Poly-Hydroxybutyrate Production by Rhodopseudomonas sp. Grown in Semi-Continuous Mode in a 4 L Photobioreactor

The synthesis of polyhydroxybutyrate (PHB) by photosynthetic non-sulfur bacteria isa potential approach for producing biodegradable plastics. In this work, acetate was used as asingle carbon source to study the effect on PHB formation in Rhodopseudomonas sp. cultured ina cylindrical four-liter photobioreactor under semi-continuous mode. The cultivation process isdivided into a symmetrical growth phase and a PHB accumulation phase separated temporally.The symmetrical growth phase (nutrient sufficient conditions) was followed by a sulfur-limitedphase to promote PHB accumulation. The main novelty is the progressive lowering of the sulfurconcentration into Rhodopseudomonas culture, which was obtained by two concomitant conditions:(1) sulfur consumption during the bacterial growth and (2) semi-continuous growth strategy. Thiscaused a progressive lowering of the sulfur concentration into Rhodopseudomonas culturedue to thesulfur-free medium used to replace 2 L of culture (50% of the total) that was withdrawn from thephotobioreactor at each dilution. The PHB content ranged from 9.26% to 15.24% of cell dry weight.At the steady state phase, the average cumulative PHB was >210 mg/L. Sulfur deficiency proved tobe one of the most suitable conditions to obtain high cumulative PHB in Rhodopseudomonas culture.