The smallest solution to one of our biggest problems - Tierney Thys & Christian Sardet
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:
plastics.
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.
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 microplastic
fragments that pollute the food chain.
Fortunately, there are microbes
that may be able
to take a bite out of this
growing problem.
In 2016, a team of Japanese researchers
sampling sludge
at 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 microorganisms,
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.
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 plasticwaste
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 stopproducing 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.
