Seagrass meadows help remove dangerous bacteria from ocean water

Greening their surroundings, and not just in the obvious way

Reinhard Dirscherl/Ullstein Bild/Getty Images

By Chelsea Whyte

Come on in, the water’s fine – at least, where seagrass beds still line the coast. A microbe found in sewage seems far less common in these areas than elsewhere, although it’s not yet clear why.

Joleah Lamb, a marine disease ecologist at Cornell University in Ithaca, New York, sampled waters off four small islands in the Spermonde archipelago, Indonesia. Her study areas included some with seagrass meadows and others without.

She found that the level of Enterococcus bacteria in seagrass areas was just one-third that in areas lacking the underwater meadows. In seagrass-free areas the bacteria were present at 10 times the limit for recreational water set by the US Environmental Protection Agency.

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These Indonesian islands lack sanitation systems, but in areas where seagrass meadows flourish, the bacteria seem to be kept in check to some degree.

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Lamb is now trying to pin down how this happens. One possibility is that the sediment beneath the grasses is locking the pathogens away, although she says early results don’t point in this direction.

Or aquatic __life colonising the seagrass might be involved. “We’re looking at microbial communities that are on the surface of the seagrass blades,” Lamb says.

But so far, it looks like the seagrass itself is doing the job, she says. Seagrasses could act in the same way as wastewater treatment facilities, releasing oxygen – made via photosynthesis, in the case of the plants – which is toxic to pathogens.

There are other potential mechanisms that could explain the effect, says Andrew Juhl, an environmental microbiologist at Columbia University, New York City.

“One of the biggest sources of mortality for pathogens in the water is light exposure,” he says. Ultraviolet light could penetrate the water, damaging the bacteria’s DNA. “It could be that it’s not the seagrass per se, but the water clarity that’s causing the effect.”

Read more: Seagrass gardens are needed to cap the carbon bomb in the oceans

Seagrass meadows often grow beside coral reefs. Lamb’s team surveyed 8000 reef-building corals and found that those next to seagrass beds had half the levels of diseases associated with bleaching than corals without a nearby meadow.

Lamb says the corals and seagrasses could have a quid pro quo arrangement. “It’s plausible they work together. Coral reefs are protecting the seagrass meadows from waves and in turn, the seagrass could be protecting them from [bacteria-rich shore] run-off,” she says.

Globally, seagrass meadows have been losing ground at the rate of 7 per cent each year since 1990, due largely to coastal development, destructive fishing practices and climate change. So, save the seagrass and we will protect the coral and ourselves from disease, right? Maybe, but it might not be the best approach.

Juhl sees the clean-up effect associated with seagrasses as encouraging, but he stresses that it doesn’t excuse us from treating sewage. “Probably your first line of defence would be to do something about the sewage and preserve the seagrass beds for other reasons,” says Juhl.

We also shouldn’t ignore the benefit to corals and humans the seagrass beds offer, including sequestering carbon more efficiently than rainforests. “If you preserve the seagrass beds, which have their own benefits that are not necessarily related to microbial water quality, you get some side benefits,” he says.

Journal reference: Science, DOI: 10.1126/science.aal1956