Methanol as a co-substrate with CO₂ 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 CO₂. Acetate production from CO₂ 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 CO₂ to enhance butyrate production in MES. Feeding with CO₂ 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 CO₂ 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, CO₂ and methanol, to produce mainly butyrate. KEY POINTS: • Butyrate was the main product from methanol and CO₂ in MES • Methanol acted as both carbon and electron source in MES • Eubacterium dominating microbial culture was enriched in MES.