Metabolic Engineering to Improve Biohydrogen Production by Rhodobacter capsulatus JP91
Energy demand is increasing day by day, resulting in global attention towards alternative eco- friendly fuels. Biohydrogen is considered the most promising energy carrier to replace conventional fossil fuels because its production and combustion is not harmful to the environment. Biological hydrogen production is an alternative method for hydrogen production, by which hydrogen is produced under mild conditions, making it environmentally friendly. Photo- biological hydrogen production by purple non-sulfur photosynthetic bacteria is a promising process in which bacteria can capture light energy to drive H2 production with their N2ase system. Some metabolic pathways, such as CO2 fixation and PHB biosynthesis, compete with nitrogenase for electrons (reducing equivalents). Recently, in attempts to improve the yield of H2, metabolic engineering has been applied to increase electron flow to N2ase. The purpose of the present study was to improve the yield of photosynthetic H2 production by Rhodobacter capsulatus JP91 using a metabolic engineering approach. The hypothesis was that inactivation of PHB synthase (phbC) would block PHB biosynthesis, thus directing more electron flux towards nitrogenase to catalyze increased H2 production. In previous studies, PHB synthase mutants were developed in other photosynthetic bacteria, including R. sphearoides and Rhodopseudomonas palustris but not Rhodobacter capsulatus. In this study, a new doubly mutated R. capsulatus strain RS15 (hup‾, phbC‾), a derivative of R. capsulatus JP91 (hup‾) was created. The results show that the newly created strain, R. capsulatus RS15, could grow in different carbon and nitrogen sources. This mutant has a long lag phase. However, once it reached log phase, growth was similar to the parental strain. From hydrogen production studies, the new developed strain RS15 is able to convert acetate, lactate, and glucose to hydrogen. Acetate was shown to be the best carbon source for H2 production by RS15 but not for its growth. Overall, R. capsulatus RS15 (hup‾, phbC‾) seems to be a promising candidate for use in two - stage hybrid systems since the main dark fermentation effluents are acetate and butyrate. Such a process could potentially make biohydrogen production technology feasible on an industrial scale.
R. capsulatus JP91 R. capsulatus RS15 Photofermentation Polyhydroxy butyrate PHB synthase Biohydrogen production Metabolic engineering Purple none sulfur bacteria