Effect of pH and salinity on photofermentative hydrogen and poly-hydroxybutyric acid production via Rhodobacter capsulatus YO3

The production of poly-hydroxybutyrate (PHB) and H₂ via Rhodobacter capsulatus YO3, an uptake hydrogenase deficient (hup⁻) mutant, was investigated in this study. The pH and salinity of the bioreactor medium were investigated as potential factors to enhance production of PHB and H₂. Initial controlled pH and salinity ranges of 7.0–8.5 and 1500–4000 mgNa⁺/L, respectively, were studied. Initial set demonstrated that under a constant pH of 7.7, increasing the salinity of the medium enhanced maximum PHB production slightly. The highest PHB accumulation (57.5 ± 0.2 %) occurred at pH 8.5 under 1500 mgNa⁺/L, but was unstable and degraded rapidly. Further sets were run to uncouple the effect of pH and salinity. Higher initial pH and salinity levels hindered H₂ productivity, with PHB production sustained only under moderate stress. The most significant drop in both H₂ and PHB production occurred with high pH (>8.0) and Na⁺ (>3000 mg/L) levels applied together. The study suggests that relatively lower pH (7.0–7.5) and Na⁺ concentration (≈1500 mg/L) favor production of both products. Under moderate stress of either higher pH or salinity, cells accumulate and sustain PHB, but under combined stress of high pH and Na⁺, production shifts towards H₂. Severe conditions inhibit both processes. For practical applications with saline wastewater, pH control is critical, as a pH of 7.0–7.5 can reduce salinity's adverse effects. Gradual increases in salinity may help cultures adapt, enhancing efficient substrate use for H₂ or PHB production.