Methanol as a co-substrate with CO2 enhances butyrate production in microbial electrosynthesis

Abstract

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.

Keywords: Butyrate; CO2 utilisation; Electron donor; Methanol utilisation; Microbial electrosynthesis.

Fig. 1

Production of VFAs at the end of each enrichment batch. Error bars indicate the standard deviations in duplicate reactors

Fig. 2

Concentrations of VFAs in the co-substrate addition experiments with a the overview of all detected VFAs (acetate and butyrate) and b the acetate production. ME-CD (fed with methanol and CO₂); ME (fed with only methanol); CD (fed with only CO₂). Error bars are the standard deviations in duplicate reactors

Fig. 3

Cathodic pH (a), H₂ production rates (b), and cathodic potential (c) during the co-substrate addition experiments. ME-CD (fed with methanol and CO₂); ME (fed with only methanol); CD (fed with only CO₂). Error bars indicate the standard deviations in duplicate reactors

Fig. 4

Substrate consumption and VFA production with different carbon sources. Figures a, b, and c represent the reactors fed with CO₂ + methanol (ME-CD), methanol (ME), and CO₂ (CD), respectively. Error bars indicate the standard deviations in duplicate reactors

Fig. 5

Electrons (a) and carbons (b) present in the inputs and in the products. Green blocks represent the electrons/carbon provided by overall consumption of methanol, grey blocks the carbon provided by consumed CO₂, and orange blocks the electrons provided by electrode. ME-CD (fed with methanol and CO₂); ME (fed with only methanol); CD (fed with only CO₂). The percentages of carbon or electron recovery in H₂ or butyrate are shown the figures. The electron recoveries (a) in acetate were 3.8 ± 0.5% (ME-CD) and 1.7 ± 0.9% (CD). The carbon recoveries (b) in acetate were 15.5 ± 0.5% (ME-CD) and 5.5 ± 2.8% (CD). Error bars indicate the standard deviations in duplicate reactors

Fig. 6

Heat map and the relative abundance of the ten most dominant genera fed with CO₂ (CD), methanol (ME), and CO₂ + methanol (ME-CD) in the co-substrate experiment (B, biofilms; P, planktonic cells, 1 and 2 represent the microbial culture from each duplicate). f__Ruminococceae is an unknown genus belonging in the family Ruminococceae. Inoculum represent the average of five enrichment batches (samples were taken from the planktonic cells at the end of each enrichment culture, error bars are not shown in the heatmap)