Biorefrineries for a Sustainable Plastic Value Chain

Plastic pollution is a major environmental challenge because most plastics are not biodegradable and strategies to prevent accumulation in ecosystems have been insufficient. Biorefineries for plastic waste upcycling offer a solution by diversifying the portfolio of products made from plastic waste and enabling a circular economy to disrupt the plastics value chain. To make this possible, the biorefinery would require the breakdown of plastic polymers and the subsequent bioconversion by microbial cell factories. This research proposes a hybrid thermal and biochemical process to upcycle polyethylene (PE), the most common type of plastic. The process combines thermal oxo-degradation (TOD) to transform PE into viable carbon feedstock for the nonconventional yeast Candida maltosa. This yeast was selected for microbial cell factory development from a screening of 20 species against model compounds representative of the feedstock and it grew from PE feedstock at rates comparable to glucose. The proof-of-concept results were remarkable, considering that the PE carbon feedstock is insoluble in water and solid. Adaptive Laboratory Evolution increased the growth rate by >200% and provided insights into the uptake mechanisms used by C. maltosa to overcome the mass transfer limitations in liquid culture. Characterizing the parent and evolved phenotypes identified canal structures formed for hydrophobic feedstock utilization, an increase in the secretion of biosurfactants and their ability to solubilize hydrophobic molecules instead of emulsifying them. These findings may inform rational strain engineering strategies to optimize industrial strains for plastic biorefineries and refineries using other hydrophobic feedstocks. In the meantime, this work is entering the metabolic engineering phase with the goal of engineering C. maltosa to produce value-added chemicals from PE waste.