Polyhydroxyalkanoates: Basics, Production and Applications of Microbial Biopolyesters

Abstract

As a result of their useful material properties and the fact that their production and degradation is embedded into nature's closed carbon cycle, polyhydroxyalkanoates (PHAs) are attracting increasing attention as ‘green plastics’. Biologically, these polyesters of hydroxyalkanoates act as microbial reserve compounds for carbon and energy; from the plastic-industrial point of view, they exhibit the potential to replace their petrol-based competitors in several bulk and niche segments of the plastic market in the foreseeable future. Polyhydroxyalkanoates displaying various properties are accessible from renewable resources by the biosynthetic action of selected prokaryotes. This opens the door for substituting petrol-based thermoplasts, elastomers, latexes, and even high-performance, functional polymers by PHAs. To achieve a breakthrough on the market, these ‘green plastics’ have to compete with well established petrol-based polymers, not only in material performance but also in economic terms. Until now, PHA production started from prized substrates of high nutritional value like sugars and oils. Carbon-rich industrial surplus streams and byproducts can be applied as feedstock for many PHA-producing microbes; this makes PHAs economically competitive without interfering with human nutrition or animal feeding. As intracellular inclusions, PHAs have to be recovered from biomass after their biosynthesis. For efficient downstream processing, innovative and sustainable methods are scrutinized to recover PHAs from the surrounding microbial cell material at high yields and purity, with the aim of minimizing the requirements for chemicals, energy and time. Designing an adequate process configuration in terms of type of bioreactors and equipment, adapting the cultivation regime (batch versus continuous cultivation) to kinetic aspects, and triggering the substrate supply are decisive factors not only for PHA productivity but also for final material quality. Material features can be adjusted by fine tuning of the polyester composition during biosynthesis and by postsynthetic PHA modification by chemical or enzymatic means. An increasing number of scientists consider genetic engineering to be a powerful tool, firstly, to improve partial aspects of PHA biosynthesis and product quality, such as productivity, substrate conversion, microbial oxygen uptake, or molecular masses and, secondly, to facilitate downstream processing for product recovery. The production of blends of PHAs and compatible materials of natural or synthetic origin is of increasing significance. This opens the door for confecting composites of PHAs and compatible filler materials, which display enhanced performance during processing and a broader range of possible applications. This chapter illustrates contemporary strategies to make, on the one hand, PHA biopolyesters competitive in terms of costs and material performance and, on the other hand, it presents the wide spectrum of application of these future-oriented biopolymers. Finally, attempts, successes, and drawbacks in the realization of (semi)industrial PHA production in different global areas are recapitulated.