Post-consumer plastics degraded in this study. A) From left to right: LDPE plastic grocery bag, HDPE milk jug, LDPE laboratory squeeze bottle, Pacific gyre waste collected from Santa Catalina Island, California. B) The distribution of diacid products after post-consumer polyethylene waste degradation using the optimized reaction. Credit: Rabot, et al.
Polyethylenes are the most commonly used plastics but are difficult to recycle, which has resulted in millions of tons of polyethylene waste ending up in the environment, including in ocean waters. Upcycling strategies that efficiently convert plastic products into valuable chemicals could aid in the management of polyethylene waste, with genetically engineered microorganisms being a promising route for conversion. Researchers at the University of Kansas have now engineered the soil fungus Aspergillus nidulans to upcycle polyethylene products into pharmacologically active compounds at unprecedented speed.
The first step in the upcycling process is to break polyethylene into diacids using an oxidative process, which was achieved using N-hydroxyphthalimide (NHPI), cobalt and manganese catalysts. The resulting diacids, containing long carbon chains (C10 and C12), were then “fed” to A. nidulans to be digested into valuable secondary metabolites. The metabolism of A. nidulans has previously been used to produce pharmaceutical components for antibiotics, cholesterol medications, immunosuppressants and antifungals, and the researchers hypothesized that the fungus could use the diacids from polyethylene as a carbon source to generate such products. The team leveraged previous genetic sequencing data and gene-targeting techniques developed for the fungus to enhance the expression of biosynthetic gene clusters for boosted production of the desired metabolites.
The team conducted their experiments using various sources of plastic waste, including a plastic grocery bag, milk jug, laboratory squeeze bottle and plastics collected from the Pacific Ocean. The researchers found that the fungus was incredibly efficient, converting 42% of the diacid mass introduced into the culture into valuable final products, including asperbenzaldehyde, citreoviridin and mutilin. Additionally, the digestion process was rapid, producing the final compounds within a week, according to study co-author Berl Oakley. Previous attempts to use fungus digest plastic products took months to yield a final product. This research was published in Angewandte Chemie.
“If our technique was a car, it would be doing 200 miles per hour, getting 60 miles per gallon, and would run on reclaimed cooking oil,” said Oakley about the “bizarrely” efficient process.
The researchers hope to eventually apply fungal upcycling techniques to all plastics, making recycling easier by removing the need for sorting.