Potential contribution of acetogenesis to anaerobic degradation in methanogenic rice field soils

Acetate is an important intermediate in the anaerobic degradation of organic matter. It is not only produced by fermentation but also by the reduction of CO2 via the acetyl-CoA pathway (acetogenesis). However, the interplay of this process in methanogenic rice field soils is not fully understood. Chemolithotrophic acetogenesis results in a rather strong depletion of the 13C of acetate. Therefore, we measured the δ13C of acetate, CO2 and CH4 that were produced during methanogenic degradation of organic matter in rice paddy soils from two geographical origins (Philippines and Italy) and used three different strategies to estimate the contribution of acetogenesis to acetate formation: (1) Incubation of soil slurries under elevated concentrations of H2/CO2 to specifically activate the H2-dependent communities; (2) incubation at three different temperatures (15, 30, 50 °C) to shift the conditions for H2 consumption; (3) incubation in the presence of inhibitors presumed to inhibit acetogenesis (KCN) or methanogenesis (BES). Only incubations under elevated H2/CO2 resulted in13C-depleted acetate (δ13C of −68 to −65‰) compared to the control (δ13C of −25‰). Temperature and presence of inhibitors also affected the δ13C of acetate, CO2 and/or CH4, but δ13C of acetate was never as low as after addition H2/CO2. A significant 13C enrichment of acetate at 15 °C in presence of BES and KCN indicated that H2-dependent acetogenesis is a favoured process at low temperature. Copy numbers in the Philippine soil of the fhs gene coding for the formyl-tetrahydrofolate synthetase of the acetyl-CoA pathway were on a similar order of magnitude (106 per gram dry soil) irrespectively of the different incubation conditions. Our results indicate that chemolithotrophic acetogenesis was operative in methanogenic rice soil at 15 °C but was more important at elevated H2/CO2 concentrations.