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

Purple non-sulfur photosynthetic bacterium Rhodobacter sphaeroides KD131 wild type (wt) and its PHB synthase deleted-mutant P1 were evaluated for hydrogen (H₂) 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 H₂ yield (HY) of 0.65 mol H₂/mol acetate- and 2.50 mol H₂/mol butyrate-consumed, while 30 mM succinate exhibited an HY of 3.29 mol H₂/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 P1 was used, in spite of depressed cell growth and lower substrate degradation compared to those observed in strain KD131 wt, higher H₂ production was observed, achieving around two-fold 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 H₂ production. When pH was controlled to 7.0, strain KD131 wt evolved more H₂ by 2.36 and 1.70 folds in the acetate- and succinate-medium, respectively, compared to those observed in without pH control. The highest H₂ production was observed when the mutant P1 was photo-fermented with a pH control to 7.0 in the medium containing acetate-(NH₄)₂SO₄. 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 H₂ production in expending reducing power.