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Lake Fryxell, McMurdo Dry Valleys, Antarctica contains a constantly cold water column and perennial icecover. Although carbon and sulfur cycling in this amictic lake have been studied previously, a paired investigation of 16S rRNA gene based microbial diversity and geochemistry of Lake Fryxell is lacking. Here, we used a combination of radiotracer-based rate measurements, geochemical measurements, and molecular microbial community analysis to investigate the anaerobic oxidation of methane (AOM) and associated processes in Lake Fryxell. The results show that while AOM and sulfate reduction appear coupled in the upper regions of the anoxic water column, in deep anoxic waters, where AOM rates are highest, sulfate is unlikely to be the electron acceptor for AOM. Despite significant rates of AOM in these waters, no putative AOM-associated Archaea or Bacteria were observed. Due to a lack of documented AOM electron acceptors and putative ANMEs, we suggest novel modes of AOM dominate in this extreme environment. First, the notable abundance of the bacterial genus Dehalococcoides suggests that reductive dehalogenation could fuel AOM. Further, taxa of the candidate phylum OP9, the Atribacteria, and the Bathyarchaeota (formerly known as the Miscellaneous Crenarchaeotal Group) both commonly observed at cold methane-seeps globally, may mediate AOM, possibly using humic acids as electron shuttles, in Lake Fryxell.Studying the microbial ecology and geochemistry of extreme environments provides an opportunity to discover novel geochemical pathways and microorganisms. Of particular interest are globally important geochemical processes, especially those that are well described from other environments but may exhibit novel modes under extreme conditions (Antranikian et al. 2005). The perennially ice-covered lakes of the McMurdo Dry Valleys, Antarctica, are wellstudied extreme environments. Lake Fryxell, which lies in the Taylor Valley, is a relatively shallow (20 m maximum depth) lake carved out by glacial erosion. The water column of Lake Fryxell remains constantly cold and lies beneath 4-7 m of perennial ice cover. Lake water lost through ice ablation is replaced from seasonal melt waters flowing in from the adjacent Canada Glacier.As a result of the permanent ice cover and low inflow, Lake Fryxell is limnologically amictic (Hutchinson and L€ offler 1956;Boehrer and Schultze 2008). The lake has a very stable water column; the movement of dissolved constituents is governed solely by molecular diffusion, creating steep geochemical gradients and stratified microbial populations (Horowitz et al. 1972;Howes et al. 1992;Smith et al. 1993). Molecular oxygen (O 2 ) is supersaturated in the upper waters of Lake Fryxell as a result of oxygenic photosynthesis by phytoplankton but is consumed deeper in the water column and is undetectable (< 0.2 mg/L) below 9 m depth. Hydrogen sulfide (H 2 S) is produced in the anoxic waters and sediments of Lake Fryxell and significant amounts of methane (CH 4 ) diffuse from the sediments (Fig. 1, Lawrence

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Despite most lakes in the Arctic being perennially or seasonally frozen for at least 40% of the year, little is known about microbial communities and nutrient cycling under ice cover. We assessed the vertical microbial community distribution and geochemical composition in early spring under ice in a seasonally ice-covered lake in southwest Greenland using amplicon-based sequencing that targeted 16S rRNA genes and using a combination of field and laboratory aqueous geochemical methods. Microbial communities changed consistently with changes in geochemistry. Composition of the abundant members responded strongly to redox conditions, shifting downward from a predominantly heterotrophic aerobic community in the suboxic waters to a heterotrophic anaerobic community in the anoxic waters. Operational taxonomic units (OTUs) of Sporichthyaceae, Comamonadaceae, and the SAR11 Clade had higher relative abundances above the oxycline and OTUs within the genus Methylobacter, the phylum Lentisphaerae, and purple sulfur bacteria (PSB) below the oxycline. Notably, a 13-fold increase in sulfide at the oxycline was reflected in an increase and change in community composition of potential sulfur oxidizers. Purple non-sulfur bacteria were present above the oxycline and green sulfur bacteria and PSB coexisted below the oxycline, however, PSB were most abundant. For the first time we show the importance of PSB as potential sulfur oxidizers in an Arctic dimictic lake.