Carboxylates and alcohols production in an autotrophic hydrogen-based membrane biofilm reactor

Microbiological conversion of CO2 into biofuels and/or organic industrial feedstock is an excellent carbon-cycling strategy. Here, autotrophic anaerobic bacteria in the membrane biofilm reactor (MBfR) transferred electrons from hydrogen gas (H-2) to inorganic carbon (IC) and produced organic acids and alcohols. We systematically varied the H-2-delivery, the IC concentration, and the hydraulic retention time in the MBfR. The relative availability of H-2 versus IC was the determining factor for enabling microbial chain elongation (MCE). When the H-2:IC mole ratio was high (>2.0 mol H-2/mol C), MCE was an important process, generating medium-chain carboxylates up to octanoate (C8, 9.1 +/- 1.3 mM C and 28.1 +/- 4.1 mmol C m(-2) d(-1)). Conversely, products with two carbons were the only ones present when the H-2:IC ratio was low (<2.0 mol H-2/mol C), so that H-2 was the limiting factor. The biofilm microbial community was enriched in phylotypes most similar to the well-known acetogen Acetobacterium for all conditions tested, but phylotypes closely related with families capable of MCE (e.g., Bacteroidales, Rhodocyclaceae, Alcaligenaceae, Thermoanaerobacteriales, and Erysipelotrichaceae) became important when the H-2:IC ratio was high. Thus, proper management of IC availability and H-2 supply allowed control over community structure and function, reflected by the chain length of the carboxylates and alcohols produced in the MBfR.

Automated summary

Microbiological conversion of CO2 into biofuels and/or organic industrial feedstock using autotrophic anaerobic bacteria in membrane biofilm reactor has shown that controlling the ratio of H-2 to IC influences microbial chain elongation process and the types and quantities of products produced. Proper management of IC availability and H-2 supply allows control over community structure and function, affecting the chain length of the carboxylates and alcohols produced.