Fractionation of stable carbon isotopes during formate consumption in anoxic rice paddy soils and lake sediments

Abstract. Formate is energetically equivalent to hydrogen and thus, is an important intermediate during the breakdown of organic matter in anoxic rice paddy 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 mainly converted to acetate both under methanogenic and sulfidogenic conditions. Methane was only a minor product and was mainly formed from the acetate. 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. The isotopic enrichment factors (ε_form) of formate consumption, determined by Mariotti plots, were in the low range of -8 ‰ to -2.5 ‰ when sulfate was absent and formate was mainly converted to acetate and CH₄. However, no enrichment factor was detectable when formate was degraded with sulfate to mainly CO₂. The δ¹³C of acetate was by about 25–50 ‰ more negative than that of formate indicating acetate production by chemolithotrophic homoacetogenesis. Hence, formate seems to be an excellent substrate for homoacetogenesis in anoxic soils and sediments, so that this process is competing well with methanogenesis and sulfate reduction.