Alternative photosynthetic electron transport pathways during anaerobiosis in the green alga Chlamydomonas reinhardtii

Oxygenic photosynthesis uses light as energy source to generate an oxidant powerful enough to oxidize water into oxygen, electrons and protons. Upon linear electron transport, electrons extracted from water are used to reduce NADP+ to NADPH. The oxygen molecule has been integrated into the cellular metabolism, both as the most efficient electron acceptor during respiratory electron transport and as oxidant and/or “substrate” in a number of biosynthetic pathways. Though photosynthesis of higher plants, algae and cyanobacteria produces oxygen, there are conditions under which this type of photosynthesis operates under hypoxic or anaerobic conditions. In the unicellular green alga Chlamydomonas reinhardtii, this condition is induced by sulfur deficiency, and it results in the production of molecular hydrogen. Research on this biotechnologically relevant phenomenon has contributed largely to new insights into additional pathways of photosynthetic electron transport, which extend the former concept of linear electron flow by far. This review summarizes the recent knowledge about various electron sources and sinks of oxygenic photosynthesis besides water and NADP+ in the context of their contribution to hydrogen photoproduction by C. reinhardtii. This article is part of a Special Issue entitled: Regulation of Electron Transport in Chloroplasts.

Research Highlights
► Research on photosynthetic hydrogen production by Chlamydomonas reinhardtii provided new insights into photosynthetic electron transport pathways.
► The two pathways of cyclic electron flow proposed to operate in C. reinhardtii have different functions during hydrogen photoproduction.
► The recently discovered monomeric plastidic NADPH-dehydrogenase Nda2 of Chlamydomonas is involved in non-photochemical PQ-reduction during photosynthetic hydrogen evolution.
► Starch is the major electron source for hydrogen production by sulfur deprived wild type Chlamydomonas cells, while in starchless mutants, photosystem 2 provides most of the electrons.
► The major ferredoxin isoform PetF is the natural electron donor of the FeFe hydrogenase HydA1 of C. reinhardtii.