The East Siberian Arctic Shelf: towards further assessment of permafrost-related methane fluxes and role of sea ice.

Sustained release of methane (CH(4)) to the atmosphere from thawing Arctic permafrost may be a positive and significant feedback to climate warming. Atmospheric venting of CH(4) 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 CH(4) emissions from this shelf are likely to be determined by the state of subsea permafrost degradation. We observed CH(4) 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. CH(4) 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 CH(4) emissions from the ESAS.

A halophilic bacterium inhabiting the warm, CaCl2-rich brine of the perennially ice-covered Lake Vanda, McMurdo Dry Valleys, Antarctica.

Lake Vanda is a perennially ice-covered and stratified lake in the McMurdo Dry Valleys, Antarctica. The lake develops a distinct chemocline at about a 50-m depth, where the waters transition from cool, oxic, and fresh to warm, sulfidic, and hypersaline. The bottom water brine is unique, as the highly chaotropic salts CaCl2 and MgCl2 predominate, and CaCl2 levels are the highest of those in any known microbial habitat. Enrichment techniques were used to isolate 15 strains of heterotrophic bacteria from the Lake Vanda brine. Despite direct supplementation of the brine samples with different organic substrates in primary enrichments, the same organism, a relative of the halophilic bacterium Halomonas (Gammaproteobacteria), was isolated from all depths sampled. The Lake Vanda (VAN) strains were obligate aerobes and showed broad pH, salinity, and temperature ranges for growth, consistent with the physicochemical properties of the brine. VAN strains were halophilic and quite CaCl2 tolerant but did not require CaCl2 for growth. The fact that only VAN strain-like organisms appeared in our enrichments hints that the highly chaotropic nature of the Lake Vanda brine may place unusual physiological constraints on the bacterial community that inhabits it.

Microbial life at -13 °C in the brine of an ice-sealed Antarctic lake.

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.

On the relationship between methane production and oxidation by anaerobic methanotrophic communities from cold seeps of the Gulf of Mexico.

The anaerobic oxidation of methane (AOM) in the marine subsurface is a significant sink for methane in the environment, yet our understanding of its regulation and dynamics is still incomplete. Relatively few groups of microorganisms consume methane in subsurface environments--namely the anaerobic methanotrophic archaea (ANME clades 1, 2 and 3), which are phylogenetically related to methanogenic archaea. Anaerobic oxidation of methane presumably proceeds via a 'reversed' methanogenic pathway. The ANME are generally associated with sulfate-reducing bacteria (SRB) and sulfate is the only documented final electron acceptor for AOM in marine sediments. Our comparative study explored the coupling of AOM with sulfate reduction (SR) and methane generation (MOG) in microbial communities from Gulf of Mexico cold seep sediments that were naturally enriched with methane and other hydrocarbons. These sediments harbour a variety of ANME clades and SRB. Following enrichment under an atmosphere of methane, AOM fuelled 50-100% of SR, even in sediment slurries containing petroleum-associated hydrocarbons and organic matter. In the presence of methane and sulfate, the investigated microbial communities produce methane at a small fraction ( approximately 10%) of the AOM rate. Anaerobic oxidation of methane, MOG and SR rates decreased significantly with decreasing concentration of methane, and in the presence of the SR inhibitor molybdate, but reacted differently to the MOG inhibitor 2-bromoethanesulfonate (BES). The addition of acetate, a possible breakdown product of petroleum in situ and a potential intermediate in AOM/SR syntrophy, did not suppress AOM activity; rather acetate stimulated microbial activity in oily sediment slurries.