Biotechnological Production of Polyhydroxyalkanoates from Agri food R residues: Sustainable Approaches

Polyhydroxyalkanoates (PHAs) are polyesters accumulated by specific microorganisms as cytoplasmic carbon storage granules; PHAs have plastic-like properties, are completely biodegradable and biocompatible, and thus are considered among the most promising alternatives to fossil plastics. Unfortunately, the replacement of conventional plastics with PHAs is limited by their high price, mainly due to the expensive raw materials used as a carbon source for microbial growth. To reduce costs, the production of PHAs should be based on available and cheap feedstocks such as food and crop residues. Cheese whey, slaughterhouse, and agro-food wastes are obtainable in large amounts as co-products of the dairy and meat industry and agriculture. Moreover, they can pose a threat to the environment if not disposed of correctly. Unfortunately, in nature, it is not common to find microbial strains capable of both efficiently metabolizing the complex carbon sources contained in residual biomasses and accumulating PHAs at high yields. For example, Cupriavidus necator DSM 545, one of the most proficient PHAs accumulators, cannot grow on such substrates. Among the possible strategies to bypass this problem, in the last 15 years, the Microbiology group of DAFNAE pursued the following approaches: A) C. necator DSM 545 was engineered to acquire the ability to grow on complex substrates. With this purpose: i) lacZY genes from Escherichia coli or lipH and lipC genes from Pseudomonas stutzeri BT3 were cloned into C. necator DSM 545 and Delftia acidovorans. The resulting recombinant strains were able to grow and accumulate PHAs from wastes containing lactose or lipids. ii) specific amylolytic genes, such as the glucodextranase G1d from Arthrobacter globiformis I42 -amylase amyZ from Zunongwangia profunda SM-A87, were co-expressed into C. necator DSM 545. The selected recombinant C. necator DSM 545 showed high hydrolytic activity on starch and, for the first time, demonstrated the one-step processing of starchy broken rice and sweet potato waste into PHAs; B) the acidogenesis phase of the anaerobic digestion was exploited as an efficient hydrolysis step to convert starchy substrates into volatile fatty acids (VFAs), to be then used as a carbon source by C. necator DSM 545 to both grow and store PHAs; C) the gaseous effluent, containing a mixture of H2 and CO2, and the liquid stream, rich in volatile fatty acids (VFAs), originating from an acetogenic reactor fed with agro-food wastes, were efficiently used by C. necator DSM 545 to accumulate PHAs under autotrophic and heterotrophic conditions, respectively. Although future research is necessary to improve yields, the obtained outcomes pave the way to future microbiological and biotechnological solutions to process organic waste into PHAs at large scale. References [1] S. Povolo et al. Bioresource Technology 101 (2010) 7902. [2] S. Brojanigo et al. Science of The Total Environment 825 (2022) 153931. [3] S. Brojanigo et al. Bioresource Technology, 347 (2022) 126383. Acknowledgement This work was partially supported by the University of Padova with the following research projects BIRD234877/23, DOR2352129, DOR2251254/22;