Ability of the phototrophic bacterium Rhodospirillum rubrum to produce various poly (beta-hydroxyalkanoates): potential sources for biodegradable polyesters.

Studies have been carried out in order to optimize growth and culture conditions for the intracellular formation of poly(beta-hydroxyalkanoates) (PHA) in the phototrophic, purple, non-sulphur bacterium Rhodospirilum rubrum. Its potential to produce novel copolymers was investigated. Recently, it has become of industrial interest to evaluate these polyesters as potentially biodegradable plastics for a wide range of possible applications. On an industrial scale, the use of photosynthetic bacteria could harness sunlight as an energy source for the production of these materials. R. rubrum was grown anaerobically in the light on different linear and branched beta-hydroxycarboxylic acids and various n-alkanoic acids. Under nitrogen-limiting conditions a PHA content of up to 45% of cellular dry weight was detected. When R. rubrum was grown on different concentrations of various n-alkanoic acids, intracellular PHA production was detected on all acids used. In most of the cases, the storage polymer contained beta-hydroxybutyrate (HB) and beta-hydroxyvalerate (HV) monomer units. Grown on n-alkanoic acids with a chain length of four carbon atoms and more, R. rubrum produced a copolymer containing the beta-hydroxyhexanoate (HC) repeating unit in addition to the HB and HV monomer. Using beta-hydroxyheptanoic acid as the carbon source, a polyester which contained HB, HV, HC, and beta-hydroxyheptanoate was formed. These copolyesters represent a novel class of biodegradable thermoplastics. The results demonstrate the metabolic flexibility of R. rubrum to form many different types of polyesters which might substitute plastics synthesized from petrochemicals.