Abstract
BackgroundSolar 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 (H2) production by hydrogenase in the periplasmic space. In the present work, we investigated the effect of the co-expression of proteorhodopsin and hydrogenase on H2 production yield under light conditions.ResultsRecombinant 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%.ConclusionHere, 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.
Highlights
Solar energy is the ultimate energy source on the Earth
From the harvested cell pellet, we observed that the cells expressing proteorhodopsin with endogenous retinal have a distinctively reddish color compared to wild-type cells (Figure 2A)
Co-expression effect of proteorhodopsin and hydrogenase on H2 production After confirmation of proteorhodopsin function in recombinant E. coli, we investigated the effect of coexpressing proteorhodopsin and H. marinus [NiFe]hydrogenase on H2 production
Summary
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 g-proteobacteria has made it possible to construct recombinant Escherichia coli with the function of light-driven proton pumps. In the field of biotechnology, the photosynthetic process in algae and cyanobacteria has been actively investigated for the conversion of solar energy to useful biofuels [3,4,5]. Proteorhodopsin can be heterologously expressed in Escherichia coli to possess proton-pumping activity [7], which is different from bacteriorhodopsin found in halobacteria [1,8]. This property of proteorhodopsin enables the investigation of its impact on cellular energy and phototrophy [8]. There have been no substantial applications in biofuel production using proteorhodopsin, this potential has been mentioned recently [1]
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