Seagrass meadows that protect humans from deadly bacteria in alarming global decline

Underwater meadows that protect humans from deadly bacteria in alarming global decline
'The beautiful oceanside water looked blue-green, but truly it was filled with dangerous pollution,' says scientist after entire team became sick
Ian Johnston The Independent 16 Feb 17;

Beautiful underwater gardens massively reduce levels of potentially deadly bacteria in the sea — preventing humans from getting diseases like dysentery and typhoid — researchers have discovered.

But these idyllic meadows of seagrasses, the most common coastal ecosystem on Earth, are in serious trouble with dramatic, year-on-year declines recorded since 1990.

And that could mean further problems for coral, upon which much of marine life depends, which is also suffers from bacterial infections. Coral is already experiencing major bleaching events as the temperature of seas around the world rises.

It is known that seagrasses have an antibacterial effect, but the findings of the new study of heavily populated islands in the Spermonde archipelago in Indonesia were unexpectedly dramatic.

For in areas near the meadows, the amount of bacteria was about half the levels found in places with no seagrass.

For the scientists involved this was not simply a matter of science, but their own health.

Professor Drew Harvell, of Cornell University, had been investigating the health of corals with colleagues in the archipelago when the entire research team fell ill with dysentery and one scientist even got typhoid.

“I experienced firsthand how threats to both human health and coral health were linked,” he said.

“The beautiful oceanside water looked blue-green, but truly it was filled with dangerous pollution — some really bad stuff in the water close to shore.

“The genetic sequencing work pinpointed the kinds of bacteria —all in difficult, arduous conditions. It showed exactly what was in the water.”

In tests, the water was found to contain 10 times the level of one type of bacteria, called Enterococcus, that is considered safe.

Dr Joleah Lamb, also of Cornell University, and colleagues to return to investigate further and this led them to the discovery of just how effective seagrasses are at reducing harmful bacteria.

Writing in the journal Science, they said: “We found that when seagrass meadows are present, there was a 50 per cent reduction in the relative abundance of potential bacterial pathogens capable of causing disease in humans and marine organisms.

“Moreover, field surveys of more than 8,000 reef-building corals located adjacent to seagrass meadows showed two-fold reductions in disease levels compared to corals at paired sites without adjacent seagrass meadows.

“These results highlight the importance of seagrass ecosystems to the health of humans and other organisms.”

Further studies revealed the numbers of several pathogens that affect fish and invertebrates were also about 50 per cent lower in seagrass gardens.

But Dr Lamb warned the meadows themselves were in sharp decline.

”Global loss of seagrass meadows is about seven per cent each year since 1990," she said.

“Hopefully this research will provide a clear message about the benefits of seagrasses for human and marine health that will resonate globally.

“Our goal is to stop measuring things dying and find solutions. Ecosystem services like seagrass meadow habitats are a solution to improve the health of people and the environment. Biodiversity is good for our health.”

The antibiotic properties of seagrasses have already been used to save billions of pounds.

For example, New York City decided to buy and restore a wetland area instead of building an $8bn (£6.4bn) water treatment plant.

Ocean meadows scrub seawater of harmful bacteria
Seagrasses keep waterborne pathogens in check, potentially benefiting people and coral reefs.
Jason Bittel Nature 16 Feb 17;

Seagrass meadows are the most widespread coastal ocean ecosystems in the world. Research now finds that these plants can reduce the load of disease-causing bacteria such as Enterococcus in the surrounding seawater by up to 50%. What’s more, coral reefs also show a 50% reduction in disease when seagrasses live nearby.

The meadows act as nurseries that shelter young animals, and provide permanent homes for creatures including fish, crabs and shrimp. The plants are also superstars when it comes to carbon sequestration. Now findings published 16 February in Science1 add a health-care component to the long list of ecosystem services that seagrasses provide.

“This study touches on something that is often ignored or forgotten,” says Lina Mtwana Nordlund, a marine and environmental researcher at Stockholm University. That’s the ability of seagrasses to ameliorate the effects of terrestrial pollution on the marine environment.

The study’s authors didn’t investigate how exactly seagrasses neutralize bacteria. But lead author Drew Harvell, a marine ecologist at Cornell University in Ithaca, New York, suggests several possible mechanisms: oxygen produced by the plants could kill certain bacteria; filter-feeding animals living in seagrass meadows might strain out pathogens; or microbes could end up physically stuck to seagrass blades.

“Since seagrasses remove sediment and particulates from the water, it is not a stretch to expect bacteria and surface-associated pathogens to also be removed from the overlying waters,” says Frederick Short, director of SeagrassNet, a global monitoring and information network for seagrass meadows. “It’s a major finding to have convincing data on yet another important function of seagrass habitat.”


A mass illness affecting participants of a workshop in 2011 inspired Harvell to look into the effects that seagrass meadows could have on pathogens. During a programme on coral health in Indonesia, everyone who went into the water, including Harvell, came down with amoebic dysentery. One researcher caught typhoid fever.

This is because relatively small islands like those in the Spermonde Archipelago, where the dives took place, can have thin, poor soil that does not soak up wastewater. The island communities often lack basic sanitation systems, so human waste and the accompanying bacteria can end up in the waves.

Harvell’s team returned in 2014 to take samples, and found that levels of Enterococcus — which can cause vomiting, diarrhea and nausea — in some areas were about ten times what the US Environmental Protection Agency considers safe. But contamination was lower in areas with seagrass meadows. The team also found drastic reductions in disease among corals near seagrasses.

“I think this research is a huge contribution to helping us understand the demise of coral reefs occurring in many locations,” says Esther Peters, a marine biologist who studies coral disease at George Mason University in Fairfax, Virginia. “I did not have any idea that seagrasses could be so important to corals by reducing potential bacterial pathogens.”

In decline

Although the findings are promising, Nordlund would like to see the research expanded to a larger scale, with a broader range of seagrass species and densities.

Species come in a range of sizes: from hovering just a centimetre or two above the sea bed to towering several metres into the water column. And seagrass roots penetrate to different depths, depending on the species and the make-up of the sediment. This probably means that different species can scrub varying amounts of bacteria out of the water, Nordlund says.

Nevertheless, Short is pleased to see the results of the study, especially given that seagrass habitats are in decline around the world — mostly owing to the effects of human activity, including pollution, nutrient-rich run-off from farms and lawns and damage from boats. “It may help to convince people worldwide of the need to protect and restore seagrasses,” he says.

'Seagrasses' vital to coastal health
Jonathan Amos BBC 17 Feb 17;

The importance of seagrasses to the health of coastal ecosystems is underlined in new research conducted around Indonesian atolls.
These underwater flowering plants, which have been with us since the age of the dinosaurs, have long been known to have anti-microbial properties.

But the latest study demonstrates that their presence really does help to suppress pollution.

Coral reefs also seem to be in a better condition when the grasses are nearby.

Although these plants grow in vast meadows, fringing every continent except Antarctica, they are also being damaged on a large scale by human activities, with global losses estimated at 7% each year since 1990.

Dr Joleah Lamb and colleagues tell this week's Science Magazine that the "ecosystem services" provided by the grasses should be valued more highly.

"The plants play so many important roles and what we've shown is just another reason to support their conservation," the Cornell University, New York, researcher said.

Dr Lamb described her work here in Boston at the annual meeting of the American Association for the Advancement of Science (AAAS).
Her team sampled seawater in the vicinity of four islands close to Sulawesi where untreated wastewater is allowed to get into the ocean.

Using a genetic probe, the researchers measured the levels in the water of Enterococcus-type bacteria, which can cause infections in humans, fish and invertebrates.

They found the load of these potentially pathogenic marine bacteria to be reduced by half when seagrass meadows were present, compared with sites that did not have the plants.

"We don't really understand the exact mechanisms that are driving the reduction in the load of harmful bacteria, but it could be the result of the seagrasses themselves and their natural chemistry, or the other organisms that are filtering the water within the seagrass meadows," she told BBC News.

"But it could also be that because they're plants, they're adding a lot of oxygen to the water through photosynthesis. That's interesting because wastewater treatment facilities will often use pulses of oxygen to deactivate bacterial pathogens."

Seagrass meadows and coral reefs are tightly linked habitats, and the team also examined more than 8,000 reef-building corals at the atolls for visual signs of the tissue loss that is characteristic of active disease lesions.

The scientists did this along reefs with and without adjacent seagrass meadows. And, again, the prevalence of disease was 50% less on those reefs paired with seagrass meadows.

The mechanism here could have something to do with the way the plants anchor sediment, preventing it from moving over corals. Other studies have suggested pathogens could be transmitted this way on sediment particles.

Seagrass meadows are being damaged worldwide, through coastal development, port development, destructive fishing practices, and excessive sediment run-off from land.

But the hope is that this study will make people sit up and take notice of what seagrasses offer.

And co-author Jeroen van de Water, from the Scientific Center in Monaco, put forward one suggestion that would certainly raise their "dollar value".

"Aquaculture is undergoing a big increase worldwide because of the global food shortage. But because marine organisms (in fish farms and the like) are densely populated, disease outbreaks are quite a problem.

"Maybe it would be interesting to integrate seagrass treatment systems with aquaculture, to reduce the cost on the environment but also economically."

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