Consider the plastic foam cup. Every year, Americans throw away 2.5
billion of them. And yet, that waste is just a fraction of the 33
million tons of plastic Americans discard every year. Less than 10
percent of that total gets recycled, and the remainder presents
challenges ranging from water contamination to animal poisoning.
Enter the mighty mealworm. The tiny worm, which is the larvae form of
the darkling beetle, can subsist on a diet of Styrofoam and other forms
of polystyrene, according to two companion
studies co-authored by
Wei-Min Wu,
a senior research engineer in the Department of Civil and Environmental
Engineering at Stanford. Microorganisms in the worms' guts biodegrade
the plastic in the process – a surprising and hopeful finding.
"Our findings have opened a new door to solve the global plastic pollution problem," Wu said.
The papers, published in
Environmental Science and Technology,
are the first to provide detailed evidence of bacterial degradation of
plastic in an animal's gut. Understanding how bacteria within mealworms
carry out this feat could potentially enable new options for safe
management of plastic waste.
"There's a possibility of really important research coming out of bizarre places," said
Craig Criddle,
a professor of civil and environmental engineering who supervises
plastics research by Wu and others at Stanford. "Sometimes, science
surprises us. This is a shock."
Plastic for dinner
In the lab, 100 mealworms ate between 34 and 39 milligrams of
Styrofoam – about the weight of a small pill – per day. The worms
converted about half of the Styrofoam into carbon dioxide, as they would
with any food source.
Within 24 hours, they excreted the bulk of the remaining plastic as
biodegraded fragments that look similar to tiny rabbit droppings.
Mealworms fed a steady diet of Styrofoam were as healthy as those eating
a normal diet, Wu said, and their waste appeared to be safe to use as
soil for crops.
Researchers, including Wu, have shown in earlier research that
waxworms, the larvae of Indian mealmoths, have microorganisms in their
guts that can biodegrade polyethylene, a plastic used in filmy products
such as trash bags. The new research on mealworms is significant,
however, because Styrofoam was thought to have been non-biodegradable
and more problematic for the environment.
Researchers led by Criddle, a senior fellow at the
Stanford Woods Institute for the Environment,
are collaborating on ongoing studies with the project leader and
papers' lead author, Jun Yang of Beihang University in China, and other
Chinese researchers. Together, they plan to study whether microorganisms
within mealworms and other insects can biodegrade plastics such as
polypropylene (used in products ranging from textiles to automotive
components), microbeads (tiny bits used as exfoliants) and bioplastics
(derived from renewable biomass sources such as corn or biogas methane).
As part of a "cradle-to-cradle" approach, the researchers will
explore the fate of these materials when consumed by small animals,
which are, in turn, consumed by other animals.
Marine diners sought
Another area of research could involve searching for a marine
equivalent of the mealworm to digest plastics, Criddle said. Plastic
waste is a particular concern in the ocean, where it fouls habitat and
kills countless seabirds, fish, turtles and other marine life.
More research is needed, however, to understand conditions favorable
to plastic degradation and the enzymes that break down polymers. This,
in turn, could help scientists engineer more powerful enzymes for
plastic degradation, and guide manufacturers in the design of polymers
that do not accumulate in the environment or in food chains.
Criddle's plastics research was originally inspired by a
2004 project
to evaluate the feasibility of biodegradable building materials. That
investigation was funded by the Stanford Woods Institute's
Environmental Venture Projects seed grant program. It led to the launch of a
company that is developing economically competitive, nontoxic bioplastics.
Co-authors of the papers, "Biodegradation and Mineralization of
Polystyrene by Plastic-Eating Mealworms. 1. Chemical and Physical
Characterization and Isotopic Tests" and "Biodegradation and
Mineralization of Polystyrene by Plastic-Eating Mealworms. 2. Role of
Gut Microorganisms," include Yu Yang, Jun Yang, Lei Jian, Yiling Song
and Longcheng Gao of Beihang University, and Jiao Zhao and Ruifu Yang of
BGI-Shenzhen.
For more Stanford experts on engineering and other topics, visit Stanford Experts.
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