The widespread use of plastic materials has resulted in a staggering accumulation of plastic waste, projected to reach approximately 33 billion tons by 2050. This alarming trend has led to the presence of plastic waste in various environments, including marine and freshwater ecosystems, sediments, and soils.
Microplastics and nanoplastics, defined as plastic fragments smaller than 5 mm and 1 μm, respectively, pose a significant threat to both aquatic and terrestrial ecosystems due to the ease with which organisms can ingest these tiny particles.
Now, Northwestern-led researchers have made a groundbreaking discovery regarding the breakdown of plastic by cells of a Comamonas bacterium. These cells first chew the plastic into nanoplastics before secreting a specialized enzyme to further break down the material. The bacteria then utilize a ring of carbon atoms from the plastic as a food source.
“We have systematically shown, for the first time, that a wastewater bacterium can take a starting plastic material, deteriorate it, fragment it, break it down, and use it as a source of carbon,” said Northwestern‘s Ludmilla Aristilde, who led the study, which was published in the journal Environmental Science & Technology. “It is amazing that this bacterium can perform that entire process, and we identified a key enzyme responsible for breaking down the plastic materials.”
The discovery paves the way for innovative bacteria-based engineering solutions to combat the pervasive issue of plastic waste. This type of waste poses a significant threat to our environment, contaminating drinking water and endangering wildlife.
Building upon previous research, Aristilde’s team has delved into the mechanisms that empower Comamonas testosteroni to metabolize simple carbons derived from decomposed plants and plastics. Their latest study focuses on C. testosteroni’s ability to thrive on polyethylene terephthalate (PET), a resilient type of plastic commonly found in food packaging and beverage bottles. Given PET’s resistance to degradation, it remains a major contributor to plastic pollution.
“PET plastics represent 12% of total global plastics usage,” Aristilde said. “And it accounts for up to 50% of microplastics in wastewaters.”
To gain a deeper understanding of how C. testosteroni interacts with and consumes plastic, Aristilde and her team employed a range of theoretical and experimental methods. They cultivated the bacteria on PET films and pellets, carefully observing the changes in the plastic material over time. Additionally, they analyzed the surrounding water for signs of plastic degradation into smaller nano-sized particles. Furthermore, the researchers delved inside the bacteria to identify the mechanisms employed in the breakdown of PET.
“In the presence of the bacterium, the microplastics were broken down into tiny nanoparticles of plastics,” Aristilde said. “We found that the wastewater bacterium has an innate ability to degrade plastic all the way down to monomers, small building blocks that join together to form polymers. These small units are a bioavailable source of carbon that bacteria can use for growth.”
Aristilde was determined to unravel the mysteries of C. testosteroni’s plastic degradation capabilities. Leveraging omics techniques capable of measuring all enzymes within the cell, her team pinpointed a specific enzyme expressed by the bacterium when exposed to PET plastics.
To further explore the enzyme’s significance, Aristilde collaborated with researchers at Oak Ridge National Laboratory in Tennessee to create bacterial cells lacking the ability to express the enzyme. Surprisingly, the absence of this enzyme resulted in a substantial loss or complete cessation of the bacteria’s plastic degradation capability.
Aristilde envisions that this breakthrough could be a game-changer for environmental solutions. Moreover, she emphasizes that this newfound knowledge can significantly enhance our understanding of the evolution of plastics in wastewater.
“Wastewater is a huge reservoir of microplastics and nanoplastics,” Aristilde said. “Most people think nanoplastics enter wastewater treatment plants as nanoplastics. But we’re showing that nanoplastics can be formed during wastewater treatment through microbial activity. That’s something we need to pay attention to as our society tries to understand the behavior of plastics throughout its journey from wastewater to receiving rivers and lakes.”
Journal reference:
- Rebecca A. Wilkes, Nanqing Zhou, Austin L. Carroll, Ojaswi Aryal, Kelly P. Teitel, Rebecca S. Wilson, Lichun Zhang, Arushi Kapoor, Edgar Castaneda, Adam M. Guss, Jacob R. Waldbauer, Ludmilla Aristilde. Mechanisms of Polyethylene Terephthalate Pellet Fragmentation into Nanoplastics and Assimilable Carbons by Wastewater Comamonas. Environmental Science & Technology, 2024; DOI: 10.1021/acs.est.4c06645