On the pathways feeding the H2 production process in nutrient-replete, hypoxic conditions. Commentary on the article “Low oxygen levels contribute to improve photohydrogen production in mixotrophic non-stressed Chlamydomonas cultures”, by Jurado-Oller et al., Biotechnology for Biofuels, published September 7, 2015; 8:149

BACKGROUND Under low O2 concentration (hypoxia) and low light, Chlamydomonas cells can produce H2 gas in nutrient-replete conditions. This process is hindered by the presence of O2, which inactivates the [FeFe]-hydrogenase enzyme responsible for H2 gas production shifting algal cultures back to normal growth. The main pathways accounting for H2 production in hypoxia are not entirely understood, as much as culture conditions setting the optimal redox state in the chloroplast supporting long-lasting H2 production. The reducing power for H2 production can be provided by photosystem II (PSII) and photofermentative processes during which proteins are degraded via yet unknown pathways. In hetero- or mixotrophic conditions, acetate respiration was proposed to indirectly contribute to H2 evolution, although this pathway has not been described in detail. MAIN BODY Recently, Jurado-Oller et al. (Biotechnol Biofuels 8: 149, 7) proposed that acetate respiration may substantially support H2 production in nutrient-replete hypoxic conditions. Addition of low amounts of O2 enhanced acetate respiration rate, particularly in the light, resulting in improved H2 production. The authors surmised that acetate oxidation through the glyoxylate pathway generates intermediates such as succinate and malate, which would be in turn oxidized in the chloroplast generating FADH2 and NADH. The latter would enter a PSII-independent pathway at the level of the plastoquinone pool, consistent with the light dependence of H2 production. The authors concluded that the water-splitting activity of PSII has a minor role in H2 evolution in nutrient-replete, mixotrophic cultures under hypoxia. However, their results with the PSII inhibitor DCMU also reveal that O2 or acetate additions promoted acetate respiration over the usually dominant PSII-dependent pathway. The more oxidized state experienced by these cultures in combination with the relatively short experimental time prevented acclimation to hypoxia, thus precluding the PSII-dependent pathway from contributing to H2 production. CONCLUSIONS In Chlamydomonas, continuous H2 gas evolution is expected once low O2 partial pressure and optimal reducing conditions are set. Under nutrient-replete conditions, the electrogenic processes involved in H2 photoproduction may rely on various electron transport pathways. Understanding how physiological conditions select for specific metabolic routes is key to achieve economic viability of this renewable energy source.