The plants also draw fertilizer runoff and other pollutants out of the water, locking them safely away in meadow soil. Scientists have estimated that an acre of seagrass provides more than $11,000 worth of filtering every year.
These services alone would make seagrass meadows among the most economically valuable ecosystems on Earth. But now Dr. Harvell, Dr. Lamb and their colleagues have found that these plants may help us in another way: by wiping out pathogens.
Their new study, published in the journal Science, began with a scuba diving trip in Indonesia. The scientists were inspecting coral reefs for infections with bacteria and fungi; some reefs are increasingly falling prey to these diseases.
“By the end of the four-day workshop, we all came down with amoebic dysentery,” recalled Dr. Harvell. “One scientist developed typhoid, and we had to ship her out.”
The experience left Dr. Harvell wondering if the illnesses of the scientists might somehow be connected to those of the reefs. “It just got me thinking about human health and environmental health and how they’re linked,” she said.
She discussed the idea with Dr. Lamb, who began to investigate.
Seagrass meadows can release so much oxygen that the surrounding water fizzes like champagne. That oxygen might be able to kill pathogens, Dr. Lamb realized. The plants also host fungi, which are known to producing bacteria-killing compounds.
Dr. Lamb and her colleagues decided to study sites around four islands in Indonesia with and without nearby seagrass. The scientists devised two strategies to search for pathogens.In one survey, they collected seawater and put it in petri dishes to see if colonies of disease-causing bacteria known as Enterococcus grew from the samples. Levels of the bacteria in water from seagrass meadows, they found, were a third of the levels in water from other sites.
In a second search, the scientists grabbed fragments of DNA floating in seawater. By examining the sequences, they identified 18 kinds of disease-causing bacteria. Water from the seagrass meadows had only half the level of this DNA, compared with water collected at other sites.
The scientists next turned their attention to coral reefs around the islands. Reefs next to seagrass meadows, they found, were half as diseased as those without meadows.
Thorsten B. Reusch, a marine ecologist at the GEOMAR Helmholtz Center for Ocean Research in Germany who was not involved in the study, called it “truly fascinating and innovative.” He also warned that “it remains to be seen what the exact mechanisms of the pathogen removal are.”
Vast mixed-species seagrass meadows in front of a village in the Spermonde Archipelago, Indonesia. Credit Joleah Lamb