Physiological studies and approaches to maximize the hydrogen production in the cyanobacterium Synechocystis sp. PCC 6803
This work shows approaches to produce hydrogen sustainably and environmentally friendly by exploiting cyanobacteria. The cyanobacterium Synechocystis sp. PCC 6803 (hereafter Synechocystis) possesses an oxygen-sensitive bidirectional [NiFe]-hydrogenase, which is either able to produce hydrogen at the onset of photosynthesis (photohydrogen) or during carbohydrate oxidation in darkness (fermentative hydrogen). Previous experiments already showed that the hydrogenase is essential for the growth of Synechocystis cells on arginine and glucose in the presence of oxygen. However, the hydrogenase is oxygen-sensitive and is not able to either produce or take up hydrogen under these conditions (Burgstaller, 2017). It must, therefore, fulfill a different function at the given condition, which led to the hypothesis that the hydrogenase might function as an oxygenase in the presence of oxygen by working as an electron valve. Experimental results, however, indicated that the hydrogenase could have the opposite function and might feed electron into the electron transport chain at the thylakoid membrane, probably through complex 1 (NDH1). The hydrogenase obviously fine-tunes the respiratory and photosynthetic electron transfer under these conditions. In order to increase the electron flux to the hydrogenase and enhance the hydrogen production, fusion constructs were designed and constructed, which directly link photosystem I (PSI) to the hydrogenase subunits (HoxYH) of Synechocystis to maximize the photohydrogen production. Measurements of the production in those mutants revealed an increase in hydrogen production in comparison to the Synechocystis wild-type (WT) in the absence of oxygen. These results show a successful generation of a fusion protein in Synechocystis cells in vivo and could be the first step towards a sustainable and CO2 neutral way to produce hydrogen as an alternative and renewable energy source.