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Background  Solar energy is the ultimate energy source on the Earth. The conversion of solar energy into fuels and energy sources can be an ideal solution to address energy problems. The recent discovery of proteorhodopsin in uncultured marine γ-proteobacteria has made it possible to construct recombinant  Escherichia coli  with the function of light-driven proton pumps. Protons that translocate across membranes by proteorhodopsin generate a proton motive force for ATP synthesis by ATPase. Excess protons can also be substrates for hydrogen (H 2 ) production by hydrogenase in the periplasmic space. In the present work, we investigated the effect of the co-expression of proteorhodopsin and hydrogenase on H 2  production yield under light conditions.    Results  Recombinant  E. coli  BL21(DE3) co-expressing proteorhodopsin and [NiFe]-hydrogenase from  Hydrogenovibrio marinus  produced ~1.3-fold more H 2  in the presence of exogenous retinal than in the absence of retinal under light conditions (70 μmole photon/(m 2 ·s)). We also observed the synergistic effect of proteorhodopsin with endogenous retinal on H 2  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/(m 2 ·s) led to an increase (~1.8-fold) in H 2  production from 287.3 to 525.7 mL H 2 /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 H 2  achieved in this study was ~3.4%.    Conclusion  Here, we report for the first time the potential application of proteorhodopsin for the production of biohydrogen, a promising alternative fuel. We showed that H 2  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.

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