The plants keep waterborne pathogens in check, potentially benefitting people and coral reefs.
Seagrass meadows are the most widespread coastal ocean environment in the world. And new research finds that these plants can reduce the load of bacteria, such as Enterococcus, in the surrounding seawater by up to 50%. What’s more, coral reefs also show a 50% reduction in the prevalence of disease when seagrasses live nearby.
The plants act as nurseries that shelter young animals, and provide permanent homes for creatures including fish and manatees. Seagrass meadows are also superstars when it comes to carbon sequestration. But the new findings, published 16 February in the journal Science1, add a healthcare component to the long list of ecosystem services 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 in Sweden. And that’s the ability of seagrasses to help ameliorate the effects of terrestrial pollution on the marine environment.
Researchers didn’t investigate the mechanism by which seagrasses neutralize bacteria. But lead study author C. Drew Harvell, a marine ecologist at Cornell University in Ithaca, New York, explains several ways in which it could happen. 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.”
Sick to their stomachs
A mass illness at a workshop in 2011 inspired Harvell to look into the effect seagrass meadows could have on pathogens. While conducting a program on coral health with several other scientists 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 found in the Spermonde Archipelago, where the dives took place, can have thin, poor soil that does not hold on to wastewater. Since the island communities often lack basic sanitation systems, human waste and the accompanying bacteria can wind up in the waves.
The research team returned in 2014 to take their samples and found that bacterial levels in some areas were about ten times higher than what the US Environmental Protection Agency considers safe. Amounts were lower in areas with seagrass meadows. The team also found drastic reductions in coral disease levels in areas with nearby 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 studying 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 a decline
While the study’s findings are promising, Nordlund would like to see the research conducted on a larger scale, with a broader range of seagrass species and densities.
Different species range in size from hovering just a centimeter or two above the seabed to towering several meters into the water column. And their roots penetrate to different depths depending on the species and sediment makeup. This would probably result in some variation in the amount of bacterial scrubbing any given species might be capable of, Nordlund says.
Nevertheless, Short is pleased to see the results of the study, especially since seagrass habitats are in decline around the world — mostly from human actions including pollution, runoff and damage from boats. “It may help to convince people worldwide of the need to protect and restore seagrasses,” he says.
Lamb, J. et al. Science 355, 731-733 (2017).
Related links in Nature Research
Related external links
About this article
Cite this article
Bittel, J. Seagrasses can scrub seawater of harmful bacteria. Nature (2017). https://doi.org/10.1038/nature.2017.21504