Acetate turnover and methanogenic pathways in Amazonian lake sediments

Conrad, Ralf; Klose, Melanie; Enrich-Prast, Alex

Lake sediments in Amazonia are a significant source of inline-formulaCH4, a potential greenhouse gas. Previous studies of sediments using inline-formula13C analysis found that the contribution of hydrogenotrophic versus acetoclastic methanogenesis to inline-formulaCH4 production was relatively high. Here, we determined the methanogenic pathway in the same sediments (inline-formulan=6) by applying inline-formula14Cbicarbonate or 2-inline-formula14Cacetate and confirmed the high relative contribution (50 %–80 %) of hydrogenotrophic methanogenesis. The respiratory index (RI) of 2-inline-formula14Cacetate, which is inline-formula14CO2 relative to inline-formula14CH4+14CO2, divided the sediments into two categories, i.e., those with an RI < 0.2 consistent with the operation of acetoclastic methanogenesis and those with an RI > 0.4 showing that a large percentage of the acetate-methyl was oxidized to inline-formulaCO2 rather than reduced to inline-formulaCH4. Hence, part of the acetate was probably converted to inline-formulaCO2 plus inline-formulaH2 via syntrophic oxidation, thus enhancing hydrogenotrophic methanogenesis. This happened despite the presence of potentially acetoclastic Methanosaetaceae in all the sediments. Alternatively, acetate may have been oxidized with a constituent of the sediment organic matter (humic acid) serving as oxidant. Indeed, apparent acetate turnover rates were larger than inline-formulaCH4 production rates except in those sediments with a inline-formulaR<0.2. Our study demonstrates that inline-formulaCH4 production in Amazonian lake sediments was not simply caused by a combination of hydrogenotrophic and acetoclastic methanogenesis but probably involved additional acetate turnover.

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Conrad, Ralf / Klose, Melanie / Enrich-Prast, Alex: Acetate turnover and methanogenic pathways in Amazonian lake sediments. 2020. Copernicus Publications.

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