As tiny pieces of plastic clog our oceans, natural meadows of seagrass are bundling up microplastics and spitting them back out onto beaches in the form of "Neptune balls".

Neptune balls, or Posidonia oceanica, are round, compact bundles of seagrass mostly found in the Mediterranean Sea. For centuries, Posidonia has been used for packaging, bedding and even insulation for houses.

But researchers from the University of Barcelona have found these spongy balls are spontaneously performing another function: mopping up ocean plastic from the seafloor.

In the ocean, microplastics – particles under 5mm in size – often originate from items such as plastic bags, bottles and fishing nets. These plastic fragments can harm our health, affecting everything from bone and brain function to hormones (read more about how microplastics affect our health).

Although most plastic pollution originates on land, the ocean – including seagrass meadows – acts as a sink.

Posidonia leaves slow water down as it flows past, says Anna Sanchez-Vidal, lead author of the Barcelona study. "There are fewer currents in seagrass meadows, so they trap carbon and sediment, and act as a refuge for biodiversity."

But these swaying underwater meadows also accumulate higher concentrations of plastic. Each year, between 1.15 and 2.41 million tonnes of plastic flows from rivers to the sea. If a river enters the sea where Posidonia is growing, some of that plastic becomes caught, and accumulates. But not all of this plastic stays trapped in swaying Posidonia meadows.

Each autumn, Posidonia sheds its leaves. These fibrous strands, rich in the tough organic polymer lignin, tangle together in dense balls. "As they move, they transport plastic intertwined within the fibres," says Sanchez-Vidal. The researchers estimated that seagrass meadows may catch nearly 900 million plastic fragments in the Mediterranean every year.

In 2018 and 2019, Sanchez-Vidal's team examined seagrass balls washed up on four beaches on........

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