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.