Simazine Enhances Dark Fermentative H₂ Production by Unicellular Halotolerant Cyanobacterium Aphanothece halophytica

The halotolerant cyanobacterium Aphanothece halophytica is a potential H₂ producer that induces H₂ evolution under nitrogen deprivation. H₂ is mainly produced via the catabolism of stored glycogen under dark anaerobic condition. H₂ evolution is catalyzed by O₂-sensitive bidirectional hydrogenase. The aim of this study was to improve H₂ production by A. halophytica using various kinds of inhibitors. Among all types of inhibitors, simazine efficiently promoted the highest H₂ production under dark conditions. High simazine concentration and long-term incubation resulted in a decrease in cell and chlorophyll concentrations. The optimal simazine concentration for H₂ production by A. halophytica was 25 µM. Simazine inhibited photosynthetic O₂ evolution but promoted dark respiration, resulting in a decrease in O₂ level. Hence, the bidirectional hydrogenase activity and H₂ production was increased. A. halophytica showed the highest H₂ production rate at 58.88 ± 0.22 µmol H₂ g^-1 dry weight h^-1 and H₂ accumulation at 356.21 ± 6.04 μmol H₂ g^-1 dry weight after treatment with 25 µM simazine under dark anaerobic condition for 2 and 24 h, respectively. This study demonstrates the potential of simazine for the enhancement of dark fermentative H₂ production by A. halophytica.