Five years ago, scientists digging through soil and sludge around a plastics recycling center in Japan discovered a bacterium that was feeding on the popular packaging material PET as an energy source, with help from a pair of purposely evolved enzymes. A fascinating discovery at the time, a new study has shown this to be part of a much wider trend in which such plastic-degrading enzymes are increasing in numbers and diversity in response to plastic pollution around the world.
Since that discovery in Japan in 2016, we have seen a couple of other interesting advances in this space. The initial hope was that by unearthing those enzymes that enabled the bacterium to quickly reduce PET plastic into environmentally benign building blocks, even more efficient versions could be engineered in the lab to form new weapons in the fight against plastic pollution.
In 2018, a group of scientists in the US built on this research to produce an engineered enzyme that could consume plastics with around 20 percent greater efficiency. That same team then developed an even more advanced version in 2020, which they described as a super-enzyme, that could digest plastic waste at six times the speed.
A range of enzymes with similar abilities have been uncovered, and new research by a team from Sweden's Chalmers University of Technology shows just widespread the phenomenon is. The researchers analyzed samples of environmental DNA sourced from hundreds of ocean and terrestrial locations around the world, using computer modeling to screen them for microbial enzymes with the potential to break down plastics.
This revealed a total of 30,000 enzymes with the potential to degrade 10 types of common plastics, with the scientists then cross-referencing this with official data on concentration of plastic pollution across different countries and oceans. It turned out that some of the locations with the highest amount of enzymes were the most heavily polluted areas, such as the Mediterranean Sea and South Pacific Ocean.
“Using our models, we found multiple lines of evidence supporting the fact that the global microbiome's plastic-degrading potential correlates strongly with measurements of environmental plastic pollution – a significant demonstration of how the environment is responding to the pressures we are placing on it,” says study author Aleksej Zelezniak.
The enzymes were found to be distributed widely across ocean and terrestrial locations, but with a few interesting insights. The team found higher concentrations of plastic-degrading enzymes at deeper levels in the ocean, indicating a connection with the greater concentrations of microplastics seen at these depths. Similarly, the land samples were found to contain many more phthalate-based plastic additive compounds, along with enzymes known to be capable of degrading them, indicating a connection between the two.
“Currently, very little is known about these plastic-degrading enzymes, and we did not expect to find such a large number of them across so many different microbes and environmental habitats. This is a surprising discovery that really illustrates the scale of the issue,” explains Jan Zrimec, first author of the study.
The mass production of plastic has skyrocketed from around two million tonnes annually to a staggering 380 million in the last seven decades or so, and around eight million tonnes wash into the ocean each year. While this has given microbes a sizable window to develop evolutionary responses to the waste in their environments, it would take a lot of highly efficient enzymes to eat away at the problem. But with significant findings like this, the scientists' playbook continues to expand.
“The next step would be to test the most promising enzyme candidates in the lab to closely investigate their properties and the rate of plastic degradation they can achieve," says Zelezniak. "From there you could engineer microbial communities with targeted degrading functions for specific polymer types."
The research was published in the journal mBio.
