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 harm of a single piece of plastic 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 chips (1). 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.
One way to solve this problem is by taking a cue from nature. Plants also developed an incredibly sturdy material many hundreds of millions of years ago. When plants evolved from water-based organisms to living on land, they had many new problems to adapt to: drying out in the air, withstanding 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, and is held together by strong bonds so it resists degradation. At the time lignin 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 the fungus, 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 milk we drink into parts we can absorb for energy. Similarly, it was useful for fungi to be able to break lignin apart to get at the food stored in plants. Under this strong selection pressure, a fungus with an enzyme that could even partially break lignin apart 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 type 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 – can 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 already known bacteria and fungi for their ability to degrade plastic. They also try to find new candidates by sifting through organisms found around 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. 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. So, 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.
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 we can solve our plastic problem.
References:
1. Kaiser, J. “The Dirt on Ocean Garbage Patches.” Science 328.5985 (2010): 1506. Web.
About The Author: Marcia is a final year PhD student at the University of Cambridge with Angeleen Fleming and Roger Keynes in the Department of Physiology, Development and Neuroscience. She is studying how the vertebral column develops using zebrafish as a model system and is broadly interested in evolution and development.
