Appearance of a reversible hydrogenase activity in Anabaena cylindrica grown in high light

Abstract

In vivo H2 evolution by Anabaena cylindrica Lemm. strain PCC 7122 grown in the presence of ammonia at low and high light intensities was studied. We found that after 2 h of anaerobic incubation, H2 evolution [at a rate of 0.5 μmol (mg dry weight)1 h−1] via reversible hydrogenase occurred in high light grown cells, while this kind of activity was not found in low light grown cells. H2 evolution was inhibited by 3‐(3′. 4′‐dichlorophenyl‐1, 1‐dimethylurea (DCMU). Illuminating the cells in the phycocyanin absorption region resulted in a higher rate of H2 evolution than illuminating the cells in the chlorophyll absorption region. The results indicate that reversible hydrogenase receives reducing equivalents from photosynthetic water photolysis and that both photosystem II and photosystem I participate in the H2 production. Hydrogenase activity was found in the soluble fraction after mild sonication in the case of low light grown cells. After this treatment high light grown cells retained 70% of their hydrogenase activity in the particulate fraction, but released it into the soluble fraction in the presence of 2% deoxycholic acid.In vitro H2 evolution did not differ significantly in the low and high light grown cells. Hence, the differences in the in vivo H2 evolution reflect the different availability of endogenous reductants for hydrogenase in the two kinds of cells. On the basis of our results it is suggested that high light grown Anabaena cells eliminate part of the photosynthetically produced excess electrons via an induced reversible hydrogenase activity. This is the first report of H2 evolution associated with water photolysis and catalyzed by hydrogenase in cyanobacteria.