A Review on Established and Emerging Fermentation Schemes for Microbial Production of Polyhydroxyalkanoate (PHA) Biopolyesters

Abstract

Polyhydroxyalkanoates (PHA) are microbial biopolyesters utilized as "green plastics". Their production under controlled conditions resorts to bioreactors operated in different modes. Because PHA biosynthesis constitutes a multiphase process, both feeding strategy and bioreactor operation mode need smart adaptation. Traditional PHA production setups based on batch, repeated batch, fed-batch or cyclic fed-batch processes are often limited in productivity, or display insufficient controllability of polyester composition. For highly diluted substrate streams like is the case of (agro) industrial waste streams, fed-batch enhanced by cell recycling has recently been reported as a viable tool to increase volumetric productivity. As an emerging trend, continuous fermentation processes in single-, two-and multi-stage setups are reported, which bring the kinetics of both microbial growth and PHA accumulation into agreement with process engineering and allow tailoring PHA's molecular structure. Moreover, we currently witness an increasing number of CO 2-based PHA production processes using cyanobacteria; these light-driven processes resort to photobioreactors similar to those used for microalgae cultivation and can be operated both discontinuously and continuously. This development is parallel to the emerging use of methane and syngas as abundantly available gaseous substrates, which also calls for bioreactor systems with optimized gas transfer. The review sheds light on the challenges of diverse PHA production processes in different bioreactor types and operational regimes using miscellaneous microbial production strains such as extremophilic Archaea, chemoheterotrophic eubacteria and phototrophic cyanobacteria. Particular emphasis is dedicated to the limitations and promises of different bioreactor-strain combinations and to efforts devoted to upscaling these processes to industrially relevant scales.

Switching from petro-plastics to microbial polyhydroxyalkanoates (PHA): the biotechnological escape route of choice out of the plastic predicament?

“…PHA production under controlled conditions, especially on a larger scale, is typically carried out in bioreactors (77,78); such bioreactors (previously also known as "fermenters") can be operated in different modes, such as batch (substrate feed only at the beginning, product harvest only at the end of the The EuroBiotech Journal process), fed-batch (repeated substrate supply according to substrate analysis, harvest only at the end), and continuous (permanent supply of substrates and products harvest) setups. In this context, PHA biosynthesis has to be understood as a multiphase process, because product (PHA) accumulation as a intracellular secondary metabolite is typically not coupled to the preceding phase of microbial growth (formation of active biomass).…”

“…Therefore, both feeding strategy and bioreactor operation mode need to be carefully adapted to the selected production strain, substrate, and kinetics of growth and product formation (79). Traditional PHA production setups operated in batch (80), repeated batch ("fill-and-empty" or "drain-and-fill" cultivation) (81), fed-batch (82), or cyclic fed-batch mode (83) often show unsatisfactory control of PHA composition, and are often characterized by modest volumetric productivity (77,78).…”

“…The above discussed use of (agro)industrial waste streams as feed streams, which often contain the carbon sources in a considerably diluted form, makes the operation in batch-or fed-batch mode impracticable; each re-feed drastically increases the volume of the bioreactor content (78). In such cases, fedbatch feeding processes coupled to cell recycling were recently developed, where a membrane module is directly coupled with the bioreactor due retain active biomass, but to discard substrate-poor fermentation broth; promising volumetric PHA productivities are reported for such setups, which also avoids an accumulation of inhibiting compounds in the fermentation broth (84).…”

Chemical and Biochemical Engineering Approaches in Manufacturing Polyhydroxyalkanoate (PHA) Biopolyesters of Tailored Structure with Focus on the Diversity of Building Blocks

“…[42][43][44][45][46] Moreover, the use of different bioreactor systems, operated continuously or discontinuously, was exhaustively reviewed recently, indicating that bioreactor setup, process regime, and feeding strategy have major impact on PHA productivity, composition, and microstructure. 47 F i g . 1 -Electron microscope image of PHA-rich C. necator cells.…”

Advances in Polyhydroxyalkanoate (PHA) Production, Volume 2

“…Still, experts in this field emphasize that sustainable and efficient PHA production requires the understanding and optimization of all individual process steps [2]. The holistic improvement of PHA production, applicable also on an industrial scale, inter alia calls for: optimized bioprocess engineering and adapted fermentation modes [3], consolidated knowledge about the enzymatic, metabolic, and genetic ongoings in PHA accumulating organisms in the context of "Next Generation Industrial Biotechnology" [4], the multi-facetted role of PHA granules in living cells and the impact of environmental stress factors on PHA formation [5], an in-depth understanding of the kinetics of the bioprocess [6], the selection of ethically clear, inexpensive feedstocks [7,8], tailoring the composition of PHA on the level of the monomeric constituents [9], and efficient and ecologically benign strategies for PHA recovery from biomass [10].…”