Microplastics Found in Deep-Sea Hydrothermal Vent Animals

A new study has detected microplastics inside animals inhabiting deep-sea hydrothermal vents more than 6,500 feet (2,000 metres) beneath the ocean’s surface.

Researchers found the particles in roughly nine out of every ten snails and mussels they investigated in one of Earth’s most remote ecosystems.

This is the first direct evidence that plastic entering the sea at the surface can descend thousands of feet and become lodged in animals that live entirely without sunlight.

Hydrothermal vents were once regarded as unspoilt outposts. Now, they bear signs of the same pollution seen along coastlines far above.

Microplastics in the depths

Hydrothermal vents are openings in the seabed through which water warmed by magma beneath streams out. They sustain dense animal communities that depend on chemical energy instead of sunlight.

Located thousands of feet below the surface and far from land, these sites were assumed to be beyond the reach of pollution originating at the surface.

A team headed by Dr Se-Joo Kim of the Korea Research Institute of Bioscience and Biotechnology (KRIBB), in collaboration with the Korea Institute of Ocean Science and Technology (KIOST), put that belief to the test.

The researchers collected snails and mussels from hydrothermal vents deeper than 6,500 feet (2,000 metres) in two widely separated oceans, before analysing what the animals had absorbed.

Microplastics were present in about nine out of ten animals taken from the hydrothermal vents. Each animal contained, on average, slightly more than three particles. Polystyrene-the light, rigid plastic used for disposable cups and food packaging-was the most frequently identified material.

Hydrothermal vents accumulate microplastics

Exactly how plastic reaches such depths has not yet been established. Surface particles may attach to sinking debris and the continual fall of dead plankton, travelling with it towards the seabed.

A previous study had found plastic at one vent field, but researchers had not compared its accumulation in animals from separate oceans. This survey did so and identified contamination at both locations.

“Plastic pollution has now spread even to deep-sea hydrothermal vent ecosystems that were once considered among the most isolated environments on Earth,” said Kim, one of the study’s corresponding authors. Their remoteness no longer prevents plastic from reaching them.

Where plastic collects

Plastic was not located in the same parts of every animal. The decisive factor was how each species fed. In snails that graze on microbial mats covering rocks, most particles were found in the gut and digestive gland.

Mussels showed a contrasting pattern. They feed by taking in seawater and filtering whatever drifts through it. Their plastic was distributed relatively evenly across the body, including the gills, digestive organs and muscle.

This distinction remained regardless of the animals’ age or size. The researchers concluded that feeding behaviour and body structure, rather than age or size, determine where plastic accumulates.

Two animals within the same hydrothermal vent ecosystem can therefore become contaminated with microplastics in entirely different ways.

Microplastics compared in two oceans

The study’s clearest contrast arose from geography rather than biology.

After adjusting for body weight, the team found that animals from the Central Indian Ridge in the Indian Ocean contained up to 14.7 times more plastic than those from the North Fiji Basin in the south-western Pacific.

The difference probably reflects the conditions surrounding each location. The Indian Ocean receives substantial plastic inputs from densely populated coastlines and major river systems across South Asia.

According to a review of plastic pollution in the area, much of this debris originates in rivers flowing into the Bay of Bengal. Ocean currents and seasonal monsoon winds then redistribute the material.

The south-western Pacific vent, which lies farther from major industrial coastlines, had lower contamination levels. The contrast indicates that the quantity of plastic reaching a deep-sea community is strongly influenced by human activity thousands of feet above and hundreds of miles away.

What comes next

The effects of plastic after it enters these animals remain unknown.

In separate research, hydrothermal vent crabs were given polyethylene, a widely used everyday plastic. The crabs were later found to have damaged gills, immune-system changes and evidence of cellular stress. This suggests that plastic may have effects beyond simply passing through the body.

The findings come at a difficult time, as governments and companies consider mining the deep seabed for metals. Such work would disturb sediment and disrupt the same habitats now known to be accumulating plastic.

Baseline surveys such as this provide a reference point for future monitoring, documenting what these communities already contain before any excavation starts.

It is now clear that no area of the ocean is beyond plastic pollution, and that an animal’s diet determines where that plastic settles within its body.

“Our findings provide important scientific evidence for establishing future deep-sea environmental monitoring systems and conservation policies,” said Kim.