Isolation and evaluation of polyhdroxyalkanoate producing bacteria from mangrove soil

Abstract:

[Background] Bacteria can synthesize polyhydroxyalkanoates (PHA) as intracellular carbon and energy storage compounds in order to better adapt to the environment. In mangroves, the soil is periodically submerged by seawater, which forms a special habitat with abundant nutrients and fluctuating contents, providing conditions for bacteria to evolve a special PHA synthesis pathway. [Objective] In order to improve the understanding of PHA-producing bacterial resources, we isolated and identified the PHA-producing bacteria, and evaluated their ability to produce PHA. [Methods] We collected soil samples from shoal and Sonneratia caseolaris rhizosphere, isolated bacteria during the 5 weeks of incubation, identified them by 16S rRNA gene sequence similarity and phylogenetic tree, and determined the PHA producing ability by testing the PHA synthase gene (phaC). Based on the draft genome sequence data, we determined the class of phaC gene, the metabolic pathways and the phylogenetic relationship among different strains. We analyzed the PHA content and composition using gas chromatography. [Results] A total of 97 strains of bacteria were isolated, among which 13 strains were PHA-producing bacteria, including Cytobacillus firmus, Bacillus flexus, Marinobacter hydrocarbonoclasticus and Microbacterium esteraromaticum. B. flexus MN15-19 using pyruvate as the sole carbon source could accumulate 11% PHA per dry cell weight. This strain had reverse tricarboxylic acid cycle, which is considered as carbon fixation pathway. Therefore, based on B. flexus MN15-19, PHA production technique directly using CO₂ could be developed. Mi. esteraromaticum was a PHA-producing bacterium, whose phaC gene was too unique to be identified as any known phaC genes. [Conclusion] Bacteria isolated from mangrove ecosystem have unknown PHA synthase pathway, which indicates that mangrove bacterial biological resources deserve further study.