Want clean water? Plant a lot of seagrass

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A plethora of species lives alongside seagrass meadows, such as these fish and invertebrates in Indonesia.
Credit: Margaux Hein

While researching in Indonesian coastal waters, several members of biologist Joleah Lamb’s team came down with gastroenteritis. Her determination to find out why resulted in the discovery of previously unknown antibacterial properties of seagrass meadows.

Studying the waters around four adjacent Indonesian islands, Lamb and her team (by then, recovering) discovered that seagrass meadows reduced the number of harmful pathogens in surrounding seawater by half.

What’s more, this protective effect extended to adjacent coral reefs, which also carried half the harmful bacterial load of those without seagrass neighbours.

Writing in Science, Lamb – from Cornell University in the US – and colleagues report that “despite being the most widespread coastal ecosystem on the planet, seagrass meadows have not been evaluated as a system for pathogen removal or disease mitigation”.

By selecting paired sites that either did or didn’t have seagrass meadows, the researchers sampled the water and checked the levels of Enterococcus – a large genus of harmful bacteria that cause diseases including urinary tract infections, diverticulitis and meningitis.

Samples from sites with seagrass meadows showed threefold lower levels than those without.

The team then set about testing whether seagrass meadows were having an effect on other pathogens, too. They compiled a list of 42 bacteria species known to be harmful to fish, mammals, and invertebrates.

Taking samples of seawater from paired sites, the researchers used genetic sequencing to isolate a particular RNA gene and then compared their finds against type samples for the same gene in each of the target pathogens.

They found 27 of the target species in the water, but their prevalence was 50% lower in the seagrass meadows and adjacent reefs, compared to areas where the plants hadn’t established.

Carolyn Ewers Lewis, a marine and coastal ecologist from Deakin University, Australia, notes the significance of the discovery. “Seagrasses fringe nearly every continent on Earth, so finding that they again have another benefit is one more reason we need to pay attention to how we’re conserving them,” she says.

Lamb and colleagues hope that further research will weed out how seagrass manifests its antibiotic properties. They note that coral conservation is vital for the wellbeing and livelihood of 275 million people living within 30 kilometres of a coral reef, as well as countless reef-dwelling species.

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