Controls and mechanisms of formate turnover in anoxic sediments

Short-chain organic acids (SCOAs) are important intermediates in the anaerobic degradation of organic matter. Yet, their turnover mechanisms in lake sediments are not well understood. We investigate formate turnover in sediments of five freshwater lakes in central Switzerland that differ in trophic status. These show formate oxidation to H2 and HCO3- to be thermodynamically favorable (-8 to -22 kJ (mol of reaction)-1) in natural samples and in batch experiments involving an initial 13C-formate pulse (100 µM). Batch experiments furthermore indicate that formate oxidation is the dominant turnover mechanism and leads to 13C-formate depletion within days. Transfer and inhibition tests show that formate conversion requires presence of metabolically active microbial cells, is O2-sensitive, and does not depend on syntrophy. Genetic analyses based on a new assay targeting the alpha subunit of formate dehydrogenase (fdhA) indicate that an abundant (>108 gene copies g-1 sediment) and diverse community of uncultivated microorganisms is involved in the turnover of formate to H2 and HCO3-. Additional tests involving anoxic marine sediment and digester sludge, oxic river water, and activated sludge, suggest that formate oxidation is the main turnover mechanism in other anoxic, but not in oxic samples.