Regulation of photosynthesis under dynamic light conditions in Chlamydomonas reinhardtii : Impact on hydrogen production

Abstract

The development of renewable biofuels to ensure sustainable energy supply is one of the biggest global challenges of modern society. Hydrogen has great potential to become the fuel of the future, since it provides energy without CO2 emission. The green alga Chlamydomonas reinhardtii possesses hydrogenase enzymes and is able to photoproduce hydrogen under specific conditons. Hydrogenases, together with flavodiiron proteins (FDPs), PROTON GRADIENT-REGULATION 5 (PGR5), and PGR5-LIKE PHOTOSYNTHETIC PHENOTYPE 1 (PGRL1), represent key players of alternative electron transport (AET) in C. reinhardtii. Photosynthetic organisms have evolved AET routes such as these to adjust the photosynthetic apparatus under dynamic environmental conditions, like changing carbon supply or fluctuating light (FL) intensity. The interplay of all of these AET routes was the subject of my doctoral research, which was performed in order to understand AET regulation and to identify possible bottlenecks towards commercially profitable hydrogen production by C. reinhardtii.

In the first part of my thesis, I demonstrated that all three proteins; FDPs, PGR5, and PGRL1, contribute to the photoprotection of C. reinhardtii under FL. FDPs form a rapid electron sink downstream of PSI and are absolutely crucial for survival under FL conditions. PGR5 operates on a slower time-scale than FDPs and is the next important protein to act under FL stress. A lack of PGR5 inhibits cell growth, even under mild FL conditions. It is possible that PGR5 acts as a redox-dependent regulator of photosynthesis. The importance of PGRL1 on the growth performance is only observed under severe FL conditions, despite the importance of PGRL1-mediated cylic electron transport during high light transients.

In the second section of my thesis, I studied the role of FDPs in H2-photoproduction during the transition to anaerobiosis. Anoxic culture conditions are necessary to enable the function of oxygen-sensitive hydrogenases. Here, it is shown that FDPs may accelerate the transition of C. reinhardtii cells to anaerobiosis during S-deprivation. Furthermore, application of a magnesium (Mg)-deprivation protocol resulted in prolonged H2 production and improved cell viability. High accumulation levels of FDPs during the H2 production phase suggested a contribution of these proteins to the maintenance of anoxia when Mg-deprivation is employed to induce H2 production.