Evaluation of the influence of environmental conditions on acetate production from syngas by an Acetobacterium woodii-dominated consortium

The bioconversion of syngas by acetogenic bacteria, with acetate as the main product, is a promising route for the valorisation of recalcitrant organic waste. This work evaluated (i) the impact of temperature and pH on acetate production by an Acetobacterium woodii -dominated consortium in gas-tight serum bottles and (ii) the long-term operation in a bubble column bioreactor with a continuous syngas feed (10 mL min -1 ). The initial results showed that temperature and pH governed carbon distribution between biomass growth and acetate synthesis, with 30ºC and a pH range of 6.3 – 6.8 being the optimal values to maximize acetate production (Y P/S ≈ 1 g acetate g -1 syngas), while pH 5.8 maximized biomass synthesis. When long-term operation was evaluated, the results showed CO was primarily used as carbon and energy source by the mixed culture regardless the pH, while H 2 consumption was negligible. Despite a pH of 6.3 supported the highest acetate concentration (14.2 g L -1 ), this condition favoured Clostridium magnum and Clostridium kluyveri growth, leading to the synthesis of butyrate and ethanol, and reducing acetate selectivity (65%). Conversely, a pH of 6.8 favoured the dominance of A. woodii and a comparable acetate accumulation (13.7 g L -1 ), but with a 100% acetate selectivity and CO removals up to 93.4%. Thus, this condition was identified as the most favourable for selective acetate production, where the high acetate production and the use of syngas as sole carbon and energy source showed the potential of this mixed culture compared to A. woodii strain. • Optimal enrichment conditions for acetate production were 30º C and pH 6.3 – 6.8. • Acetate yields up to 1.1 g acetate g -1 syngas were reached. • Bioreactor tests showed high CO removals but negligible H 2 consumption. • pH 6.3 increased acetate accumulation, but also butyrate and ethanol production. • pH 6.8 favoured A. woodii growth and a selective acetate production.