Enhancement of dark fermentative hydrogen production in nitrogen-deprived halotolerant unicellular cyanobacterium Aphanothece halophytica by treatment with reducing agents

Abstract

To enhance H2 production by the halotolerant unicellular cyanobacterium Aphanothece halophytica, effect of various kinds of reducing sugar and reducing agent on H2 production was investigated. The highest H2 production rate of 55.80 ± 0.50 μmol H2 g dry weight−1 h−1 was obtained when the cells were incubated in BG110 medium containing 0.189 mmol C-atom L−1 glucose under dark anaerobic condition. This rate was 1.5 folds higher than that without glucose. Among ten reducing agents tested, β-mercaptoethanol, dithiothreitol, l-cysteine and sodium sulfide had high potential as an effective reducing agent to increase H2 production by A. halophytica. Cells treated with 50 mM sodium sulfide showed the highest H2 accumulation with 4815.59 ± 194.78 μmol H2 g dry weight−1 after 24 h of dark anaerobic incubation. An increase in H2 production was ascribed to an increase of hydrogenase activity and a decrease of O2 production rate. This H2 production yield was approximately 20 folds higher than that without reducing agent. Furthermore, 50 mM sodium sulfide appeared to be non-toxic to A. halophytica cells, since the IC50 of sodium sulfide was higher than 100 mM. The reduced ferredoxin at 1.5 μM-1.5 mM, NADH and NADPH at 0.15–1.5 mM could support in vitro [NiFe]-H2ase activity, demonstrating the ability of these compounds to provide electrons towards [NiFe]-H2ase in A. halophytica.