Seagrass Reduces Marine Pollution and Disease

Drew Harvell discusses her team’s research, revealing how the presence of seagrass mitigates the effects of human sewage in ocean ecosystems, in an edited video clip.  | Carla Schaffer/AAAS

BOSTON — Ocean seagrass meadows reduce bacteria unhealthful to humans and marine organisms by up to 50%, a new study shows, and they also decrease the likelihood of disease in coral reefs by half.

“These findings are extremely important for ensuring the conservation of seagrass ecosystems,” said Joleah Lamb, a postdoctoral researcher in the department of ecology and evolutionary biology at Cornell University. “Global loss of seagrass meadows is estimated at 7% each year since 1990, but when compared to other endangered marine ecosystems, seagrass meadows receive less than 2% of media coverage. I hope these results will send a clear message to other regions of the world about the potential benefits that these ecosystems can have on human and marine health.”

Not only could seagrasses help improve water quality and maintain the health of marine ecosystems in increasingly populated coastal zones, where an estimated 1 billion people are projected to reside by 2060, but these plants also play a key role in sustaining the rapid increase in the aquaculture industry occurring in recent years in the face of global food shortages.

“The aquaculture industry is highly susceptible to disease outbreaks from bacterial infections,” Lamb said. “Integrating seagrass treatment systems with aquaculture could lower the economic and environmental costs accompanying routine use of vaccines, pesticides and therapeutics for treatment of marine disease outbreaks.”

The study, published in the 17 February issue of the journal Science, was the focus of a press briefing at the 2017 AAAS Annual Meeting in Boston.

Removing human pathogens from water is essential for human health. Plants, with their natural microbe-killing compounds, play a vital role – one that can offer significant economic benefits. For example, in recent years, New York City opted to purchase and restore wetland habitat in a nearby catchment area instead of building an $8 billion treatment plant, saving the city billions in capital and ongoing costs.

Although seagrasses are known to produce natural antibiotics, they have not been evaluated for their ability to remove pathogens from the ocean, or for their ability to mitigate disease among organisms living there.

“Levels of disease in many of our oceans’ animals are increasing,” said Drew Harvell, senior author on the study, and a professor in the department of ecology and evolutionary biology at Cornell University. “It is vital to start examining new ways to reduce or stop what is making our critical marine resources susceptible to disease outbreaks.”

Working in waters off four Indonesian islands, Harvell, Lamb and colleagues sought to assess the influence of seagrass on marine microbial pathogens and disease – an effort in part inspired by their team members first getting sick in this location while studying coral beds.

“When our entire research team became ill in Indonesia,” said Lamb, “Professor Harvell was really eager to do something about the wastewater pollution. One night, I was watching YouTube and stumbled across a time-lapse video of clams rapidly removing algae from experimental aquariums. I thought this was very exciting. We immediately contemplated whether the vast seagrass ecosystems around the islands we worked on … in Indonesia could perform this same service.” 

In shore waters off these islands, the researchers studied bacterial composition. The group used Enterococcus tests, the EPA standard to determine beach closures, to see whether seagrass meadows influenced bacterial levels. Seawater samples of Enterococcus exceeded EPA human health risk exposure levels by a factor of 10 for bacteria in recreational waters.

“120 is the cutoff in U.S. waters for beach closures, for Enterococcus,” Harvell said, “and these levels were over 2,000 in the non-seagrass beds.”

Levels of Enterococcus were reduced three-fold, however, in areas where seagrass was abundant, the researchers found.

Further studies by Harvell, Lamb and their team revealed that pathogens of fish and marine invertebrates were also much lower in number when seagrass was present – by 50%.

Because seagrass meadows and coral reefs are usually linked habitats, the team also examined more than 8,000 reef-building corals for disease. Disease levels were two-fold lower on reefs with adjacent seagrass beds than on reefs without nearby grasses, they report.

“This is the first study to assess whether a coastal ecosystem is capable of alleviating diseases that affect and associated marine organisms,” said Lamb.

At this point, Lamb and colleagues do not fully understand the exact mechanisms that are driving the reductions in bacterial loads of seawater within seagrass ecosystems. An intact seagrass ecosystem comprises a diversity of bivalves like clams, sponges and tunicates like sea squirts that could further remove bacteria from the water column. Lamb and colleagues are now considering this further.

“Our findings are the beginning of a solution to dialing back the pollution that can cause disease in both ocean wildlife, and humans,” said Harvell.

[Associated image: Margaux Hein]