Stable carbon isotope biogeochemistry of propionate and acetate in methanogenic soils and lake sediments

In anoxic environments, degradation of organic matter (OM) results in strong fractionation of carbon isotopes, with formation of ¹³C-depleted CH₄. Propionate and acetate are important products of OM fermentation. Propionate is further fermented to acetate. Acetate is a direct precursor of CH₄, the remainder usually being produced from H₂-mediated CO₂ reduction. There is a paucity of data for the turnover of acetate and, even more so, propionate. We therefore analyzed the δ¹³C values of organic carbon, propionate, acetate and the methyl (Me) group of acetate (acetate-Me) during the production of CH₄ in anoxic incubations of various flooded and non-flooded soils and various lake sediments. Incubation in the presence of CH₃F, which inhibits CH₄ production from acetate, allowed exclusion of isotope effects during aceticlastic methanogenesis. Despite the variation inherent in the wide diversity of sample type and origin, the data collectively showed that the δ¹³C value of acetate was only marginally different (-2±5‰) from that of OM, while propionate was depleted in ¹³C relative to total acetate (-6±5‰). Acetate-Me was generally depleted in ¹³C relative to total acetate (-8±5‰). Thus, isotopic enrichment factors during the degradation of OM to total propionate and acetate were much smaller than those during hydrogenotrophic and aceticlastic methanogenesis or the intramolecular difference in δ¹³C between the carboxyl (CO₂H) and Me of acetate, so that the δ¹³C value of OM may be used as a proxy when data for acetate are not available.