Deep underground microbes are generating methane in Swedish impact crater
Scientists have identified active microbial communities producing methane in fractured rocks 400 meters below Sweden's Siljan impact crater, offering new insights into how life survives in extreme subsurface environments. The findings have implications for understanding potential microbial activity on Mars and for assessing methane emissions from similar geological structures on Earth.
Originaltitel: Microbial methanogenesis fueled by freshwater infiltration and oil biodegradation in the Siljan impact structure, Sweden
<p>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 δ<sup>13</sup>C<sub>CH4</sub> and positive δ<sup>13</sup>C<sub>CO2</sub> values along with multiply substituted isotopologues (Δ<sup>13</sup>CH<sub>3</sub>D) that indicated disequilibrium fractionation due to microbial kinetic isotope effects. The presence of C<sub>2</sub> to C<sub>5</sub> hydrocarbons 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.</p>