Electron pathways involved in H-2-metabolism in the green alga Scenedesmusobliquus

The green alga Scenedesmus obliquus is capable of both uptake and productio n of H-2 after anaerobic adaptation (photoreduction of CO2 or photohydrogen production). The essential enzyme for H-2-metabolism is a NiFe-hydrogenase with a [2Fe-2S]-ferredoxin as its natural redox partner. Western blot anal ysis showed that the hydrogenase is constitutively expressed. The K-m value s were 79.5 muM and 12.5 muM, determined with ferredoxin and H-2, respectiv ely, as electron donor for the hydrogenase. In vitro, NADP(+) was reduced b y H-2 in the presence of the hydrogenase, the ferredoxin and a ferredoxin-N ADP reductase. From these results and considerations on the stoichiometry w e propose that this light-independent electron transfer is part of the phot oreduction of CO2 in vivo. For ATP synthesis, necessary for the photoreduct ion of CO2, light-dependent cyclic electron transfer around Photosystem (PS ) I accompanies this 'dark reaction'. PS II fluorescence data suggest that (a) in S. obliquus H-2-reduction might function as the anaerobic counterpar t of the O-2-dependent Mehler reaction, and (b) the presence of either a fe rredoxin quinone-reductase or NAD(P)-dehydrogenase (complex I) in S. obliqu us chloroplasts. (C) 2001 Elsevier Science B.V. All rights reserved.