Competition Between Chemolithotrophic Acetogenesis and Hydrogenotrophic Methanogenesis for Exogenous H2/CO2 in Anaerobically Digested Sludge: Impact of Temperature

Anaerobic digestion is a widely applied technology for sewage sludge treatment. Hydrogen and CO2 are important degradation products, which serve as substrates for both hydrogenotrophic methanogenesis and chemolithotrophic acetogenesis. In order to understand the competition between these processes for H-2/CO2, sludge samples were incubated under H-2/CO2 headspace at different temperatures, and analyzed with respect to turnover of H-2, CO2, CH4 and acetate including their delta C-13 values. At 15 degrees C, C-13-depleted acetate (delta C-13 of -41 to -43 parts per thousand) and transient acetate accumulation were observed under H-2/CO2, and CH4 accumulated with delta C-13 values increasing from -53 to -33 parts per thousand. The copy numbers of the fhs gene, which is characteristic for acetogenic bacteria, were at 15 degrees C one order of magnitude higher in the H-2/CO2 incubations than the N-2 control. At 30 degrees C, however, acetate did not accumulate in the H-2/CO2 incubation and the delta C-13 of CH4 was very low (-100 to -77 parts per thousand). At 50 degrees C, isotopically enriched acetate was transiently formed and subsequently consumed followed by the production of C-13-depleted CH4. Collectively, the results indicate a high contribution of chemolithotrophic acetogenesis to H-2/CO2 utilization at 15 degrees C and 50 degrees C, while H-2/CO2 was mainly consumed by hydrogenotrophic methanogenesis at 30 degrees C. Fermentative production and methanogenic consumption of acetate were active at 50 degrees C.