Title: Valorization of wastes via a one-step microbial fermentation process
Abstract:
Pollution from plastic waste represents one of the most relevant environmental problems that our society currently faces. Most conventional plastics are extremely recalcitrant, with estimated decomposition times of decades to centuries. However, the production of plastics increases every day due to the great versatility and performance of these polymers in their different applications. The main source of plastic pollution originates from product packaging. Only 10% of this plastic waste is currently recycled worldwide, 30% in developed countries. Current recycling systems are largely thermo-mechanical and are limited to polyethylene (PE), polyethylene terephthalate (PET) and polypropylene (PP). Biotechnology represents an interesting alternative to the treatment of plastics. In particular, plastic polyesters (whose bonds can be hydrolyzed by different types of enzymes; e.g. cutinases, lipases, proteases, etc.) are the most susceptible to being eliminated or transformed into other chemical products through biological processes. Among the most used polyesters, in addition to PET (8% of total plastics of any type of origin), are PBAT (polybutylene adipate co-terephthalate), PBS (polybutylene succinate), PCL (polycaprolactone), PLA (poly lactic acid) or PHA (polyhydroxyalkanoate). These polyesters are biodegradable to a greater or lesser extent (although their biodegradation processes are extremely slow in some cases and are limited to composting and anaerobic digestion). Previous work, carried out in our group, has revealed that the denitrifying bacteria, Paracoccus denitrificans, is capable of using as carbon and energy sources practically all of the monomers that constitute the most used commercial plastic polyesters, in addition to C1 compounds. Moreover, this bacterium can produce PHAs in a wide range of environmental conditions However, one of the main obstacles for using PHAs as commercial products is their high production cost when compared with conventional plastics. Any cost-competitive alternative for PHA production should display fast growth with cheap carbon sources and high conversion efficiency of substrate into product. These cheap carbon sources can be bioplastic wastes, thus allowing bio-recycling of mixed bioplastics. In this study, a method to directly produce PHAs via a one-step microbial process, using different genetic modified P. denitrificans strains and a wide variety of polyester polymers as a sole carbon and energy source, was developed. Mechanically and chemically pretreated polyester waste was also shown to be a suitable substrate for the production of PHA using P. denitrificans. Optimization of the medium by reducing the nitrogen concentration allowed PHA contents of 30 % of the cell biomass with some of the carbon sources tested. In summary, a direct and renewable method for producing PHAs from polyester waste was herein developed and validated.
