Poly(3-hydroxybutyrate) production by Rhodopseudomonas sp. S16-VOGS3 cells grown in digested sludge

The biomass produced by cultivating photosynthetic bacteria could be used for a variety of applications, such as feedstock for laying hens and/or bioplastic production. To achieve high productivity, it is important to optimize certain criteria such as the availability of water, nutrients, and light. Due to the high price of nutrients, alternatives such as the use of digested sludge as a nutrient source have been explored. In this study, digested sludge was tested as cultivation medium in order to promote Rhodopseudomonas sp. S16-VOGS3 growth and poly(3-hydroxybutyrate) (PHB) production. A cylindrical photobioreactor (0.22-L working volume) was sufficient to ensure optimal mixing of the culture and adequate exposure of the bacterial cells to the light. By using 8% of the digested sludge diluted in sterile deionized water, Rhodopseudomonas cells grew fast and stabilized at a value of 0.37 ± 0.01 g L⁻¹ of cell dry weight (CDW). Amounts higher than 8%–12% of digested sludge should be investigated to improve the process efficiency. PHB production started as soon as phosphate was limited in the culture and continued over time to a maximum concentration of 18.5 mg L⁻¹ and 5.2 ± 0.2% of CDW. This study demonstrated that digested sludge can be effectively used as a suitable feedstock for the cultivation of Rhodopseudomonas sp. S16-VOGS3.