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In 2021, what sounded like a good news story hit the media: in the Mediterranean, seagrasses were trapping plastic waste, capturing fragments in their leaves and locking microplastics in seafloor sediments. The news cycle was spurred by a study of Neptune grass, which showed that when this seagrass species sheds its leaves each autumn some of that plastic debris is jettisoned back to shore, slightly cleaning the marine environment. At the time, scientists and reporters billed the Mediterranean seagrass as a potent ally in the fight against marine plastic pollution. But that hopeful narrative is, unfortunately, too optimistic and only tells part of the story.

In a recent meta-analysis, Alice Rotini, a marine biologist at the Institute for Environmental Protection and Research in Italy, and her colleagues comprehensively reviewed and analyzed 26 existing studies that examine how plastic pollution affects seagrasses. Rotini says that rather than filtering plastic pollution out of the water, seagrass meadows and the diverse life that depends on them may be suffering under plastic’s influence.

“I am very concerned that we don’t know the real impact,” says Rotini.

The individual studies included in Rotini’s review, for instance, show that microplastics settling on seagrass leaves can reduce plant growth, cause leaf loss, lower levels of photosynthesis and respiration, and potentially disrupt nutrient cycling. Another recently published study shows that Neptune grass meadows with high levels of microplastic pollution also have fewer crustaceans and mollusks.

For Alyssa Novak—a coastal ecologist at Boston University who was not involved in Rotini’s work but has carried out her own research on the topic—the previously lauded plastic trapping mechanism is too rife with complications to be hailed as a solution to humanity’s ocean trash problem. “It’s a benefit in terms of how it’s trapping those particles in the beds,” she says. But that benefit is undone when fish, sea turtles, and other animals that graze on seagrass accidentally eat a bunch of stored plastic, for example.

Seagrass meadows provide habitat for numerous species and filter the water. Plastic’s influence, Novak says, could disrupt these vital services.

Plastic’s interference could also upset the ability of seagrass to capture and store carbon, says Mohammad Abu Noman, a PhD candidate in the Blue Carbon Lab at Australia’s Deakin University. That potential would be a major problem in the fight against climate change and requires further investigation, he says.

The bottom line of her meta-analysis, Rotini says, is that many unknowns swirl around the effects plastic has on seagrass. Neptune grass may trap plastic, but it comes at a cost. In Abu Noman’s view, it’s possible that with further research scientists may find that plastic’s potential disruption of seagrass ecosystems is the lesser evil compared with its toll on open ocean or terrestrial ecosystems. But framing seagrass’s propensity to trap and store plastic as an ecosystem service is a stretch.

“I definitely don’t want people to move in the direction of saying, ‘We have seagrass beds and they’re trapping microplastics so that’s resolving the problem,’” Novak says. “That’s not resolving the problem—it’s just moving and concentrating it in a different location.”