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, the capacity of Chlorella sorokiniana immobilized in alginate to produce hydrogen (H₂) over an extended period 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 a 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; the total gas yield was 0.1 mol H₂/m². In an 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 the electron-transport chain in microalgae cells revealed a decrease in the rate of electron transport, an increase in the size of the PSII antenna and the development of non-photochemical quenching processes, while the activity of PSII remained moderately high (Fv/Fm = 0.4-0.6). Inhibitor analysis using 10^–5 M DCMU demonstrated that the contribution of PSII to the hydrogenase reaction increased from 30% on the first day of the experiment to 50% by the fourth day. The addition of 10^–5 M DBMIB led
to a 90% reduction in the rate of H₂ formation on both day 1 and day 4.