Abstract
Abstract. Acetate is an important intermediate during the degradation of organic matter in anoxic flooded soils and sediments. Acetate is disproportionated to CH4 and CO2 by methanogenic or is oxidized to CO2 by sulfate-reducing microorganisms. These reactions result in carbon isotope fractionation, depending on the microbial species and their particular carbon metabolism. To learn more about the magnitude of the isotopic enrichment factors (ε) involved, acetate conversion to CH4 and CO2 was measured in anoxic paddy soils from Vercelli (Italy) and the International Rice Research Institute (IRRI, the Philippines) and in anoxic lake sediments from the northeastern and the southwestern basins of Lake Fuchskuhle (Germany). Acetate consumption was measured using samples of paddy soil or lake sediment suspended in water or in phosphate buffer (pH 7.0), both in the absence and presence of sulfate (gypsum), and of methyl fluoride (CH3F), an inhibitor of aceticlastic methanogenesis. Under methanogenic conditions, values of εac for acetate consumption were always in a range of −21 ‰ to −17 ‰ but higher in the lake sediment from the southwestern basin (−11 ‰). Under sulfidogenic conditions εac values tended to be slightly lower (−26 ‰ to −19 ‰), especially when aceticlastic methanogenesis was inhibited. Again, εac in the lake sediment of the southwestern basin was higher (−18 ‰ to −14 ‰). Determination of εCH4 from the accumulation of 13C in CH4 resulted in much lower values (−37 ‰ to −27 ‰) than from the depletion of 13C in acetate (−21 ‰ to −17 ‰ ), especially when acetate degradation was measured in buffer suspensions. The microbial communities were characterized by sequencing the bacterial 16S rRNA (ribosomal ribonucleic acid) genes as well as the methanogenic mcrA and sulfidogenic dsrB genes. The microbial communities were quite different between lake sediments and paddy soils but were similar in the sediments of the two lake basins and in the soils from Vercelli and the IRRI, and they were similar after preincubation without and with addition of sulfate (gypsum). The different microbial compositions could hardly serve for the prediction of the magnitude of enrichment factors.
Highlights
Organic matter degradation under anaerobic conditions results in the production of CO2, when electron acceptors such sulfate are available, and in the production of CH4 and CO2, when they are absent
Acetate consumption was measured using samples of paddy soil or lake sediment suspended in water or in phosphate buffer, both in the absence and presence of sulfate, and of methyl fluoride (CH3F), an inhibitor of aceticlastic methanogenesis
Incubation of unbuffered suspensions of rice field soil from the International Rice Research Institute (IRRI) in the Philippines with acetate under anoxic conditions resulted in the depletion of acetate and the release of
Summary
Organic matter degradation under anaerobic conditions results in the production of CO2, when electron acceptors such sulfate (sulfidogenic conditions) are available, and in the production of CH4 and CO2, when they are absent (methanogenic conditions). Carbon dioxide is an end product but can serve as an intermediate It can for example be converted by chemolithotrophic microorganisms to CH4 or to acetate. These conversion reactions of CO2 have an isotope effect and result in products that are strongly depleted in 13C, expressing isotope enrichment factors (ε) being on the order of −70 ‰ to −50 ‰ (Blaser and Conrad, 2016). The conversion of acetate to CO2 by sulfidogenic reactions or to CH4 plus CO2 by methanogenic reactions can have quite substantial enrichment factors, often being on the order of about −20 ‰ (Goevert and Conrad, 2009, 2010)
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