Pollution | Life in marine plastic

The accumulation of plastic in the oceans has been in the news for a decade, with images of turtles with straws in their noses. More recently, studies are looking at the microorganisms that live on and feed on plastic. Their results are surprising.

Mathieu Perreault
The Press

Biodiversity

Bivalve shells. Jellyfish and coral larvae. Translucent verses. Hermit crab-like crustaceans. The density of species observed by the Chinese research submarine Shenhai Yongshi (Warrior of the Depths) as it dives in the Xisha Canyon in the South China Sea is 10 times higher than the surrounding seabed.

Researchers from the Chinese Academy of Sciences believe that this is due to the presence of a buildup of plastic, due to the currents. “At this depth, there are few sources of energy and food,” says Xikun Song, lead author of a study published last fall in Environmental Science and Technology Letters. “The plastic may be consumed by bacteria which are then ingested by other microorganisms. The large pieces can also be used as a substrate, especially for the larvae. »

Located at a depth of 3200 m, this canyon contains an average of 52,000 plastic objects per square kilometer. Chinese researchers counted 1,200 organisms from 49 species out of the 33 plastic objects studied. However, this biodiversity does not compensate for the deleterious effects of plastic accumulation zones: the same team showed in 2020 that PCBs, a very toxic chemical found on Chinese beaches, sometimes come from contaminated plastics accumulating in the abyssal canyons.

Corals

The ability of large pieces of plastic to serve as substrate is also being tested to repopulate damaged or unhealthy coral reefs. “The idea would be to use it to collect corals from healthy reefs and then transport them to reefs to be repopulated,” says Alessandro Cau, a biologist from the University of Cagliari, Sardinia, who published a study on the subject last summer in the Marine Pollution Bulletin.

On the other hand, the pieces of plastic abandoned in the oceans cannot be used as a substrate, underlines Mr. Cau. “They accumulate in the canyons, and some studies show that there is a lot of current in these canyons. The corals and larvae that settle on the large pieces of plastic will be lost, because this plastic will be shredded by the currents. Julien Gigault, a chemist from Laval University who specializes in marine nanoplastics, observes that in the French Caribbean, plastic “trees” are used in the lagoons as a substrate for corals. In Xikun Song’s study, none of the organisms observed on the plastic had reached sexual maturity.

Invasive species

This ability of microorganisms to use plastic as a substrate can lead to problems, according to a Chilean study. “The growth of bugula, a zooplankton that is very damaging to maritime structures, is very high on the pieces of plastic,” explains Mauricio Urbina, a biologist from the University of Concepción, lead author of a study published in 2020 in the journal Maritime Pollution Bulletin.

The plastisphere

The life that forms on the surface of plastic has been dubbed the “plastisphere” by a Dutch microbiologist, Linda Amaral, from the Royal Netherlands Institute for Maritime Research. “We are beginning to understand that organisms in the plastisphere can unbalance ecosystems,” says Ms.^me Amal. They can disrupt food chains, favor certain types of plankton and invertebrates over others, modify marine flora. We also see that the bacteria that tend to form biofilms on the surface of plastics also have a great pathogenic potential. »

A geomicrobiologist from the University of Copenhagen, Nicole Posth, has just shown that bacteria in plastic-rich marine environments are different from elsewhere. “I was recently in Tanzania, where SMEs are collecting marine plastic to sell to recycling plants,” says Ms.^me Posth. Studies have found cholera bacteria on plastic. One can wonder if the plastic promotes the transmission of cholera. »

Bacterial degradation

In 2003, an American study concluded that 53% of the hydrocarbons that enter the oceans come from natural oil leaks. These leaks total 12,000 barrels per day, the equivalent of 0.01% of global consumption. These natural leaks have generated the appearance of oil-eating bacteria, in particular alcanivorax. The latter can also attack plastic, according to Robyn Wright, who published in 2020 in the journal Environmental Science & Technology a summary of plastisphere studies.

“In the past two years, the number of studies on the subject has exploded,” says M.^me Wright. More and more researchers are looking into the bacterial degradation of plastic, according to M^me Posth. Early research on the plastisphere focused on the bioaccumulation of plastic along the food chain (large organisms eat smaller organisms that have ingested plastic).

Promising development

The plastisphere has already generated new pathways in evolution. “In 2016, we identified a bacterium capable of degrading PET plastic, thanks to a new enzyme,” says Robyn Wright. This bacterium is called Ideonella sakaiensis. “It’s probably the result of evolution. It was detected next to a plastic bottle factory in Japan. Since then, efforts have been made to improve this enzyme to degrade plastic. »