Polyhydroxyalkanoates (PHA) are a family of biodegradable polyesters naturally synthesised by some bacteria and archaea. PHA have high industrial value as bioplastics for packaging and biomedical applications. However, their broader use is hindered by high production costs and uncontrolled variation of polymer properties. The extreme halophile Haloferax mediterranei shows bioprocess advantages that can be exploited for the low cost production of the PHA copolymer Poly(3-hydroxbutyrate-co-3-hydroxyvalterate) (PHBV). The focus of this thesis is to identify process variables responsible for the uncontrolled variation of PHA properties in order to progress towards the robust production of PHBV using H. mediterranei. The outcome of the investigation is a novel cultivation strategy for the reliable synthesis of PHBV copolymers with controlled composition and microstructure showing minor differences in material characteristics. Initially, growth kinetics and PHBV synthesis were characterised under nitrogen-excess and nitrogen-limiting conditions in ammonium and for the first time, nitrate. The nitrogen source and concentration influenced PHBV accumulation and variations in polymer composition were observed with ammonium, highlighting the importance of the control of cultivation conditions. Volatile fatty acids (VFA) were found to be a more direct approach to determine PHBV composition and for the first time were used as substrates in H. mediterranei cultures. When the cells were grown in C4:0/C5:0 mixtures, the 3HV fraction in the PHBV was proportional to the percentage of C5:0 in the feed mixture, allowing the synthesis of copolymers with a predefined composition ranging from pure PHB to pure PHV. The cultivation strategy proved effective for the synthesis of HV rich PHBV, which is not easily obtained due to the 3HV precursor toxicity. The polymer microstructure was controlled using different feeding strategies: co-feeding generated random copolymers, while sequential feeding created block and blend copolymers. The synthesis of block copolymers is of interest because the materials show enhanced yield strength and mechanical strength, making such materials more suitable for commodity uses. Bespoke random, block, and blend copolymers with 0â100 mol% 3HV were synthesized and their thermal and mechanical properties studied. Lastly, high temperature cultivation and several surfactants were tested to enhance the production of bespoke PHBV from VFA. PHBV productivity and accumulation was greatly improved in a fed-batch bioreactor fermentation at 37°C with Tween-80 and the maximum PHBV content 58.9% was obtained. The polymers from shake-flasks and from bioreactors showed minor variations in their material properties, demonstrating the scalability and the robustness of the process developed. Further understanding of the different process variables affecting polymer synthesis and composition was gained in this thesis. It is now possible to produce PHBV with controllable composition, microstructure and minor differences in material characteristics. The novel and robust production strategy developed address the bioprocess challenge of minimising the uncontrolled variation of polymer characteristics that is currently hindering the wider use of PHA hence allowing the production of high quality polymers for commodity goods, packaging and biomedical applications.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:764624 |
| Date | January 2018 |
| Creators | Ferré, Anna |
| Contributors | Webb, Colin ; Winterburn, James |
| Publisher | University of Manchester |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | https://www.research.manchester.ac.uk/portal/en/theses/process-development-for-the-robust-production-of-polyhydroxyalkanoates(91fd1ca5-b907-4061-b232-99528b0862c0).html |
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