Enhanced hydrogen production by green alga Scenedesmus obliquus TISTR 8546 under atmospheric air through potassium deprivation and cysteine supplementation

Abstract

Hydrogen is a promising alternative energy carrier that can be produced by green microalgae. However, the key enzyme catalyzing this process, [FeFe]‑hydrogenase, is highly sensitive to O2 generated during photosynthesis, which severely restricts its activity. This study aimed to screen the microalgal strain capable of producing high levels of H2 in the presence of O2 and to enhance H2 production yields using O2-scavenging strategies to levels comparable to or exceeding those achieved under anaerobic conditions. Among the 24 strains tested, Scenedesmus obliquus TISTR 8546 demonstrated the highest H2 production under both aerobic and anaerobic conditions. The O2I50 of S. obliquus TISTR 8546 cells for H2 evolution was 15.93 ± 0.24%. Potassium deprivation significantly enhanced H2 production and hydrogenase activity by lowering O2 levels through reduced photosynthetic O2 evolution and increased dark respiration. Moreover, potassium deprivation promoted intracellular starch accumulation, providing reducing equivalents for H2 generation. Cysteine supplementation further stimulated H2 production by serving as a reducing agent. S. obliquus TISTR 8546 exhibited a maximum H2 production rate of 22.92 ± 1.05 μmol H2 mg Chl−1 h−1 and achieved a maximum cumulative H2 production of 1153.78 ± 52.65 μmol H2 mg Chl−1 when incubated in potassium-deprived TAP medium supplemented with 0.1 mM cysteine under atmospheric air for 6 days. This rate was 23.6- and 2.7-fold higher than those obtained in TAP and TAP-K media, respectively. These findings demonstrate the potential of S. obliquus TISTR 8546 as a robust microalgal strain for sustainable H2 production under atmospheric air, highlighting its promise for future industrial applications.