Doctoral Research Project : Sustainable production of bacterial polymers

The persistence of plastic pollution, and dependence on fossil fuels call for innovative solutions. The current thesis explored circular and bioeconomy strategies, with a particular focus on biodegradable and biocompatible plastics, such as Polyhydroxyalkanoates (PHAs). The research aimed to optimise PHA production throughout the entire lifecycle, from upstream to downstream processes, and end-of-life phase. Firstly, PHA production by Pseudomonas mendocina CH50 was enhanced by screening low-cost carbon substrates and using the Plackett-Burman Design. This led to two novel mcl-PHA copolymers, P(3HO-co-3HD) (29% dcw) and P(3HO-co-3HD-co-3HDD) (62% dcw), produced using saponified sunflower oil with fructose, and coconut oil respectively, which displayed properties comparable to thermoplastic polymers. Moreover, the study investigated the ability of an unexplored extremophile bacterium, named Pseudohalocynthiibacter aestuariivivens P96, to accumulate 78% dcw of P(3HB). This research, published in collaboration with Stazione Zoologica A. Dohrn, marks a significant step towards commercialising PHAs' use of extremophiles, addressing challenges related to contamination prevention procedures, wastewater treatment costs, and low productivity. To promote environmental sustainability, the study investigated green methods for extracting P(3HB) and P(3HO-co-3HD-co-3HDD). Dispersion extraction proved to be the most eco-friendly method, with dimethyl carbonate emerging as an excellent non-halogenated solvent, yielding 41% dcw of P(3HO-co-3HD-co-3HDD) and demonstrating good solvent recovery. However, chloroform emerged as the only solvent capable of extracting P(3HB), contrary to prevailing literature. Noteworthy, a novel biological extraction method was developed, using Myxococcus xanthus as a predatory bacterium to lyse P. aestuariivivens 96 cells, though further improvements are still needed for optimal polymer recovery. Finally, biodegradability testing was conducted on P(3HB), P(3HO-co-3HD-co-3HDD), and two control materials, PLLA and PET, under marine aerobic and anaerobic conditions, involving AD and MEC-AD reactors. The results showcased promising marine biodegradability for P(3HO-co-3HD-co-3HDD), but anaerobic conditions posed a challenge. In contrast, P(3HB) demonstrated slower marine biodegradability yet decomposed in less than 8 weeks under anaerobic conditions, signifying its potential for food packaging. Although PLLA did not degrade within 8 weeks, UV pre-treatment exhibited promise in accelerating its anaerobic decomposition, hinting at potential future industrial applications