Bicarbonate-dependent production and methanogenic consumption of acetate in anoxic paddy soil

Abstract

Acetate turnover was measured in slurries of anoxic methanogenic paddy soil after addition of carrier-free [2- 14C]-acetate. Acetate concentrations stayed fairly constant for about 1–2 days indicating steady state between production and consumption reactions. Depending on the experiment, acetate concentrations were between 100 and 3000 μM. Turnover rates were determined from the logarithmic decrease of [2- 14C]-acetate or from the accumulation of acetate in the presence of chloroform resulting in similar values, i.e. 12–13 nmol h −1g −1d.w. soil at 17°C and 36–88 nmol h −1g −1d.w. at 30°C. Acetate consumption was completely inhibited by chloroform. The respiratory index (RI) was < 0.27. Hence, acetate was apparently consumed by methanogenic bacteria. About 80–90% of the CH 4 produced originated from the methyl group of acetate. The role of homoacetogenesis for acetate production was studied by measuring the incorporation of radioactive bicarbonate into acetate. In different experiments, CO 2 incorporation accounted for fractions of 1–60% of the acetate produced, about 10% being the most likely value for steady-state conditions. The fraction increased at high H 2 concentrations and decreased at high acetate concentrations. The rate of H 2 production that was required for chemolithotrophic acetate production from CO 2 was two orders of magnitude higher than the actually measured rate. Hence, most of the CO 2 incorporation into acetate was caused by electron donors other than H 2 (e.g., carbohydrates) and/or by exchange reactions.