Light-enhanced polyhydroxyalkanoate accumulation in the aerobic anoxygenic phototroph Roseibium alexandrii MAR44

Marine phototrophic bacteria represent promising platforms for sustainable microbial biopolymer production. Here, we report the screening and functional characterization of MAR44, a Mediterranean isolate identified as the aerobic anoxygenic phototrophic bacterium (AAPB) Roseibium alexandrii. This strain carries a complete set of genes for polyhydroxyalkanoate (PHA) metabolism, including phaC, phaAB, phaR, phaZ and multiple phaP homologues. Phenotypic screening confirmed substantial intracellular PHA accumulation, and gravimetric determination revealed intracellular polymer contents of up to approximately 50% of cell dry weight. The purified polymer exhibited the characteristic FTIR absorption bands and ¹H NMR signals associated with PHAs. A key finding is that illumination significantly (p < 0.05) increased glucose-induced intracellular PHA accumulation. Under continuous illumination and long-day photoperiods, MAR44 showed higher mean intracellular PHA accumulation levels, with increases of up to approximately two-fold compared with short-day conditions. These results suggest an association between illumination and enhanced intracellular PHA accumulation in this AAPB, although the underlying mechanisms remain to be elucidated. This study also introduces MAR44 as a promising candidate for further research into light-supported biopolymer production, expanding the current set of organisms beyond traditional soil-derived producers such as Cupriavidus necator.