, as well as high concentrations of oxidized species of nitrogen (i.e., supersaturated nitrous oxide and ∼1 mmol·L −1 nitrate) and sulfur (as sulfate). The existence of this system, with active biota, and a suite of reduced as well as oxidized compounds, is unusual given the millennial scale of its isolation from external sources of energy. The geochemistry of the brine suggests that abiotic brine-rock reactions may occur in this system and that the rich sources of dissolved electron acceptors prevent sulfate reduction and methanogenesis from being energetically favorable. The discovery of this ecosystem and the in situ biotic and abiotic processes occurring at low temperature provides a tractable system to study habitability of isolated terrestrial cryoenvironments (e.g., permafrost cryopegs and subglacial ecosystems), and is a potential analog for habitats on other icy worlds where water-rock reactions may cooccur with saline deposits and subsurface oceans.astrobiology | geomicrobiology | microbial ecology | extreme environment T he observation of microbes surviving and growing in a variety of icy systems on Earth has expanded our understanding of how life pervades, functions, and persists under challenging conditions (e.g., refs. 1-3). Studies of the physical characteristics, the geochemical properties, and microbes in ice (triple point junctions, brine channels, gas bubbles) have also changed our perceptions of the environments that may contain traces of, or even sustain, life beyond Earth [e.g., Mars (4), Europa (5), and Enceladus (6)].Solute depression of ice crystal formation or solar radiation melting of water ice are key processes that provide liquid waterthe key solvent that makes life possible-within icy systems. Microbial communities in these conditions are often sustained by a supply of energy that ultimately derives from photosynthesis (present or past). The understanding of ecosystems based on energy sources other than the Sun comes mainly from realms where hydrothermal processes have provided reduced compounds necessary to fuel chemosynthetically driven ecosystems. Methane derived from thermogenic or biogenic sources can also support microbial communities in deep sea (7) and high arctic cold saline seeps (8). More recently, discoveries of life and associated processes in deep terrestrial subsurface ecosystems (9) provide compelling evidence of subsurface life that in some cases is fueled by nonphotosynthetic processes. Ou...

The role of anaerobic oxidation of methane (AOM) in wetlands, the largest natural source of atmospheric methane, is poorly constrained. Here we report rates of microbially mediated AOM (average rate ¼ 20 nmol cm À 3 per day) in three freshwater wetlands that span multiple biogeographical provinces. The observed AOM rates rival those in marine environments. Most AOM activity may have been coupled to sulphate reduction, but other electron acceptors remain feasible. Lipid biomarkers typically associated with anaerobic methane-oxidizing archaea were more enriched in 13 C than those characteristic of marine systems, potentially due to distinct microbial metabolic pathways or dilution with heterotrophic isotope signals. On the basis of this extensive data set, AOM in freshwater wetlands may consume 200 Tg methane per year, reducing their potential methane emissions by over 50%. These findings challenge precepts surrounding wetland carbon cycling and demonstrate the environmental relevance of an anaerobic methane sink in ecosystems traditionally considered strong methane sources.

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We found unprecedentedly high abundances of microbially produced CH 4 in the anoxic deep waters of Lake Untersee, an oligotrophic, perennially ice-covered Antarctic freshwater lake. The maximum CH 4 concentration (approaching 21.8 Ϯ 1.4 mmol L Ϫ1 ) is one of the highest observed so far in a natural aquatic ecosystem. Although surficial lake sediments are the predominant source of CH 4 in Lake Untersee, methanogenesis occurs also within the anoxic waters. Radiocarbon labeling experiments show that H 2 /CO 2 reduction is the predominant methanogenic pathway (90-100%) both in the sediments and the water column, whereas acetate is only a minor CH 4 precursor. This result is consistent with the stable carbon isotope fractionation between coexisting CH 4 and CO 2 . In the water column, CH 4 is partly consumed by both aerobic and anaerobic microbial oxidation as evidenced by CH 4 concentration patterns, stable isotope analyses ( 13 C, 2 H), and 14 C-CH 4 assays. Dissimilatory sulfate reduction also occurs and peaks at 84 m water depth (1.83 mol SO 4 L Ϫ1 d Ϫ1 ). Intense methanogenesis in surficial lake sediments, diffusion of CH 4 from sediments to the water column, additional CH 4 production in the water column, gross CH 4 production higher than CH 4 consumption, and lack of mixing because of the permanent ice cover cause the exceptionally high CH 4 concentration in the lake. Our studies demonstrate that H 2 /CO 2 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.As an environmentally important greenhouse gas, methane (CH 4 ) plays a significant role in the global climatic system. Studies of CH 4 cycling in various environments is therefore of fundamental interest. Biological processes are the primary source (ca. 80-90%) of atmospheric CH 4 (e.g., Cicerone and AcknowledgmentsWe thank M. Schwab, D. Schachtschneider, and G. Müller for assistance in the field; K. Weingart, W. Städter, G. Schäfer, and L. Schönicke for technical assistance. H. Kämpf kindly provided SO 4 , NH 4 , and DIC analyses. We thank A. Mackensen for providing the ␦ 13 C-DIC analyses, and P. Harting for providing solubility data. The acetate analyses were done by L. Dulov, Institute of Microbiology, Moscow. The manuscript benefited from critical comments made