Formate consumption and stable carbon isotope fractionation in anoxic rice field soils and lake sediments

Formate is energetically equivalent to hydrogen and thus, is an important intermediate during the breakdown of organic matter in anoxic rice field soils and lake sediments. Formate is a common substrate for methanogenesis, homoacetogenesis and sulfate reduction. However, how much these processes contribute to formate degradation and fractionate carbon stable isotopes is largely unknown. Therefore, we measured the conversion of formate to acetate, CH₄ and CO₂ and the δ¹³C of these compounds in samples of paddy soils from Vercelli (Italy) and the International Rice Research Institute (IRRI, the Philippines) and of sediments from the NE and SW basins of Lake Fuchskuhle (Germany). The samples were suspended 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. In the paddy soils, formate was found to be an excellent substrate for acetate formation, while CH₄ was mainly produced from acetate. Acetate was also produced in the presence of sulfate. The produced acetate was strongly depleted in ¹³C relative to formate (about -50‰ to-25‰), but the consumption of formate itself displayed only a small isotope enrichment factor on the order of -8‰ to -6‰. Therefore, it is likely that formate was disproportionated to ¹³C-depleted acetate and ¹³C-enriched CO₂. The δ¹³C of CO₂ was indeed slightly higher than that of formate. Acetate was most likely produced by homoacetogenesis via the Wood-Ljungdahl pathway. Methane was only a minor product and was mainly produced from the acetate as its production was inhibited by CH₃F. The homoactogenic bacteria in the paddy soils apparently competed well with both methanogenic and sulfate-reducing microorganisms, when formate was the substrate. In the lake sediments, the product spectrum was similar, but only under methanogenic conditions. In the presence of sulfate, however, acetate and CH₄ were only minor products and no enrichment factor was detectable when formate was degraded to mainly CO₂. Hence, homoacetogenesis was the major anaerobic degradation pathway of formate. Formate-dependent methanogenesis was negligible, and sulfate-dependent oxidation was only operative in the lake sediments but not in the paddy soils.