Enhancing photo-fermentative hydrogen production by Rhodobacter sphaeroides KD131 and its PHB synthase deleted-mutant from acetate and butyrate

摘要

Purple non-sulfur photosynthetic bacterium Rhodobacter sphaeroides KD131 wild type (wt) and its PHB synthase deleted-mutant PI were evaluated for hydrogen (H_2) production from acetate and butyrate, the most abundant liquid end products of dark fermentation. In the presence of glutamate (8 mM), 60 mM of acetate and 30 mM of butyrate were degraded down to 41.5% and 24.0%, respectively, and achieved a H_2 yield (HY) of 0.65 mol H_2mol acetate- and 2.50 mol H_2/mol butyrate-consumed, while 30 mM succinate exhibited an HY of 3.29 mol H_2/mol substrate-consumed. The order of HY observed was inversely related to poly-(3-hydroxybutyrate) (PHB) content and pH increase in the broth. When mutant PI was used, in spite of depressed cell growth and lower substrate degradation compared to those observed in strain KD131 wt, higher H_2 production was observed, achieving around twofold increase of HY in both acetate and butyrate. A pH control to 7.0 during fermentation was effective in increasing substrate degradation and decreasing PHB content, thereby significantly increasing H_2 production. When pH was controlled to 7.0, strain KD131 wt evolved more H_2 by 2.36 and 1.70 folds in the acetate- and succinate-medium, respectively, compared to those observed in without pH control. The highest H_2 production was observed when the mutant PI was photo-fermented with a pH control to 7.0 in the medium containing acetate-(NH_4)_2SO_4. It seemed that pH control had an effect not only on the depressed production of PHB but also on soluble microbial products and secondary metabolites, which would compete with H_2 production in expending reducing power.