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About 360 Million years ago, a toxic material that could not easily be broken down arose on planet Earth. The stuff would persist for thousands of years, amassing on land (and especially in forests) in daunting piles. These mounds stuck around for millions of years, so long that a heap of them fossilized. Today, humans dig up these deposits to burn as fossil fuel.
That toxic material? It was lignin, the main ingredient in tree bark. And that fuel we now excavate and burn? It's coal.
Though it's hard to imagine tree bark as a toxic pollutant, during Earth's Carboniferous period, extending from 360 million to 300 million years ago, it really was. When trees evolved lignin to protect against herbivores, they went a little crazy adorning themselves with it. Carboniferous trees developed bark to wood ratios of 8 to 1 or higher, compared to today's average ratio of about 1 to 4. The result was that when trees died or toppled over, they wouldn't really go anywhere. This arboreal pollution persisted until microorganisms evolved that could rapidly decompose lignin. It took tens of millions of years, but Earth eventually cleaned up its tree trash.
Aside from simply being fascinating from an ecological perspective, this story sheds light on a human-caused pollution problem today. Millions of years ago, lignin was comparable to polyethylene terephthalate (PET). That's right, carboniferous trees were like today's plastic. And now that we're producing about 300 million tonnes of plastic waste each year, depositing it in rivers, oceans, and just about everywhere else, Earth's microbes are already evolving ways of dealing with it.
In 2016, a team of Japanese researchers discovered a new bacterium hanging out in sediments near a plastic bottle recycling facility in Sakai, Japan. Ideonella sakaiensis had evolved a novel enzyme, which scientists dubbed PETase, that allowed it to degrade PET into two environmentally benign compounds that it could then consume for food, releasing carbon dioxide and creating organic matter in the process. The discovery hinted that there may be many more of these plastic-eating microorganisms out there, evolving to take advantage of the new 'food' source. Five years later, this has been confirmed. Scientists soon realized they might be able to harness these bacteria and their enzymes to remove plastic from the environment and recycle it much more effectively. Researchers have since boosted PETase's ability to degrade and digest plastic, and the French startup Carbios says that is has developed a bacterium that can digest plastic bottles in a matter of hours.
Unfortunately, at the rate we're injecting plastic into the environment, we can't wait millions of years for microbes to naturally manage humanity's plastic waste before it does severe harm to global ecosystems and ourselves. Thus, scientists are rushing engineer these plastic-eating microbes and speed up the process, but it remains to be seen if these efforts will pay off in any meaningful way.
There's another, more drastic option that we might consider: releasing engineered plastic-eating microbes into the wild. But this could backfire big time. Plastic, for all its problems, is a sizable store of carbon. Breaking down the world's plastic pollution with microbes would send a lot of carbon into the atmosphere, further exacerbating global climate change. And, of course, we probably don't want to release engineered microbes into the environment haphazardly, as doing so could result in unforeseen consequences.
Earth's long-ago lignin saga showcases that our planet's natural systems are more than capable of cleaning up gigantic messes over lengthy timespans. When it comes to plastic pollution, the real question is how much damage we'll do in the meantime, both to ourselves and to the natural world. As problem-solvers attempt to harness microbes to decompose or recycle plastic waste, it might be wise for the rest of us to find ways to use less of it.