Methanol as an alternative electron donor in chain elongation for butyrate and caproate formation

Chain elongation is an emerging mixed culture biotechnology converting acetate into valuable biochemicals by using ethanol as an external electron donor. In this study we proposed to test another potential electron donor, methanol, in chain elongation. Methanol can be produced through the thermochemical conversion of lignocellulosic biowaste. Use of methanol in chain elongation integrates the lignocellulosic feedstocks and the thermochemical platform technologies into chain elongation. After such integration, the feedstocks for chain elongation are solely from 2nd generation biomass resources. A proof-of-principle study of chain elongation using methanol and acetate was performed in both a batch and a continuous experiment. In the batch experiment, butyrate (191 mMC) and caproate (3 mMC) production from methanol and acetate was observed. A mixed culture microbiome taken from a previous chain elongation reactor fed with ethanol was responsible for the observed organic acid production. The continuous experiment was performed in an upflow anaerobic bioreactor (UAB). The hydraulic retention time (HRT) was 36 h and the operational period lasted for 45 days. In the continuous experiment, butyrate production (Rate > 30 mMC/day) was observed; the caproate concentration was below the detection limit during the entire continuous operational period. In both experiments, methanol and acetate were both substrates contributing to the butyrate production. To the authors' current knowledge, this study is the first attempt at a mixed culture fermentation utilising methanol and acetate for biochemical production. Further research should focus on elevating the butyrate production rate and concentration in the continuous operation of methanol chain elongation, which may stimulate caproate formation.