Clostridium thermocellum, a heat-resistant microbe found in leaf and branch compost piles, can degrade 60% of polyethylene terephthalate, a plastic used in packaging and drink bottles, in 14 days.
Plastic pollution is a global concern, with 8300 million tons estimated to be produced by humankind since 1950, of which 79 percent are simply landfilled or carelessly disposed in nature. Polyethylene terephthalate (PET), a mass-produced synthetic polyester used to manufacture clothing and disposable consumer products, is notoriously difficult to degrade, with only a few hydrolytic enzymes reported to catalyse the polymer chain breakdown at modest efficiencies.
In a breakthrough, researchers at Qingdao Institute of Bioenergy and Bioprocess Technology (QIBEBT) of the Chinese Academy of Sciences, led by Associate Professor Liu Yajun, genetically engineered a thermophilic anaerobic bacterium, Clostridium thermocellum (C. thermocellum), that would degrade PET more efficiently than current industry methods. The findings were published as a special issue article in Microbial Biotechnology.
Recycling facilities that biodegrade PET typically deploy microbes that require oxygen and lower temperatures, making the mechanics of the process more energy-intensive and expensive. In contrast, C. thermocellum could thrive in hot, oxygen-free environments, in addition to its ability to break down plant-based fibers. “Because C. thermocellum can naturally degrade cellulose efficiently, the C. thermocellum-based strategy is expected to demonstrate great potential for application in the bio-recycling of mixed textile waste containing both cellulose and polyester fractions,” said Liu.
The team first obtained microbes that would thrive above 60°C – a temperature ideal for degrading PET into its component compounds. They then decided to use C. thermocellum, which are abundantly found inside leaf and branch compost piles in nature. The scientists submerged a thin film of PET (approximately 50 mg) in a solution rich with C. thermocellum. The test vials were kept at 60°C for 14 days, at which point more than 60 percent of the PET had broken down, indicating a markedly higher degradation performance than previously reported results.
Current processes take about six weeks to completely degrade PET, and require energy-intensive inputs such as huge electric motors to agitate the recycling vessels and inject air bubbles to maintain temperature and oxygen levels. “If further refined and adopted globally, this new process has the potential to save PET recycling energy, time and money, as well as divert a greater percentage of PET away from landfills and oceans,” Liu enthused. [APBN]