Production of Hydrogen Gas from Light and the Inorganic Electron Donor Thiosulfate by Rhodopseudomonas palustris

Abstract

A challenge for photobiological production of hydrogen gas (H ₂ ) as a potential biofuel is to find suitable electron-donating feedstocks. Here, we examined the inorganic compound thiosulfate as a possible electron donor for nitrogenase-catalyzed H ₂ production by the purple nonsulfur phototrophic bacterium (PNSB) Rhodopseudomonas palustris . Thiosulfate is an intermediate of microbial sulfur metabolism in nature and is also generated in industrial processes. We found that R. palustris grew photoautotrophically with thiosulfate and bicarbonate and produced H ₂ when nitrogen gas was the sole nitrogen source (nitrogen-fixing conditions). In addition, illuminated nongrowing R. palustris cells converted about 80% of available electrons from thiosulfate to H ₂ . H ₂ production with acetate and succinate as electron donors was less efficient (40 to 60%), partly because nongrowing cells excreted the intermediary metabolite α-ketoglutarate into the culture medium. The fixABCX operon (RPA4602 to RPA4605) encoding a predicted electron-transfer complex is necessary for growth using thiosulfate under nitrogen-fixing conditions and may serve as a point of engineering to control rates of H ₂ production. The possibility to use thiosulfate expands the range of electron-donating compounds for H ₂ production by PNSBs beyond biomass-based electron donors.