Improved production of biohydrogen in light-powered Escherichia coli by co-expression of proteorhodopsin and heterologous hydrogenase

Recombinant E. coli BL21(DE3) co-expressing proteorhodopsin and [NiFe]-hydrogenase from Hydrogenovibrio marinus produced ~1.3-fold more H2 in the presence of exogenous retinal than in the absence of retinal under light conditions (70 μmole photon/(m2·s)). We also observed the synergistic effect of proteorhodopsin with endogenous retinal on H2 production (~1.3-fold more) with a dual plasmid system compared to the strain with a single plasmid for the sole expression of hydrogenase. The increase of light intensity from 70 to 130 μmole photon/(m2·s) led to an increase (~1.8-fold) in H2 production from 287.3 to 525.7 mL H2/L-culture in the culture of recombinant E. coli co-expressing hydrogenase and proteorhodopsin in conjunction with endogenous retinal. The conversion efficiency of light energy to H2 achieved in this study was ~3.4%.Here, we report for the first time the potential application of proteorhodopsin for the production of biohydrogen, a promising alternative fuel. We showed that H2 production was enhanced by the co-expression of proteorhodopsin and [NiFe]-hydrogenase in recombinant E. coli BL21(DE3) in a light intensity-dependent manner. These results demonstrate that E. coli can be applied as light-powered cell factories for biohydrogen production by introducing proteorhodopsin.Since the Industrial Revolution, energy consumption has increased exponentially and most energy has been derived from fossil fuels. Currently, we still depend on fossil fuels for more than 80 percent of our demands for electricity, transportation, and industries, although concerns about the exhaustion of fossil fuels and global warming have led to increased attention to renewable energy [1]. Among various renewable energy sources, solar energy is the most abundant and ultimate source. The total amount of solar energy absorbed by the Earth's surface is 1.74 × 105 terawatts (TW) [2], which is a tremendous amount compared to the world's energy consumption (~13 TW) [1]. Thus, the conversi