Effect of PHA on Viability and Stress Resilience in <i>Rhodospirillum rubrum</i>

AIMS: Purple bacteria possess considerable biotechnological potential due to their versatile metabolism, enabling the production of valuable biopolymers such as polyhydroxyalkanoates (PHAs). PHAs are biodegradable biopolyesters that serve in microorganisms as intracellular carbon and energy sources and can also function as protective compounds against environmental stress. This study focuses on Rhodospirillum rubrum, a purple non-sulphur bacterium known for its metabolic flexibility and ability to accumulate PHAs. METHODS AND RESULTS: A wild-type strain (WT) Rhodospirillum rubrum (DSM 467) and a ∆phaC1∆phaC2 mutant strain incapable of PHA biosynthesis were investigated. Both strains were cultivated under chemoheterotrophic conditions with acetate as sole carbon source prior to stress exposure selected stressors such as elevated temperature, osmotic stress, and repeated freeze-thaw cycles. Cell viability was determined by flow cytometry, while morphological changes were observed using electron microscopy. Thus, we could demonstrate that PHA granules contribute to stress resilience under both freeze cycles and high osmotic pressure conditions, but not to enhanced temperatures. CONCLUSIONS: PHA plays an important role in maintaining cellular integrity of the wild-type strain during acute osmotic shock. In contrast, the mutant ΔphaC1ΔphaC2 strain exhibited pronounced structural damage, including loss of intracellular organization and release of chromatophores into the extracellular space, highlighting the importance of PHA metabolism for osmotic stress resilience. In contrast to other studies, PHA did not provide a protective effect against high or low temperature in R. rubrum wild-type and mutant strains.