Hydrogen production during stationary phase in purple photosynthetic bacteria

The merit of a hydrogen production system was investigated, where cells in the stationary phase of growth were treated as live enzymes, continually catalyzing hydrogen production in the absence of growth. Batch cultures of the purple photosynthetic bacteria Rhodospirillum rubrum UR2 were grown photoheterotrophically with succinate as the electron donor. Hydrogen evolved during growth, via the enzyme nitrogenase, at a rate of 21 mL gas L−1 culture h−1, and continued to evolve at high rates for about 70 h after cells had ceased growth. Hydrogen production stopped precisely when succinate was depleted from the medium. Upon replenishment of succinate to the cultures, hydrogen production resumed but cells did not grow further; however, the rate and yield of hydrogen production was lower with successive succinate additions than that measured during growth. These results suggest that hydrogen production is not strictly coupled to growth. Nevertheless, the results also establish the necessity for cell growth in order to maintain maximal hydrogen production rates. Supplementation of cultures with limited amounts of fresh growth medium, given in addition to the succinate replenishment, partially restored the hydrogen production rate and yield, along with a proportional increase in cell biomass. Results were confirmed in parallel experiments with Rhodopseudomonas palustris CGA009. A strategy is suggested for enhancing the biofuels to biomass production ratio under conditions of continuous cultivation with minimal cell growth (about 10% of the control), allowing a greater proportion of the cellular metabolic activity to be directed toward H2-production.