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Abstract The ongoing global temperature rise enhances permafrost thaw in the Arctic, allowing Pleistocene‐aged frozen soil organic matter to become available for microbial degradation and production of greenhouse gases, particularly methane. Here, we examined the extent and mechanism of anaerobic oxidation of methane (AOM) in the sediments of four interior Alaska thermokarst lakes, which formed and continue to expand as a result of ice‐rich permafrost thaw. In cores of surface (~ 1 m) lake sediments we quantified methane production (methanogenesis) and AOM rates using anaerobic incubation experiments in low (4°C) and high (16°C) temperatures. Methanogenesis rates were measured by the accumulation of methane over ~ 90 d, whereas AOM rates were measured by adding labeled‐13CH4and measuring the produced dissolved inorganic13C. Our results demonstrate that while methanogenesis was vigorous in these anoxic sediments, AOM was lower by two orders of magnitude. In almost all sediment depths and temperatures, AOM rates remained less than 2% of the methanogenesis rates. Experimental evidence indicates that the AOM is strongly related to methanogens, as the addition of a methanogens' inhibitor prevented AOM. Variety of electron acceptor additions did not stimulate AOM, and methanotrophs were scarcely detected. These observations suggest that the AOM signals in the incubation experiments might be a result of enzymatic reversibility (“back‐flux”) during CH4production, rather than thermodynamically favorable AOM. Regardless of the mechanism, the quantitative results show that near surface (< 1‐m) thermokarst sediments in interior Alaska have little to no buffer mechanisms capable of attenuating methane production in a warming climate.
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Microbial methanogenesis fueled by freshwater infiltration and oil biodegradation in the Siljan impact structure, Sweden
Abstract Deeply fractured rocks of meteorite impact craters are suggested as prime niches for subsurface microbial colonization. Methane can be a product of such microbial communities and seeps of methane from impact craters on Earth are of strong interest as they act as analogs for Mars. Previous studies report signs of ancient microbial methanogenesis in the Devonian Siljan meteorite impact structure in Sweden, but the proportion of microbial methane, metabolic pathways, and potential modern activity remain elusive. In this study, gas composition, hydrochemistry, oil organic geochemistry, and microbial community analyses are reported in 400 m deep fractures of the Siljan impact structure. The results showed a dominantly microbial origin for methane, which was supported by highly negative δ13CCH4and positive δ13CCO2values along with multiply substituted isotopologues (Δ13CH3D) that indicated disequilibrium fractionation due to microbial kinetic isotope effects. The presence of C2to C5hydrocarbons suggested a minor thermogenic input in the gas mix. Characterization of the microbial community via 16S rRNA gene amplicon sequencing and real-time PCR indicated a low abundance of several methanogenic archaeal populations, which is common for settings with active methanogenesis. Evidence of oil biodegradation suggested that secondary microbial hydrocarbon utilization was involved in the methanogenesis. Low sulfate and high alkalinity in the groundwaters also suggested a dominantly microbial methane formation driven by infiltration of freshwater that was coupled to sulfate reduction and secondary utilization of early mature thermogenic hydrocarbons.
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- Award ID(s):
- 2120733
- PAR ID:
- 10564555
- Publisher / Repository:
- Springer Science + Business Media
- Date Published:
- Journal Name:
- Discover Applied Sciences
- Volume:
- 7
- Issue:
- 1
- ISSN:
- 3004-9261
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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