Big Soda Lake (Nevada). 3. Pelagic methanogenesis and anaerobic methane oxidation1
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In situ rates of methanogenesis and methane oxidation were measured in meromictic Big Soda Lake. Methane production was measured by the accumulation of methane in the headspaces of anaerobically sealed water samples; radiotracer was used to follow methane oxidation. Nearly all the methane oxidation occurred in the anoxic zones of the lake. Rates of anaerobic oxidation exceeded production at all depths studied in both the mixolimnion (2–6 vs. 0.1–1 nmol liter −1 d −1 ) and monimolimnion (49–85 vs. 1.6–12 nmol liter −1 d −1 ) of the lake. Thus, a net consumption of methane equivalent to 1.36 mmol m −2 d −1 occurred in the anoxic water column. Anaerobic methane oxidation had a first‐order rate constant of 8.1±0.5 × 10 −4 d −1 , and activity was eliminated by filter sterilization. However, in situ methane oxidation was of insufficient magnitude to cause a noticeable decrease of ambient dissolved methane levels over an incubation period of 97 h.Anaerobic methane oxidation is a globally important but poorly understood process. Four lines of evidence have recently improved our understanding of this process. First, studies of recent marine sediments indicate that a consortium of methanogens and sulphate-reducing bacteria are responsible for anaerobic methane oxidation; a mechanism of 'reverse methanogenesis' was proposed, based on the principle of interspecies hydrogen transfer. Second, studies of known methanogens under low hydrogen and high methane conditions were unable to induce methane oxidation, indicating that 'reverse methanogenesis' is not a widespread process in methanogens. Third, lipid biomarker studies detected isotopically depleted archaeal and bacterial biomarkers from marine methane vents, and indicate that Archaea are the primary consumers of methane. Finally, phylogenetic studies indicate that only specific groups of Archaea and SRB are involved in methane oxidation. This review integrates results from these recent studies to constrain the responsible mechanisms.
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Light-Dependent Aerobic Methane Oxidation Reduces Methane Emissions from Seasonally Stratified Lakes
Lakes are a natural source of methane to the atmosphere and contribute significantly to total emissions compared to the oceans. Controls on methane emissions from lake surfaces, particularly biotic processes within anoxic hypolimnia, are only partially understood. Here we investigated biological methane oxidation in the water column of the seasonally stratified Lake Rotsee. A zone of methane oxidation extending from the oxic/anoxic interface into anoxic waters was identified by chemical profiling of oxygen, methane and δ13C of methane. Incubation experiments with 13C-methane yielded highest oxidation rates within the oxycline, and comparable rates were measured in anoxic waters. Despite predominantly anoxic conditions within the zone of methane oxidation, known groups of anaerobic methanotrophic archaea were conspicuously absent. Instead, aerobic gammaproteobacterial methanotrophs were identified as the active methane oxidizers. In addition, continuous oxidation and maximum rates always occurred under light conditions. These findings, along with the detection of chlorophyll a, suggest that aerobic methane oxidation is tightly coupled to light-dependent photosynthetic oxygen production both at the oxycline and in the anoxic bottom layer. It is likely that this interaction between oxygenic phototrophs and aerobic methanotrophs represents a widespread mechanism by which methane is oxidized in lake water, thus diminishing its release into the atmosphere.
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Rates of CH4 production in slurries of anoxic Italian paddy soils were higher when incubated without agitation than with shaking or stirring. Stirring resulted in a drastically reduced transformation of [2-14C]acetate to 14CH4 and increased the relative contribution of CH4 production from H14CO3− to total methanogenesis. Numbers of acetotrophic methanogens were low (103 g−1 dry soil) in stirred slurries. An anoxic suspension of sterile sand which was amended with Methanosarcina barkeri and acetate produced only CH4 if it was not stirred. In stirred anoxic paddy soil, acetate accumulated to very high concentrations (<10 mM). Propionate, butyrate and/or isopropanol also increased in stirred slurries. Hydrogen partial pressures, on the other hand, reached in all treatments a similar value of about 3–5 Pa. However, H2 production was apparently inhibited by stirring, since H2 accumulated only if slurries in which methanogenesis was inhibited by chloroform were not stirred. Our results indicate that measurements of metabolic rates in anoxic paddy soil are better conducted in non-agitated incubations to avoid the potential destruction of acetotrophic methanogens, syntrophic microbial associations and other microorganisms that are sensitive to mechanical forces.
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Rates of CH4 production in slurries of anoxic Italian paddy soils were higher when incubated without agitation than with shaking or stirring. Stirring resulted in a drastically reduced transformation of [2-14C]acetate to 14CH4 and increased the relative contribution of CH4 production from H14CO3− to total methanogenesis. Numbers of acetotrophic methanogens were low (103 g−1 dry soil) in stirred slurries. An anoxic suspension of sterile sand which was amended with Methanosarcina barkeri and acetate produced only CH4 if it was not stirred. In stirred anoxic paddy soil, acetate accumulated to very high concentrations (<10 mM). Propionate, butyrate and/or isopropanol also increased in stirred slurries. Hydrogen partial pressures, on the other hand, reached in all treatments a similar value of about 3–5 Pa. However, H2 production was apparently inhibited by stirring, since H2 accumulated only if slurries in which methanogenesis was inhibited by chloroform were not stirred. Our results indicate that measurements of metabolic rates in anoxic paddy soil are better conducted in non-agitated incubations to avoid the potential destruction of acetotrophic methanogens, syntrophic microbial associations and other microorganisms that are sensitive to mechanical forces.
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