On the pathways feeding the H(2) 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 O(2) concentration (hypoxia) and low light, Chlamydomonas cells can produce H(2) gas in nutrient-replete conditions. This process is hindered by the presence of O(2), which inactivates the [FeFe]-hydrogenase enzyme responsible for H(2) gas production shifting algal cultures back to normal growth. The main pathways accounting for H(2) production in hypoxia are not entirely understood, as much as culture conditions setting the optimal redox state in the chloroplast supporting long-lasting H(2) production. The reducing power for H(2) 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 H(2) 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 H(2) production in nutrient-replete hypoxic conditions. Addition of low amounts of O(2) enhanced acetate respiration rate, particularly in the light, resulting in improved H(2) 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 FADH(2) and NADH. The latter would enter a PSII-independent pathway at the level of the plastoquinone pool, consistent with the light dependence of H(2) production. The authors concluded that the water-splitting activity of PSII has a minor role in H(2) evolution in nutrient-replete, mixotrophic cultures under hypoxia. However, their results with the PSII inhibitor DCMU also reveal that O(2) 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 H(2) production. CONCLUSIONS: In Chlamydomonas, continuous H(2) gas evolution is expected once low O(2) partial pressure and optimal reducing conditions are set. Under nutrient-replete conditions, the electrogenic processes involved in H(2) 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.