Enhancement of biohydrogen production rate in Rhodospirillum rubrum by a dynamic CO-feeding strategy using dark fermentation

BACKGROUND: Rhodospirillum rubrum is a purple non-sulphur bacterium that produces H(2) by photofermentation of several organic compounds or by water gas-shift reaction during CO fermentation. Successful strategies for both processes have been developed in light-dependent systems. This work explores a dark fermentation bioprocess for H(2) production from water using CO as the electron donor. RESULTS: The study of the influence of the stirring and the initial CO partial pressure (p(CO)) demonstrated that the process was inhibited at p(CO) of 1.00 atm. Optimal p(CO) value was established in 0.60 atm. CO dose adaptation to bacterial growth in fed-batch fermentations increased the global rate of H(2) production, yielding 27.2 mmol H(2) l(−1) h(−1) and reduced by 50% the operation time. A kinetic model was proposed to describe the evolution of the molecular species involved in gas and liquid phases in a wide range of p(CO) conditions from 0.10 to 1.00 atm. CONCLUSIONS: Dark fermentation in R. rubrum expands the ways to produce biohydrogen from CO. This work optimizes this bioprocess at lab-bioreactor scale studying the influence of the stirring speed, the initial CO partial pressure and the operation in batch and fed-batch regimes. Dynamic CO supply adapted to the biomass growth enhances the productivity reached in darkness by other strategies described in the literature, being similar to that obtained under light continuous syngas fermentations. The kinetic model proposed describes all the conditions tested.