Methanol as a co-substrate with CO 2 enhances butyrate production in microbial electrosynthesis.

Methanol is a promising feedstock for the bio-based economy as it can be derived from organic waste streams or produced electrochemically from CO 2 . Acetate production from CO 2 in microbial electrosynthesis (MES) has been widely studied, while more valuable compounds such as butyrate are currently attracting attention. In this study, methanol was used as a co-substrate with CO 2 to enhance butyrate production in MES. Feeding with CO 2 and methanol resulted in the highest butyrate production rates and titres of 0.36 ± 0.01 g L -1 d -1 and 8.6 ± 0.2 g L -1 , respectively, outperforming reactors with only CO 2 feeding (0.20 ± 0.03 g L -1 d -1 and 5.2 ± 0.1 g L -1 , respectively). Methanol acted as electron donor and as carbon source, both of which contributed ca. 50% of the carbon in the products. Eubacterium was the dominant genus with 52.6 ± 2.5% relative abundance. Thus, we demonstrate attractive route for the use of the C1 substrates, CO 2 and methanol, to produce mainly butyrate. KEY POINTS: • Butyrate was the main product from methanol and CO 2 in MES • Methanol acted as both carbon and electron source in MES • Eubacterium dominating microbial culture was enriched in MES.