At this very moment, almost everything around you is being eaten. Invisible to the naked eye, organisms called microbes swarm every surface. Hordes of bacteria, archaea, and fungi have evolved to produce powerful enzymes that break down tough organic material into digestible nutrients. But there’s one particularly widespread type of material that almost no microbes can biodegrade: plastic.
How plastic is made?
To make most plastics, molecules from oil, gas and coal are refined and turned into long, repeating chains called polymers. This process often requires temperatures above 100˚C, incredibly high pressure, and various chemical modifications. The resulting man-made polymers are quite different from the polymers found in nature. And since they’ve only been around since the 1950s, most microbes haven’t had time to evolve enzymes to digest them. Making matters even more difficult, breaking most plastics’ chemical bonds requires high temperatures comparable to those used to create them. And such heat is deadly to most microbes.
Plastic trash around the world
This means that most plastics never biologically degrade. They just turn into countless, tiny, indigestible pieces. And pieces from the most common plastics like Polyethylene, Polypropylene, and Polyester-terephthalate have been piling up for decades. Each year humanity produces roughly 400 million more tons of plastic, 80% of which is discarded as trash. Of that plastic waste, only 10% is recycled. 60% gets incinerated or goes into the landfills, and 30% leaks out into the environment where it will pollute natural ecosystems for centuries.
An estimated 10 million tons of plastic waste end up in the ocean each year, mostly in the form of micro plastic fragments that pollute the food chain. Fortunately, there are microbes that may be able to take a bite out of this growing problem.
Microbes that can digest plastic
In 2016, a team of Japanese researchers sampling sludg eat a plastic-bottle recycling plant discovered Ideonella sakaiensis 201-F6.This never-before-identified bacterium contained two enzymes capable of slowly breaking down PET polymers at relatively low temperatures. Researchers isolated the genes coding for these plastic-digesting enzymes, allowing other bioengineers to combine and improve the pair. Creating super-enzymes that could break down PET up to 6 times faster. Even with this boost, these lab-grown enzymes still took weeks to degrade a thin film of PET, and they operated best at temperatures below 40˚C.
However, another group of scientists in Japan had been researching bacterial enzymes adapted to high temperature environments like compost piles. And within one particularly warm pile of rotting leaves and branches, they found gene sequences for powerful degrading enzymes known as Leaf Branch Compost Cutinases. Using fast-growing micro organisms, other researchers were able to genetically engineer high quantities of these enzymes.
They then enhanced and selected special variants of the Cutinases that could degrade PET plastic in environments reaching 70˚C. A high temperature that can weaken PET polymers and make them digestible. With the help of these and other tiny diehards, the future of PET recycling looks promising.
Can all forms of PEs degradable?
But PET is just one type of plastic. We still need ways to biologically degrade all the other types, including abundant PEs and PPs which only begin breaking down at temperatures well above 130˚C. Researchers don’t currently know of any microbes or enzymes tough enough to tolerate such temperatures. So for now, the main way we deal with these plastics is through energy-intensive physical and chemical processes.
Today only a small fraction of plastic waste can be biologically degraded by microbes. Researchers are looking for more heat-tolerant plastivores in the planet’s most hostile environments and engineering better plastivorous enzymes in the lab. But we can’t rely solely on these tiny helpers to clean up our enormous mess. We need to completely rethink our relationship with plastics. Make better use of existing plastics, and stop producing more of the same. And we urgently need to design more environmentally friendly types of polymers that our growing entourage of plastivores can easily break down.