Are plastic-degrading enzymes evolving in pace with plastic pollution?

Somewhat eerily, plastic waste pollution is affecting the environment in far more ways than expected, a new study shows.

Over the last 70 years or so, plastic waste has become a ubiquitous presence. It is found everywhere on earth, invading ecosystems on land, in freshwaters, and in the ocean. Now, apparently, these ecosystems are adapting. The past decades have provided sufficient evolutionary time for various microbes present in the environment to respond to these compounds. Scientists have shown in various studies that the number of enzymes that have developed the ability to degrade plastics is on the rise.

Now, using computer modelling, a team of researchers at Sweden’s Chalmers University of Technology have found ‘multiple lines of evidence’  that this expanding number of plastic-degrading enzymes is correlated with the rising rates of plastic pollution.

The researchers recently published their findings in the scientific journal mBIO.

They analysed samples of environmental DNA from hundreds of locations around the world, using computer models to search for microbial enzymes with plastic-degrading potential, which was then cross-referenced with the official numbers for plastic waste pollution across countries and oceans.

They found that strong correlation between the global microbiome's plastic-degrading potential and measurements of environmental plastic pollution – “a significant demonstration of how the environment is responding to the pressures we are placing on it,” said Aleksej Zelezniak, Associate Professor in Systems Biology at Chalmers University of Technology. 

In other words, the quantity and diversity of plastic-degrading enzymes is increasing, in direct response to local levels of plastic pollution.

The variation found between the land and ocean samples both in number and type of plastic particles, as well as plastic-degrading enzymes, was striking. The samples taking from the land, for example, contained relatively high levels of phthalate-based plastic additive compounds. These compounds are known to be particularly susceptible to leaking during production, disposal, and recycling – processes which take place mainly on land. This correlated with the occurrence of a greater number of enzymes able to degrade these compounds found in these land samples, indicating a connection between the two.

Something similar turned up in the ocean samples: the greater the depth, the higher the quantity of plastic-degrading enzymes in a sample. The dataset comprised 67 locations from 8 oceans, at 3 different depths, and this was a consistent finding, again, indicating a connection with the higher level of microplastics that have been repeatedly observed at deeper levels in the ocean.

“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 and former post-doc in Aleksej Zelezniak’s group, now a researcher at the National Institute of Biology in Slovenia. Even more notable: nearly 60% of the identified plastic-degrading enzymes did not map to any known enzyme classes, suggesting that the team had uncovered novel plastic-degrading functional content.

The researchers believe that their results could potentially be used to discover and adapt enzymes for novel recycling processes.

“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. From there you could engineer microbial communities with targeted degrading functions for specific polymer types,” said Aleksej Zelezniak.

The article, Plastic-Degrading Potential across the Global Microbiome Correlates with Recent Pollution Trends, is published in the journal mBio. It was written by Jan Zrimec, Mariia Kokina, Sara Jonasson, Francisco Zorrilla and Aleksek Zelezniak.