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Anaerobic digestion is a widely applied technology for sewage sludge treatment. Hydrogen and CO 2  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 /CO 2 , sludge samples were incubated under H 2 /CO 2  headspace at different temperatures, and analyzed with respect to turnover of H 2 , CO 2 , CH 4  and acetate including their δ 13 C values. At 15°C,  13 C-depleted acetate (δ 13 C of −41 to −43‰) and transient acetate accumulation were observed under H 2 /CO 2 , and CH 4  accumulated with δ 13 C values increasing from −53 to −33‰. The copy numbers of the  fhs  gene, which is characteristic for acetogenic bacteria, were at 15°C one order of magnitude higher in the H 2 /CO 2  incubations than the N 2  control. At 30°C, however, acetate did not accumulate in the H 2 /CO 2  incubation and the δ 13 C of CH 4  was very low (−100 to −77‰). At 50°C, isotopically enriched acetate was transiently formed and subsequently consumed followed by the production of  13 C-depleted CH 4 . Collectively, the results indicate a high contribution of chemolithotrophic acetogenesis to H 2 /CO 2  utilization at 15°C and 50°C, while H 2 /CO 2  was mainly consumed by hydrogenotrophic methanogenesis at 30°C. Fermentative production and methanogenic consumption of acetate were active at 50°C.

frontiers in microbiology

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