Improved glycerol to ethanol conversion by E. coli using a metagenomic fragment isolated from an anaerobic reactor

Crude glycerol obtained as a by-product of biodiesel production is a reliable feedstock with the potential to be converted into reduced chemicals with high yields. It has been previously shown that ethanol is the primary product of glycerol fermentation by Escherichia coli. However, few efforts were made to enhance this conversion by means of the expression of heterologous genes with the potential to improve glycerol transport or metabolism. In this study, a fosmid-based metagenomic library constructed from an anaerobic reactor purge sludge was screened for genetic elements that promote the use and fermentation of crude glycerol by E. coli. One clone was selected based on its improved growth rate on this feedstock. The corresponding fosmid, named G1, was fully sequenced (41 kbp long) and the gene responsible for the observed phenotype was pinpointed by in vitro insertion mutagenesis. Ethanol production from both pure and crude glycerol was evaluated using the parental G1 clone harboring the ethanologenic plasmid pLOI297 or the industrial strain LY180 complemented with G1. In mineral salts media containing 50 % (v/v) pure glycerol, ethanol concentrations increased two-fold on average when G1 was present in the cells reaching up to 20 g/L after 24 h fermentation. Similar fermentation experiments were done using crude instead of pure glycerol. With an initial OD620 of 8.0, final ethanol concentrations after 24 h were much higher reaching 67 and 75 g/L with LY180 cells carrying the control fosmid or the G1 fosmid, respectively. This translates into a specific ethanol production rate of 0.39 g h(-1) OD(-1) L(-1).