Short-Term Photoinhibition Induces Long-Term Hydrogen Photoproduction in a Phototrophic Culture of Chlorella sorokiniana on Complete Medium

This work demonstrates, for the first time, capacity of the Chlorella sorokiniana immobilized in alginate to produce hydrogen (H₂) over an extended period of time when cultivated under strictly photoautotropic conditions on complete mineral medium. In order to reduce photosynthetic activity, immobilized cells were subjected to a 30-minute pre-incubation period at high light intensity of 1000 μmol photons m⁻²∙s⁻¹. The ability to produce H₂ was evaluated under illumination of 40 μmol/(m²∙s). The culture not bubbled with argon produced H₂ for 9 days; total gas yield was 0.1 mol H₂/m². In the culture under argon atmosphere, the release of H₂ continued for 51 days, resulting in a total yield of 0.55 mol H₂/m². The immobilized culture was capable of H₂ production at 16% O₂ in the gas phase, which may be due to the effects of photoinhibition and activation of oxygen uptake pathways in mitochondria and chloroplast. Analysis of the functioning of electron-transport chain in the microalgae cells revealed decrease in the rate of electron transport, increase in the size of the PSII antenna, and development of non-photochemical quenching processes, while activity of PSII remained moderately high (Fv/Fm = 0.4-0.6). Inhibitor analysis using 10⁻⁵ M DCMU demonstrated that contribution of PSII to hydrogenase reaction increased from 30% on the first day of the experiment to 50% by the fourth day. Addition of 10⁻⁵ M DBMIB led to the 90% reduction in the rate of H₂ formation on both day 1 and day 4.