Homoacetogenic bacteria are strict anaerobes capable of autotrophic growth on H2/CO2 or CO, and of heterotrophic growth on a wide range of sugars, alcohols, methoxylated aromatic compounds and one carbon compounds, yielding acetate as their sole metabolic end-product. Batch activity tests on anaerobic granular sludge, using H2/CO2 as a substrate and 2-bromoethanesulfonate (BES) as a specific methanogenic inhibitor revealed that H2/CO2 conversion and concomitant acetate production commenced only after a lag period of 60–100 h. This finding suggests that the homoacetogenic population of digester sludge could be maintained by heterotrophic growth on sugars or other organic compounds, rather than by autotrophic growth on H2/CO2. In the present study, two upflow anaerobic sludge bed (UASB) reactors were operated at 37°C and 55°C for two distinct trial periods, each characterised by the application of influents designed to enrich for homoacetogenic bacteria. Specific primers designed for the amplification of the functional gene encoding formyltetrahydrofolate synthetase (FTHFS), a key enzyme in the acetyl-CoA pathway of acetogenesis, were used as a specific probe for acetogenic bacteria. The diversity of acetogens in the granular sludge cultivated in each reactor was revealed by application of FTHFS targeted PCR. Results show that biomass acetogenic composition was dependent upon the operational temperature of the reactor and the substrate supplied as influent.

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