Innovation in Plastic End-of-Life Management – “Plastic Eating” Bacteria Reveal PET Decomposition Enzymes
The use of plastics has been markedly increasing since the 1960s. In the United States (U.S.), plastic generation has increased from 0.39 to 35 million tons between 1960 and 2017. According to the U.S. Environmental Protection Agency (EPA), most of the plastics generated are directed to landfills at end-of-life (Figure 1A). In 2017, 26.8 million tons of plastics were landfilled (representing 75.8% of plastics generated), whereas only 2.96 million tons of plastics were recycled (representing 8.4% of plastics generated). Consequently, plastics are a rapidly increasing segment of municipal solid waste (EPA, 2019a). To put these numbers in a global context, annual plastics production worldwide was estimated at 381 million tons in 2015 (Figure 1B); recycling and disposal accounted for 19.5% and 55.0%, respectively. Notably, plastic containers and packaging were the main category of plastic products in the U.S. and globally (EPA, 2019b; Ritchie and Roser, 2018).
Figure 1. A. Generation and disposal of plastics in the U.S. between 1960 and 2017 [based on EPA (2019a)]. B. Generation of plastics worldwide between 1950 and 2015 [based on Ritchie and Roser (2018)].
Given the large amount of plastic waste produced globally, responsible plastic use and waste management have become a global issue. It has been estimated that the 192 countries with a coast bordering the Atlantic, Pacific, and Indian oceans, and the Mediterranean and Black seas produced 2.5 billion metric tons of solid waste, of which 275 million metric tons was plastic, and of which 4.8 to 12.7 million metric tons was mismanaged plastic waste that entered the ocean in 2010 (Jambeck et al., 2015). Further, it has been estimated that over a quarter of a million tons of plastic pieces were floating in the oceans (Eriksen et al., 2014). It is noteworthy that in 2017, the U.S. EPA highlighted literature suggesting that approximately 90% of plastics in the pelagic marine environment are microplastics (MPs), which are particles of less than 5 mm in diameter (EPA, 2017). MPs have also been detected in freshwater, sediment, soil, air, and even foodstuff (e.g. beer, tap water) (Prata et al., 2020), although work is underway to improve standardization in MP counting and detection. MPs are a complex group of environmental contaminants, which are formed through degradation (photolysis, thermooxidative environmental contaminants, which are formed through degradation (photolysis, thermooxidative processes, and biodegradation) of larger plastic pieces (Andrady, 2011). MPs can also originate from intentional use in cosmetics (e.g. exfoliants) or by industries (e.g. air blasting) (Prata et al., 2020).
Plastic end-of-life recovery and management (including MPs) approaches reflect complex environmental considerations, and efforts are currently underway to 1) reduce plastic use, 2) increase plastic recycling, and 3) remove plastic debris from oceans. Additionally, efforts are underway to invent environmentally-friendly strategies to break down plastics. One such example is the discovery of synthetic plastic-decomposing bacteria. In 2016, “plastic-eating” bacteria were discovered at a Japanese waste site, and in 2018, researchers engineered the first enzyme capable of breaking down plastic [polyethylene terephthalate (PET)]; the enzyme PETase was shown to break down plastic materials in just a few days (The Guardian, 2020). In September, a study by Knott et al. (2020) described a two-enzyme system for plastics depolymerization in an article published in the Proceedings of the National Academy of Sciences (PNAS) journal. The study built on the previous research performed on PETase; the new two-enzyme system (PETase-MHETase) is capable of breaking down plastic six times faster than prior enzyme systems. This enhanced performance is achieved by novel coordination of the two enzymes, which work together to first solubilize, then decompose the polymer chain. This new system could prove essential for plastics upcycling, especially in relation to waste originating from plastic water bottles (many of which are made from PET). It is also anticipated that such strategies could help reduce the amount of mismanaged waste, and ultimately the extent of plastic pollution (including the prevalence of MPs).
Cardno ChemRisk scientists have extensive professional experience evaluating the possible hazards posed by chemicals, including microplastics. For more information on Cardno ChemRisk's capabilities, please contact Dr. Andrey Massarsky, Sarah Brown, Dr. Jordan Kozal, or Kenneth Unice.