Breaking the unbreakable: Solving the problems of plastics and plants
We are addicted to plastics. They are used for everything, from food packaging to smart phones. But when we are done with them, they hang around for a long time, taking decades to decompose...
These hardy plastics aren't just creating litter in cities and filling up landfills. They are harmful to wildlife, especially in the sea where animals can become entangled in the plastic or mistake it for food. The damage a single piece of plastic can unleash can be long-lasting since it takes so long to degrade. A striking example of this is the Great Pacific garbage patch, which has formed from small bits of floating plastic that break into smaller and smaller pieces but haven’t fully degraded. Researchers have described ocean water taken from there as looking like a “snow globe” of plastic chips1. Though we are developing biodegradable plastics and recycling is on the rise, there is still the question of what to do with the built-up waste that's already accumulated and amounts to hundreds of plastic items for every individual on the planet.
One way to solve this problem is by taking a cue from nature. Hundreds of millions of years ago, plants developed their own incredibly sturdy material. When they adapted from a water-based existence to life on land, they had many new problems to surmount: drying out in the air, withstanding ultraviolet (UV) from sunlight, and counteracting gravity. To be able to grow upwards, they evolved a new material - lignin. Lignin becomes embedded in the wall that surrounds plant cells and gives it rigidity; it's held together by strong bonds, so it resists degradation. And when lignin first evolved, no living thing could break it apart. So why aren’t we surrounded by piles of un-decomposed trees?
We have bacteria and fungi to thank for that. Specifically, the kinds that have counter-evolved to break lignin apart. Mostly this job is done by a fungus called "white rot". Cells make proteins called enzymes that can help bring molecules together or break them apart. For example, it is the enzyme lactase that breaks down the lactose in the milk we drink into parts we can absorb for energy. Similarly, it was useful for fungi to be able to break apart lignin to get at the food stored in plants. Under this strong selection pressure, a fungus with an enzyme that could even partially break down lignin would get more food and thrive. Every change that appeared that was a small step towards improving this enzyme would be an advantage for the fungus. Eventually, they evolved a special form of peroxidase enzymes that are particularly good at using reactive chemicals to attack the lignin structure.
So, plants invented an indestructible material and then fungi figured out how to digest it. So could we do the same with plastics? Even though there is currently no known organism that can efficiently break down plastic, there are ways to search for ones that do. Scientists test known bacteria and fungi for their ability to degrade plastic. They also try to find new candidates by sifting through organisms recovered from places containing slowly-degrading plastic to pinpoint which one is actually responsible for breaking the plastic apart.
There have been plastic-degrading bacteria and fungi found in this way, but they are nowhere near as efficient as the white-rot fungus is at breaking down lignin. For example, Yoshida and colleagues took water and soil samples from a bottle recycling site and looked to see if there were bacteria or fungi that could degrade plastic. In 2016 they published a new bacterium, Ideonella sakaiensis 201-F6, which carries an enzyme that breaks plastics into parts that are not harmful to the environment2. This enzyme was named PETase, since it degrades a type of plastic known as PET. However, the PETase is not very efficient. This is probably because of the short amount of time organisms have had adapt to this new material, similar to how fungal enzymes had to evolve from less efficient enzymes. There was a lag of many millions of years between the evolution of lignin and the evolution of organisms able to degrade it thoroughly and quickly.
We do not have this kind of time. Scientists can speed up the process by directed evolution. While natural evolution depends on random mutations popping up, in directed evolution we can actively create small differences in enzymes that could make them better, and then directly test these slightly different enzymes for their ability to degrade plastic. This year, researchers made small changes to the inefficient PETase enzyme to create a mutant enzyme that degrades plastic faster3. It was only a small improvement, but it shows that we can make changes to existing enzymes in the laboratory to work towards an efficient solution to degrading plastic.
With this type of biotechnology, we can use the cells of organisms around us as a resource and learn lessons from their evolutionary history. By harnessing the ingenuity of natural systems maybe we can solve our plastic problem...
This essay won the British Society for Cell Biology science writing prize, 2017