A marine fungus can break down plastic polyethylene provided it is first exposed to ultraviolet radiation from sunlight. NIOZ researchers published their results in the journal Total Environmental Science. They expect deeper parts of the ocean to be home to many more fungi that break down plastic.

The fungus Parengyodontium album lives along with other marine microbes in thin layers on plastic debris in the ocean. Marine microbiologists from the Royal Netherlands Institute for Marine Research (NIOZ) have discovered that a fungus can break down plastic polyethylene (PE) particles, the most common type of plastic found in the ocean.

NIOZ researchers are from the University of Utrecht, the Ocean Cleanup Foundation, and researchers from Paris, Copenhagen, and St. Louis, Switzerland. He collaborated with colleagues from research institutes in the city of St. Gallen. This discovery allows the fungus to join a very short list of plastic-degrading marine fungi: only four species have been found to date. It is already known that large numbers of bacteria can decompose plastic.

Exact compliance with the degradation process

Researchers set out to find microbes that break down plastic in plastic pollution hotspots in the North Pacific. They isolated a marine fungus from collected plastic waste and grew it in the laboratory on special plastic containing labeled carbon.

Waksmaa explains: “These are so-called isotopes ¹³ C remains traceable in the food chain. It’s like a tag that allows us to track where the carbon is going. We can then trace it to its degradation products.

“What makes this study scientifically remarkable is that we were able to measure the degradation process.” In the laboratory, Waksmaa and his team observed that P. album PE degradation occurred at a rate of approximately 0.05% per day.

“Our measurements also showed that the fungus does not use much of the carbon that comes when it breaks down PE. Most of the PE used by P. album is converted to carbon dioxide, which the fungus releases again.” Although CO₂ is a greenhouse gas, this process is not something that would create a new problem: The amount released by fungi is the same as the small amount that humans release when breathing.

Only under the influence of UV

The researchers found that the presence of sunlight is necessary for the fungus to use PE as an energy source. “In the laboratory, P. album only degrades polyethylene exposed to UV radiation for at least a short period of time. This means that the fungus can only degrade plastic floating near the surface in the ocean,” – explains Vaksmaa.

“UV was already known to mechanically break down plastic, but our results show that UV also promotes the biodegradation of plastic by marine fungi.”

There are other mushrooms too

Because many different plastics are buried in deeper layers before exposure to sunlight, P. album will not be able to break them all down. Waksmaa speculates that there are other, as yet unknown, fungi that break down plastic in the deep ocean.

“Marine fungi can break down complex materials made of carbon. Since there are a large number of marine fungi present, it is likely that other species, in addition to the four species identified so far, contribute to the decomposition of plastic. There are still many questions about the dynamics of how plastic breaks down in deeper layers,” says Waksmaa.

plastic soup

Research into organisms that break down plastic is urgent. Humans produce more than 400 billion kilograms of plastic every year, and this figure is expected to at least triple by 2060.

Most plastic waste ends up in the sea: It floats in surface waters from the poles to the tropics, reaches the depths of the sea, and eventually sinks to the seabed.

NIOZ lead author Waksmaa says: “Large amounts of plastic end up in subtropical gyres, ring-shaped currents in the oceans where seawater is almost stationary. This means that once plastic gets in there, it gets trapped. Only in the North Pacific Subtropical Gyre in the Pacific Ocean does about “80 million kilograms of floating plastic have accumulated, and this is just one of six major vortices around the world.”