Acetate turnover and methanogenic pathways in Amazonian lake sediments

• Published in: Biogeosciences
• Main Authors: R. Conrad, M. Klose, A. Enrich-Prast
• Format: Article
• Language: English
• Published: Copernicus Publications 2020-02-01
• Online Access: https://www.biogeosciences.net/17/1063/2020/bg-17-1063-2020.pdf
• ISSN: 1726-4170; 1726-4189

<p>Lake sediments in Amazonia are a significant source of <span class="inline-formula">CH₄</span>, a potential greenhouse gas. Previous studies of sediments using <span class="inline-formula">¹³C</span> analysis found that the contribution of hydrogenotrophic versus acetoclastic methanogenesis to <span class="inline-formula">CH₄</span> production was relatively high. Here, we determined the methanogenic pathway in the same sediments (<span class="inline-formula"><i>n</i>=6</span>) by applying <span class="inline-formula">¹⁴C</span>bicarbonate or 2-<span class="inline-formula">¹⁴C</span>acetate and confirmed the high relative contribution (50&thinsp;%–80&thinsp;%) of hydrogenotrophic methanogenesis. The respiratory index (RI) of 2-<span class="inline-formula">¹⁴C</span>acetate, which is <span class="inline-formula">¹⁴CO₂</span> relative to <span class="inline-formula">¹⁴CH₄+¹⁴CO₂</span>, divided the sediments into two categories, i.e., those with an RI&thinsp;&lt;&thinsp;0.2 consistent with the operation of acetoclastic methanogenesis and those with an RI&thinsp;&gt;&thinsp;0.4 showing that a large percentage of the acetate-methyl was oxidized to <span class="inline-formula">CO₂</span> rather than reduced to <span class="inline-formula">CH₄</span>. Hence, part of the acetate was probably converted to <span class="inline-formula">CO₂</span> plus <span class="inline-formula">H₂</span> 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 <span class="inline-formula">CH₄</span> production rates except in those sediments with a <span class="inline-formula"><i>R</i><i>&lt;</i>0.2</span>. Our study demonstrates that <span class="inline-formula">CH₄</span> production in Amazonian lake sediments was not simply caused by a combination of hydrogenotrophic and acetoclastic methanogenesis but probably involved additional acetate turnover.</p>