Reversing methanogenesis to produce ethanol

Capturing greenhouse gases and converting them into biochemicals is beneficial. Previously, we reversed methanogenesis by cloning methyl-coenzyme M reductase (Mcr) from an unculturable population of anaerobic methanotrophic archaea into the methanogen Methanosarcina acetivorans to capture both methane and carbon dioxide to produce acetate, lactate, and electricity. In the present study, we found that the methane captured (130 ± 30 μmol) by ANME-1 Mcr in M. acetivorans is converted from acetate to ethanol (120 ± 10 μmol). Moreover, ethanol production was tripled (370 ± 20 µmol) by adding iron(III) and humic acids to facilitate electron transfer and yeast extract to stimulate growth. We further introduced heterologous carboxylic acid reductase and alcohol dehydrogenase into the same host with the goal to produce even more ethanol from the intermediate acetate, but we found that this actually leads to a drop in ethanol yield, showing that the methanogenic host converts acetate from Mcr-captured methane by utilizing its native aldehyde ferredoxin oxidoreductase and alcohol dehydrogenase. Hence, we demonstrate that M. acetivorans may be utilized to produce ethanol from methane and carbon dioxide, that this conversion is enhanced by heterologous production of ANME-1 Mcr, and that the host can produce ethanol from acetate. Acetate produced by methanogens from captured methane and carbon dioxide can be converted efficiently to ethanol, demonstrated by lab experiments on the engineered methanogen Methanosarcina acetivorans.