Effect of volatile fatty acids mixtures on the simultaneous photofermentative production of hydrogen and polyhydroxybutyrate

Purple non-sulfur bacteria generate hydrogen and polyhydroxybutyrate (PHB) as a mechanism for disposing of reducing equivalents generated during substrate consumption. However, both pathways compete for the reducing equivalents released from bacteria growing under certain substrates, thus the formation of hydrogen or PHB is detrimental to the formation of each other. The effect of mixtures of acetic, propionic and butyric acids on the formation of H-2 and PHB was evaluated using BoxBehnken design. A bacterial community mainly constituted by Rhodopseudomonas palustris was used as inoculum. It was observed that the three volatile fatty acids had a significant effect on the specific PHB production. However, only the propionic acid had a significant effect on the specific H-2 production activity and the highest value was observed when acetate was the main component in the mixture. The maximum values for the specific PHB and hydrogen production rates were 16.4 mg-PHB/g-TSS/day and 391 mL-H-2/g-TSS/day, respectively.