Polyhydroxyalkanoate (PHA) is a biopolyester/bioplastic that is produced by a variety of microorganisms to store carbon and increase reducing redox potential. Photosynthetic bacteria convert carbon dioxide into organic compounds using light energy and are known to accumulate PHA. We analyzed PHAs synthesized by 3 purple sulfur bacteria and 9 purple non-sulfur bacteria strains. These 12 purple bacteria were cultured in nitrogen-limited medium containing acetate and/or sodium bicarbonate as carbon sources. PHA production in the purple sulfur bacteria was induced by nitrogen-limited conditions. Purple non-sulfur bacteria accumulated PHA even under normal growth conditions, and PHA production in 3 strains was enhanced by nitrogen-limited conditions. Gel permeation chromatography analysis revealed that 5 photosynthetic purple bacteria synthesized high-molecular-weight PHAs, which are useful for industrial applications. Quantitative reverse transcription polymerase chain reaction analysis revealed that mRNA levels of phaC and PhaZ genes were low under nitrogen-limited conditions, resulting in production of high-molecular-weight PHAs. We conclude that all 12 tested strains are able to synthesize PHA to some degree, and we identify 5 photosynthetic purple bacteria that accumulate high-molecular-weight PHA molecules. Furthermore, the photosynthetic purple bacteria synthesized PHA when they were cultured in seawater supplemented with acetate. The photosynthetic purple bacteria strains characterized in this study should be useful as host microorganisms for large-scale PHA production utilizing abundant marine resources and carbon dioxide.

To convert lignin and lignin derivatives to biopolyesters, six types of marine bacterial strains were investigated with mineral salt media containing lignin and lignin derivatives. Our results showed that Oceanimonas doudoroffii, which was isolated from an area of the sea polluted with allantoin, directly synthesized polyhydroxyalkanoate (PHA) using lignin or several lignin derivatives as sole carbon sources. Based on the PHA accumulation data, the conversion from lignin to PHA can be carried out mainly via sinapinic acid and syringic acid rather than other lignin derivatives. This is the first study to show the direct microbial conversion from lignin to biopolyester, which will open the door for lignin-based biorefinery applications.

The production of poly(hydroxyalkanoate)s (PHAs) as ecofriendly bioplastics by various deep-sea bacteria (4 types of Colwellia spp., 11 types of Moritella spp., and 18 types of Shewanella spp.) from glucose, fructose, gluconate, or from one of the several plant oils as the sole source of carbon was examined at atmospheric pressure. Some of the deep-sea bacteria successfully accumulated PHAs that had a wide range of molecular weights and contained 3-hydroxybutyrate, 3-hydroxyvalerate, and the other hydroxyalkanoate units. Furthermore, with a plant oil as its sole source of carbon, Shewanella surugensis produced low-molecular weight oligomeric PHAs. These results provide important and basic information regarding the production of PHAs by deep-sea bacteria and on the diversity of PHA synthase enzymes in nature.

Polyhydroxyalkanoates (PHAs) are a family of biopolyesters accumulated by a variety of microorganisms as carbon and energy storage under starvation conditions. We focused on marine purple non-sulfur photosynthetic bacteria as host microorganisms for PHA production and developed a method for their isolation from natural seawater. To identify novel PHA-producing marine purple non-sulfur photosynthetic bacteria, natural seawaters were cultured in nutrient-rich medium for purple non-sulfur photosynthetic bacteria, and twelve pink- or red-pigmented colonies were picked up. Gas chromatography mass spectrometry analysis revealed that four isolates synthesized PHA at levels ranging from 0.5 to 24.4 wt% of cell dry weight. The 16S ribosomal RNA sequence analysis revealed that one isolate (HM2) showed 100% identity to marine purple non-sulfur photosynthetic bacteria. In conclusion, we have demonstrated in this study that PHA-producing marine purple non-sulfur photosynthetic bacteria can be isolated from natural seawater under nutrient-rich conditions.

Marine purple photosynthetic bacteria are ideal organisms for the production of useful materials at reduced costs and contributing to a sustainable society because they can utilize sunlight, seawater, and components of air, including carbon dioxide and nitrogen gases, for their growth. However, conjugation is the only applicable method for the transformation of marine purple photosynthetic bacteria so far. Here, we examined a calcium chloride-mediated method for the transformation of marine purple photosynthetic bacteria. Plasmid DNAs containing the kanamycin resistance gene were successfully transferred into chemically competent cells of two strains of marine purple photosynthetic bacteria (Rhodovulum sulfidophilum and Roseospira marina). Heat shock treatment increased the transformation efficiency in R. sulfidophilum, whereas the addition of cell-penetrating peptide did not improve it. We also found that prolonged incubation in agar plates containing kanamycin led to spontaneous mutation of the 16S rRNA, resulting in kanamycin resistance in R. marina. Thus, we developed an efficient and facile transformation method using chemically competent cells of marine purple photosynthetic bacteria with calcium chloride.