Photoproduction of hydrogen from acetate by a chemoheterotrophic bacterium Rhodopseudomonas palustris P4

Rhodopseudomonas palustris P4 can produce H2 either from CO by water-gas shift reaction or from various sugars by anaerobic fermentation. Fermentative H2 production by P4 is fast, but its yield is relatively low due to the formation of various organic acids. In order to increase H2 production yield from glucose, P4 was investigated for the photo-fermentation of acetate which is a major by-product of fermentative H2 production. Experiments were performed in batch modes using both light-grown and dark-grown cells. When the dark-grown P4 was challenged with light and acetate, H2 was produced with the consumption of acetate after a lag period of 25 h. H2 production was inhibited when a nitrogen source, especially ammonium, is present. When the dark-fermentation broth containing acetate was adopted for photo-fermentation with light-grown cells, H2 production and concomitant acetate consumption occurred without a lag period. The H2 yield was estimated as 2.4–2.8 mol H2/mol acetate and the specific H2 production rate was as 9.8 ml H2/g cell h. The fact that a single strain can perform both dark- and light-fermentation gives a great advantage in process development. Compared to a one-step dark-fermentation, the combined dark- and light-fermentation can increase the H2 production yield on glucose by two-fold.