초록
<P><B>Abstract</B></P> <P>Purple non-sulfur bacteria (PNSB) are well known for converting short-chain organic acids to H<SUB>2</SUB>, however, a decrease in pH caused by metabolic acids production limited H<SUB>2</SUB> production during the photo-fermentation from glucose. Here we address why volatile fatty acids (VFA) excreted as fermentation products cannot be further degraded by <I>R. sphaeroides</I> that readily use them. We found that the photo-fermentation with pH controlled at 6.9 ± 0.1 resulted in a 90% increase of H<SUB>2</SUB> yield and a 107.6% increase in volume H<SUB>2</SUB> production relative to the pH-uncontrolled culture. Comparative fermentations on glucose at pH 5.8 and pH 7.1 using culture medium supplemented with 50% spent fermentation broth demonstrated that low pH alone is not the limiting factor and compounds present in the supernatants along with pH decrease were the most inhibitory to H<SUB>2</SUB> production. The impact of byproducts VFA on phototrophic H<SUB>2</SUB> production was dependent on both the pH and VFA concentrations; even 7 mM VFA addition totally inhibited H<SUB>2</SUB> production from glucose at pH 5.4. H<SUB>2</SUB> production with pH control for the Δ<I>hup</I> strain was not discernibly different from the parent strain, which are all significantly higher than high-performance strains by metabolic engineering. These results demonstrate that pH dependent VFA inhibition can be turned into a driving force for enhanced H<SUB>2</SUB> production from glucose by pH regulation.</P> <P><B>Highlights</B></P> <P> <UL> <LI> H<SUB>2</SUB> yield increased by 90% and H<SUB>2</SUB> production increased by 107.6% by pH regulation. </LI> <LI> Byproducts VFA impact on H<SUB>2</SUB> production was dependent on pH and VFA concentrations. </LI> <LI> The Δ<I>hup</I> mutant and parent strain produced similar amount of H<SUB>2</SUB> under pH control. </LI> <LI> VFA inhibition can be turned into improved H<SUB>2</SUB> production by pH regulation. </LI> </UL> </P> <P><B>Graphical abstract</B></P> <P>[DISPLAY OMISSION]</P>