Through these bacteria’s “metabolic acrobatics” as Morgan describes, PETs no longer have to wait ten centuries to degrade but be taken care by microorganisms, the heroes we can’t even see.
The three strains of bacteria include: Bacillus cereus, Pseudomonas putida, and a novel Pseudomonas strain. Morgan had the opportunity to name it Pseudomonas morganensis. And surprisingly, these three added to an already existing list of plastic-degrading organisms. In 2016, Japanese researchers isolated Ideonella sakaiensis, a strain of bacteria also found to metabolize plastic. The mechanism of these discovered strains is fairly similar: they both use an alpha-beta hydrolase, which are biological catalysts that break chemical bonds by using water, such as lipases.
With all these discoveries of plastic-degrading bacteria, Morgan points out that they are much more common than we think. And our plastic waste may benefit from having more groups start exploring and collecting samples from polluted sites. “These bacteria are already out there in the environment doing that process, why not try to put them to work.”
Let's talk plastic.
So the degradation of plastic comes down to a whole bunch of C’s – that is carbon molecules. This name gets thrown around a lot, especially when talking about climate change. But as we are seeing, carbon itself isn’t bad! In fact, it is what makes life possible. We are made of carbon, we eat carbon to grow and survive just like Morgan’s bacteria. Instead, it is the fact that plastics are a bunch of carbon polymers that makes it so bad and hard to break down.
Carbon polymers are made by connecting carbons together to form a central chain, much like your bones making the skeleton that structures your body. However, unlike the individual carbon units of sugars, these long and strong carbon-carbon chains are not naturally present in nature because it takes an abundance of energy to connect a bunch of carbons together. The production of a plastic bottle requires 3.4 megajoules of energy. That is 812,620 calories and if you eat about 2,000 calories a day, that’ll last you more than a year.
Pictured are the skeletal formulas of different backbone structures for different types of plastic that determine how they can be reused or degraded. Each unlabelled angle in the structure represents a carbon molecule. The "n" refers to the number of repeating subunits within the bracket that join to make the plastic polymer. It is the increasing value of "n" that makes plastic so hard to break down.
Because it is not naturally found in nature, our environment has not evolved to process this material. When a piece of plastic is thrown away, usual decomposing organisms can’t exactly degrade it like it can degrade the sugars of an apple. This makes plastic durability something of a double-edged sword: it is a fantastically useful product but also incredibly difficult to naturally degrade like organic material.
And this is where Morgan’s research brings us hope. We now know that when needed, some bacteria can break up those long chains and take in plastic as food.
Where do we go from here? Well, another Reed College senior is currently continuing this project for her own thesis. Knowing the main character for plastic degradation is the lipase, Cam Roberts is screening for their genetic components that control production and regulation. This is in hopes of upregulating those elements to speed up the bacteria’s metabolism of plastic. Morgan’s vision of this is a “big industrial scaled, contained, carbon free system where these bacteria can thrive on their sole food source of PET waste.”
Right now, the degrading ability of the three bacterial strains has only been tested on PET plastic. But as Professor Mellies points out, "there are many kinds of plastic and even within the same type of plastic, they are different because people use different plasticizers." This makes not only recycling more complicated, but also adds in more factors to how the mechanisms of metabolism can be used to degrade plastic. So, as we wait for a factory of bioremediation, what we can do is be cognizant of reducing our plastic use and the types of plastic being used.
-- Vicki Deng