Polyhydroxyalkanoates (PHA) constitute a group of microbial biopolyesters with important ecosystem functions and a high biotechnological potential. During the past decade, the rapid development of new molecular and microscopic techniques resulted in novel insights into the ecology of PHA‐producing bacteria in aquatic and terrestrial microenvironments. Ecosystems showing fluctuating availability of carbon or transient limitation of essential nutrients, e.g. the rhizosphere of plants or estuarine sediments, contain a broad number of various PHA producers. PHA‐producing microorganisms show a widespread phylogenetic diversity and are often characterized by a symbiotic or syntrophic life style. PHA are already produced commercially in large‐scale fermentation. However, they have to compete economically with petrol‐based polymers. Hence, the development of low‐cost production strategies on the basis of diverse renewable materials is a crucial challenge. Ecological knowledge is required for these developments, which links both parts of the review together. The article highlights how a better understanding of the ecology of PHA‐producing microorganisms can lead to a broader application of microbial biopolymers on the basis of sustainable production processes. These processes have to be evaluated by means of life cycle assessment and Cleaner Production studies prior to their industrial implementation.
Polyhydroxyalkanoates are energy reserve polymers produced by bacteria to survive periods of starvation in natural habitats. Little is known about the ecology of polyhydroxyalkanoate-producing bacteria. To analyse the occurrence of this specific group on/in seven different plant species, a combined strategy containing culture-dependent and -independent methods was applied. Using microbial fingerprint techniques (single-strand conformation polymorphism analysis with specific primers for phaC gene encoding the key enzyme of the polyhydroxyalkanoate synthesis), a high number of bands were especially found for the rhizosphere. Furthermore, cluster analysis revealed plant species-specific communities. Isolation of bacteria, recognition of brightly refractile cytoplasmatic inclusions, lipophilic stainings and a PCR strategy targeted on the phaC gene were used as a culture-dependent strategy for the detection of polyhydroxyalkanoate-producing bacteria. Results again represent a high degree of plant specificity: the rhizosphere of sugar beet contained the highest number of positive strains. This was confirmed by quantitative PCR: the relative copy number of phaC was statistically and significantly enhanced in all rhizospheres in comparison with bulk soil. New polyhydroxyalkanoate-producing bacterial species were detected: for example, Burkholderia terricola, Lysobacter gummosus, Pseudomonas extremaustralis, Pseudomonas brassicacearum and Pseudomonas orientalis. Our results confirm the hypothesis that the rhizosphere is an interesting hidden reservoir for polyhydroxyalkanoate producers.
Members of the genus Burkholderia are highly versatile bacteria that can be beneficial as well as pathogenic for their eukaryotic hosts. Furthermore, many strains exhibit a remarkable biotechnological potential. To study the ecosystem function and lifestyle of B. terricola, we analysed the interactions with plants and survival in soil as well as the mechanisms behind it. We used a combination of in vitro and ad planta assays to study Burkholderia-plant interaction and assess the role of poly-β-hydroxybutyrate (PHB). Additionally, DsRedlabelled bacteria were analysed by confocal laser scanning microscopy (CLSM) to study root colonisation. B. terricola ZR2-12 treatment resulted in enhanced growth of sugar beet plants with a more than doubled biomass relative to the non-treated control. The strain was a remarkable good root coloniser, which was found in rhizosphere as well as endorhiza of sugar beet up to 10 log 10 CFU g −1 .Using CLSM, we observed that ZR2-12 cells form large micro-colonies along the apoplastic spaces of the root. Xylem vessels were colonised by smaller aggregates and single cells, whereas in root tips mainly single cells were present. The colonisation patterns differed strongly between older and younger parts of the roots. PHB production of ZR2-12 (up to 70% (w/w) of cell dry mass) provided a competitive advantage for rhizosphere colonisation. B. terricola ZR2-12 belongs to the plant-associated Burkholderia cluster with biotechnological potential due to its excellent root colonisation and plant growth promotion.
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