Photosynthetic electron transfer pathways during hydrogen photoproduction in green algae mechanisms and limitations

International audienceSome microalgal species produce hydrogen in the light owing to a coupling between the photosynthetic electron transfer chain and a [FeFe]-hydrogenase. This reaction, which represents a waste of energy for algal cells, avoids over-reduction of photosynthetic electron carriers, particularly during the anaerobic induction of photosynthesis. Algal hydrogen photoproduction is a promising area for biotechnological applications, but represents a minor route for electron flow under most conditions. The first pathway limitation relates to the sensitivity of the [FeFe]-hydrogenase to oxygen, which is produced in the light by photosystem II (PSII). The second limitation relates to the supply of electrons to the [FeFe]-hydrogenase, since electron transfer reactions of oxygenic photosynthesis are highly regulated. Multiple pathways operate during oxygenic photosynthesis, including the linear pathway (the so-called "Z" scheme) oxidizing water at PSII and reducing NADP+ at PSI, cyclic pathways recycling electrons around PSI, and electron pathways reducing molecular oxygen. The last occurs either in the chloroplasts (using the plastid terminal oxidase PTOX or flavodiiron proteins, Flv) or in mitochondria, thanks to the operation of metabolic shuttles. We will describe in this chapter how these different pathways interact to supply electrons to the [FeFe]-hydrogenase during the process of hydrogen photoproduction and discuss future possible biotechnological improvements