Osmotic stress as a factor for regulating hydrogenase activity and enhancing H production during mixed carbon sources fermentation.

performs mixed-acid fermentation and produces molecular hydrogen (H) via reversible hydrogenases (Hyd). H producing activity was investigated during hyper- and hypo-osmotic stress conditions when a mixture of carbon sources (glucose and glycerol) was fermented at different pHs. Hyper-osmotic stress decreased H production rate (V) 30 % in wild type at pH 7.5 when glucose was supplemented, while addition of formate stimulated V ~45% compared to hypo-stress conditions. Only in in formate assays was V inhibited ~25% compared to hypo-stress conditions. In hypo-stress conditions addition of glycerol increased V ~2 and 3 fold in and mutants, respectively, compared to wild type. At pH 6.5 hyper-osmotic stress stimulated V ~2 fold in all strains except mutant when glucose was supplemented, while in formate assays significant stimulation (3 fold) was determined in mutant. At pH 5.5 hyper-osmotic stress inhibited V ~30% in wild type when glucose was supplemented, but in formate assays it was stimulated in all strains except . Taken together, it can be concluded that, depending on external pH and absence of Hyd enzymes in stationary-phase-grown osmotically stressed cells, H production can be stimulated significantly which can be applied in developing H production biotechnology.