The inhibition of competitive metabolic pathways by various inhibitors in order to redirect electron flow towards nitrogenase and bidirectional Hox-hydrogenase was investigated in Anabaena siamensis TISTR 8012. Cells grown in BG11(0) supplemented with KCN, rotenone, DCMU, and DL-glyceraldehyde under light condition for 24 h showed enhanced H-2 production. Cells grown in BG11 medium showed only marginal H-2 production and its production was hardly increased by the inhibitors tested. H-2 production with either 20 mM KCN or 50 mu M DCMU in BG11(0) medium was 22 mu mol H-2 mg chl a(-1) h(-1), threefold higher than the control. The increased H-2 production caused by inhibitors was consistent with the increase in the respective Hox-hydrogenase activities and nifD transcript levels, as well as the decrease in hupL transcript levels. The results suggested that interruption of metabolic pathways essential for growth could redirect electrons flow towards nitrogenase and bidirectional Hox-hydrogenase resulting in increased H-2 production. (C) 2012 Elsevier Ltd. All rights reserved.