Photofermentative production of poly-β-hydroxybutyrate (PHB) by purple non-sulfur bacteria using olive oil by-products

This study evaluated the ability of six purple non-sulfur bacteria (PNSB) to convert olive oil by-products into poly-β-hydroxybutyrate (PHB). Strains were first independently cultivated in synthetic media with different carbon sources (acetic, lactic and malic acid) to assess their physiology and PHB production. Subsequently, their growth and PHB production using ingested pâté olive cake (IPOC) as a substrate were investigated. Transmission electron microscopy (TEM) observations were conducted on strains cultivated on IPOC to investigate their cell morphologies and inclusion bodies presence and size. Rhodopseudomonas palustris strains accumulated up to 6.8% w PHB/w cells with acetate and 0.86% w PHB/w cells with a daily productivity of 0.54 mg PHB L⁻¹ culture d⁻¹ on IPOC. In contrast, Cereibacter johrii and Cereibacter sphaeroides reached 58.64% w PHB/w cells and 65.45% w PHB/w cells with acetate, respectively, while C. sphaeroides achieved 21.48% w PHB/w cells and a daily productivity of 10.85 mg PHB L⁻¹ culture d⁻¹ when cultivated on IPOC. All strains exhibited growth and PHB accumulation in both synthetic media and IPOC substrate. Specifically, R. palustris strains 42OL, AV33 and CGA009 displayed growth capability in all substrates, while C. johrii strains 9Cis and PISA 7, and C. sphaeroides F17 showed promising PHB synthesis capabilities. TEM observations revealed that R. palustris strains, with smaller cell and inclusion body sizes, exhibited lower PHB accumulations, while C. johrii and C. sphaeroides strains, characterized by larger cells and inclusion bodies, demonstrated higher PHB production, recognizing them as promising candidates for PHB production using olive oil by-products. Further investigations under laboratory-scale conditions will be necessary to optimize operating parameters and develop integrated strategies for simultaneous PHB synthesis and the co-production of value-added products, thereby enhancing the economic feasibility of the process within a biorefinery framework.