Characterisation of open, mixed microbial cultures for polyhydroxyalkanoate (PHA) production

Viable processes have emerged for the production of polyhydroxyalkanoates (PHAs) via open, mixed microbial cultures (MMC). MMC PHA production is increasingly integrated into process technologies for the biological management of organic waste residuals and wastewaters. Herein, the literature has been systematically and comprehensively reviewed in order to establish the current knowledge about the characterisation of microbial communities engineered for PHA production. Despite the numerous descriptions of the microbial structure of PHA-storing MMC, the microorganisms actively storing PHAs are rarely identified. The microbial community structure and population dynamics in both the biomass enrichment and the PHA accumulation process have not been equally analysed. PHA-storing bacteria have been identified predominantly within the classes of Alphaproteobacteria, Betaproteobacteria and Gammaproteobacteria. Molecular techniques and microscopy have been preferentially used to characterise microbial diversity, abundance and activity in PHA-storing MMC. Fluorescence in situ hybridization remains relevant for the spatial identification and quantification of PHA-storing bacteria, and in tandem with Nile blue A staining helps identify actively PHA-storing bacteria. 16S rRNA-based fingerprinting is useful for describing dynamics in microbial community structure rather than diversity, and new generation sequencing provides a high-throughput characterisation of microbial diversity, dynamics and relative abundance. The dominant bacteria and their dynamics in bioreactors enriching for MMC with PHA-storage capacity and actively accumulating PHA are further covered with respect to the operating conditions used to select microbial communities for the PHA-storage phenotype. The specification of the PHA-storage phenotype and metabolic activity in identified bacteria remains a challenge in the study of PHA-storing MMC.