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.