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Schlagwörter:
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Zusammenfassung:
Lake sediments in Amazonia are a significant source of CH4, a potential
greenhouse gas. Previous studies of sediments using C-13 analysis found
that the contribution of hydrogenotrophic versus acetoclastic
methanogenesis to CH4 production was relatively high. Here, we
determined the methanogenic pathway in the same sediments (n = 6) by
applying (14)Cbicarbonate or 2-(14)Cacetate and confirmed the high
relative contribution (50 %-80 %) of hydrogenotrophic methanogenesis.
The respiratory index (RI) of 2-(14)Cacetate, which is (CO2)-C-14
relative to (CH4)-C-14 +(CO2)-C-14, 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 CO2 rather than
reduced to CH4. Hence, part of the acetate was probably converted to CO2
plus H-2 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 CH4 production rates except in those
sediments with a R<0.2. Our study demonstrates that CH4 production in
Amazonian lake sediments was not simply caused by a combination of
hydrogenotrophic and acetoclastic methanogenesis but probably involved
additional acetate turnover.
