Controlling the microbial competition between hydrogenotrophic methanogens and homoacetogens using mass transfer and thermodynamic constraints

The reduction of CO₂ allows the synthesis of platform molecules for the chemical and energy industry. Anaerobic microbial consortia contain homoacetogenic microorganisms (HAC) capable of reducing CO₂ to acetate. However, one of the obstacles to their use is the understanding and management of their functional diversity. In particular, managing the competition between HAC and hydrogenotrophic methanogens (HM) that convert CO₂ into methane is crucial to selectively produce acetate.

This study contributes to bring new knowledge on the competition between HAC and HM. This microbial competition is encountered in numerous anaerobic systems, and it is necessary to know how to manage it. In this sense, mass transfer between the gas phase where the substrates are located, and the liquid phase which contains the microbial catalysts, as well as kinetic and thermodynamic aspects of biological reactions have been integrated in this work. The microbial competition between HM and HAC was studied in successive batches. The effect of temperature between 25 °C and 35 °C was investigated, as well as different states of mass transfer limitation in the system.

A clear effect of temperature between 25 °C and 35 °C on the outcome of the competition between HM and HAC was highlighted, as well as the effect of mass transfer limitation. Enrichment of Acetobacterium homoacetogens and elimination of hydrogenotrophic methanogens was possible at 25 °C without mass transfer limitation. Acetate product selectivity was of 100% by the end of the enrichment period in successive batches. On the contrary, under mass transfer limitation, and/or at 35 °C, Methanobacterium hydrogenotrophic methanogens were promoted with 100% of methane selectivity by the end of the enrichment. This study contributes to identify specific process parameters influencing the selection of HAC over HM and should help in the design of experiments depending on the target product from H₂/CO₂. Furthermore, the results obtained could be applied to continuous acetate producing systems, at a larger scale, and they can also be useful in other gas fermentation systems and processes.