Glycerol Fermentation and Molecular Hydrogen Production by Escherichia Coli Batch Cultures Affected by Some Reducing Reagents and Heavy Metal Ions

Abstract

Glycerol fermentation redox routes by E. coli wild type batch cultures and effects of Ni2+, Fe2+ and Fe3+ ions of different concentration (0.01–0.3 mM) on cell growth and H2 production were investigated. Influences of aforementioned metal ions and Cu2+, as well as Fe2+ on H2 production rate in vitro assays were also studied in wild type and hyaB hybC double mutant (with defective hydrogenase (Hyd) 1 and Hyd -2). Cell growth was shown to be followed by decrease of pH and redox potential (ORP) measured by both titanium-silicate (Eh) and platinum electrodes (E′h). After 8 h growth, at pH 7.5, Eh dropped down negative value (−120 mV) and H2 production was observed at the middle log phase. Whereas at the same pH in the presence of 0.05 mM Fe2+ both Eh and Eh′ electrodes readings dropped to more negative values ~ −170 ± 10 and −450 ± 12 mV, respectively. All ions used at 0.05–0.1 mM concentrations stimulated bacterial growth ~1.2 to ~1.4 fold at different pHs. Ni2+ enhanced H2 formation in a concentration-dependent manner: maximal stimulation, up to 1.5 fold, was observed at 0.2 mM NiCl2 only at pH 7.5. Ni2+ also promoted to high H2 yield at pH 6.5 and 7.5 in vitro. In addition, 0.05–0.1 mM Fe2+ also affected on H2 production rate and increased it ~2 fold in vitro at pH 6.5 and 7.5 whereas, 0.05–0.1 mM Cu2+ had inhibitory effect on H2 production rate. In hyaB hybC mutant H2 production rate was decreased compared with wild type and Fe2+ had no effect on H2 production rate and yield both at pH 6.5 and 7.5.