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 C-13 of acetate. Therefore, we measured the delta C-13 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 H-2/CO2 to specifically activate the H-2-dependent communities; (2) incubation at three different temperatures (15, 30, 50 degrees C) to shift the conditions for H-2 consumption; (3) incubation in the presence of inhibitors presumed to inhibit acetogenesis (KCN) or methanogenesis (BES). Only incubations under elevated H-2/CO2 resulted in(13)C-depleted acetate (delta C-13 of -68 to -65 parts per thousand) compared to the control (delta C-13 of -25 parts per thousand). Temperature and presence of inhibitors also affected the delta C-13 of acetate, CO2 and/or CH4, but delta C-13 of acetate was never as low as after addition H-2/CO2. A significant C-13 enrichment of acetate at 15 degrees C in presence of BES and KCN indicated that HZ-dependent acetogenesis is a favoured process at low temperature. Copy numbers in the Philippine soil of the flu gene coding for the formyltetrahydrofolate synthetase of the acetyl-CoA pathway were on a similar order of magnitude (10(6) per gram dry soil) irrespectively of the different incubation conditions. Our results indicate that chemolithotrophic acetogenesis was operative in methanogenic rice soil at 15 degrees C but was more important at elevated H-2/CO2 concentrations.