At this time, almost everything around you is being eaten. Invisible to the naked eye, organisms called microbes accumulate on every surface.
Groups of bacteria, archaea,
and fungi have developed powerful enzymes that break down tough organic materials into digestible nutrients.
But there's one particularly broad class of materials that almost no microbes can biodegrade: plastics. To make most plastics, oil, gas and coal molecules are refined and converted into long, repeating chains called polymers.
This process often requires temperatures in excess of 100˚C, incredibly high pressures, and various chemical modifications. The resulting man-made polymers are quite different from those found in nature.
And because they've only been around since the 1950s, most microbes haven't had time to develop the enzymes to digest them.
Making matters more difficult,
most plastics require high temperatures to break the chemical bonds used to make them—and such heat is lethal to most microbes.
This means that most plastics never biodegrade—they just break down into countless, tiny, indigestible fragments.
And fragments of very common plastics such as polyethylene, polypropylene, and polyester-terephthalate have been accumulating for decades.
Every year humanity produces about 400 million more tons of plastic, 80% of which is discarded as trash.
Only 10% of this plastic waste is recycled.
60% is burned or goes to landfills, and 30% is released into the atmosphere where it will pollute natural ecosystems for centuries.
An estimated 10 million tons of plastic waste ends up in the ocean every year, mostly in the form of microplastic fragments that contaminate the food chain. Fortunately, there are microbes that may be able to overcome this growing problem.
In 2016, a team of Japanese researchers discovered Ideonella sakaiensis 201-F6 sampling sludge at a plastic bottle recycling plant.
The bacterium contains two never-before-recognized enzymes capable of slowly breaking down PET polymers at relatively low temperatures.
The researchers isolated the genes coding for the plastic-digesting enzymes, allowing other bioengineers to combine and improve the pair—creating super-enzymes that digest PET 6 times faster.
Can break Even with this boost,
these lab-grown enzymes still took weeks to degrade a thin film of PET, and they worked best at temperatures below 40˚C.
However, another group of scientists in Japan was researching bacterial enzymes that adapted to high-temperature environments such as compost piles.
And inside a particularly warm pile of decomposing leaves and twigs, they found the gene sequence for a powerful enzyme known as leaf branch compost cutinase.
Using fast-growing microorganisms, other researchers were able to genetically engineer high amounts of these enzymes.
They then amplified and selected specific types of cutinases that could degrade PET plastics in environments reaching 70˚C—a high temperature that can weaken and digest PET polymers.
With the help of these and other small die-hards, the future of PET recycling looks promising. But PET is just one type of plastic.
We still need methods to biodegrade all other types,
including the abundant PE and PP that only begin to break down at temperatures above 130˚C. Researchers currently know of no microbes or enzymes that are hardy enough to withstand such temperatures.
So currently, the primary method of dealing 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 finding more heat-tolerant plastivores in some of the planet's most hostile environments and engineering better plastivore enzymes in the laboratory.
But we can't rely entirely on these little helpers to clean up our huge mess. We need to completely rethink our relationship with plastic, make better use of existing plastic, and stop producing more of it.
And we urgently need to design more environmentally friendly types of polymers that our growing cohort of plastiverse can easily break down.
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