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

摘要

Glycerol fermentation redox routes by E. coli wild type batch cultures and effects of Ni~(2+), Fe~(2+) and Fe~(3+) ions of different concentration (0.01-0.3 mM) on cell growth and H_2 production were investigated. Influences of aforementioned metal ions and Cu~(2+), as well as Fe~(2+) on H_2 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 (E_h) and platinum electrodes (E'_h). After 8 h growth, at pH 7.5, E_h dropped down negative value (-120 mV) and H_2 production was observed at the middle log phase. Whereas at the same pH in the presence of 0.05 mM Fe~(2+) both E_h and E_h' electrodes readings dropped to more negative value～-170 +- 10 and -450 +- 12 raV, respectively. All ions used at 0.05-0.1 mM concentrations stimulated bacterial growth ～1.2 to ～1.4 fold at different pHs. Ni~(2+) enhanced H_2 formation in a concentration-dependent manner: maximal stimulation, up to 1.5 fold, was observed at 0.2 mM NiCl_2 only at pH 7.5. Ni~(2+) also promoted to high H_2 yield at pH 6.5 and 7.5 in vitro. In addition, 0.05-0.1 mM Fe~(2+) also affected on H_2 production rate and increased it ～2 fold in vitro at pH 6.5 and 7.5 whereas, 0.05-0.1 mM Cu~(2+) had inhibitory effect on H_2 production rate. In hyaB hybC mutant H_2 production rate was decreased compared with wild type and Fe~(2+) had no effect on H_2 production rate and yield both at pH 6.5 and 7.5. The findings indicate the strengthening of reductive processes by E. coli during glycerol fermentation and point out the role of essential heavy metal ions in H_2 production. Furthermore, enhanced H_2 production by heavy metal ions probably depends on operation of Hyd- 1 and Hyd- 2 at pH 6.5 and 7.5.