This research digest is a synopsis of the study “Seagrass ecosystems reduce exposure to bacterial pathogens of humans, fishes, and invertebrate” published in Science in 2017.

Photo of seagrass

Photo: iStock

Key Findings and Implications

  • Researchers found that the relative abundance of potential disease-causing bacterial pathogens in seawater was 50 percent lower when seagrass meadows were present compared to similar sites without seagrass.
  • Coral disease prevalence was 50 percent less on reefs with adjacent seagrass meadows, as compared to reefs without seagrass meadows.
  • While other studies have examined the role of natural ecosystem services in reducing waterborne human diseases, this was the first study to assess the potential of natural ecosystems in mitigating diseases that affect marine organisms.
  • Seagrasses and coral reefs are both ecosystems in rapid global decline; at the same time people struggle to manage sewage. Studies highlighting these interconnections can help inform investments in seagrass restoration and protection, which, in turn, can benefit the health of coral reefs, species that depend on reefs, fisheries, and human health.

Throughout most of the world, sewage is discharged into the coastal environment untreated or only partially-treated,1,2 and heavily laden with disease-causing bacterial pathogens. These bacteria pose health risks to both people and marine organisms, especially coral reefs, whose complex microbiomes—made up of a diverse community of algae, fungi, archaea and bacteria—contribute to their vulnerability.

A study published in Science in 2017 points to the previously undiscovered ways that seagrass meadows mitigate disease risks by reducing pathogens in the water, adding to a long list of valuable services provided by these rapidly-declining coastal ecosystems.

In this study, researchers collected samples from the water surrounding four islands off the southwest coast of Sulawesi, Indonesia, where residents do not have access to adequate sanitation. The study area is part of the Spermonde Archipelago in Indonesia, a chain of 120 islands within what is known as the Coral Triangle, an area considered to be a hotspot of global marine biodiversity.3 The absence of sewage management for island residents was reflected in high shoreline levels of the fecal indicator bacteria Enterococci, which were present at levels ten times higher than what is recommended by the U.S. Environmental Protection Agency threshold for safe recreation.

Further from shore, the researchers compared pathogen levels in seawater taken from areas where seagrasses were present to areas without seagrass. They found that pathogens were significantly reduced in seagrass-adjacent ecosystems. For instance, in tidal mudflats near seagrass, pathogens were three times lower; in waters with coral reefs, pathogens were about half as prevalent as they were in waters without seagrasses.

Photo: iStock

Seagrasses are flowering plants that form underwater meadows, usually in shallow coastal areas where they can absorb plenty of sunlight. Among many roles, seagrass meadows stabilize the seafloor, reducing wave action by about 20 percent, limiting erosion, and protecting coasts from storm damage.4 They absorb carbon dioxide which would otherwise act as a heat-trapping gas in the atmosphere. Though they cover less than 0.2 percent of the ocean floor,5 seagrasses sequester about 10 percent of all carbon buried in ocean sediment6 at twice the rate as the world’s temperate and tropical forests, per area.7 Seagrasses also provide critical habitat for fish and harbor a wide variety of invertebrates including crabs, squid, sponges, sea anemones and worms. A single acre of seagrasses can support 40,000 fish and 50 million invertebrates.8

Though they cover less than 0.2 percent of the ocean floor, seagrasses sequester about 10 percent of all carbon buried in ocean sediment at twice the rate as the world’s temperate and tropical forests, per area.

Despite the considerable benefits conferred by seagrass meadows, human activity has led to their rapid and ongoing decline worldwide. A seminal 2009 study found that the total area of seagrass meadow globally had decreased by nearly a third since the late 1800s, and that the rate of loss was accelerating, from less than one percent per year in the 1940s to 7 percent per year since the 1990s, making seagrass meadows among the most threatened ecosystems on earth.9 Much of this decline could be attributed to deteriorating water quality associated with coastal development.10

The impetus for this study came about during a research trip to Indonesia several years ago. While surveying coral reefs for disease, the entire dive team got sick, said the study’s lead author, Dr. Joleah Lamb, who was then a research fellow in the Department of Ecology and Evolutionary Biology at Cornell University.

“Five scientists spent several days sitting near a bathroom, and one had to be urgently flown back to Australia,” said Dr. Lamb, currently based at the University of California, Irvine. While recovering, Lamb pondered the expansive seagrass meadows she had observed while diving and wondered if these might play a role in controlling pathogens.

“It’s sort of an out-of-sight, out-of-mind problem,…and I think that speaks more broadly to why this is such a pervasive problem globally. People don’t like to think about sewage. People don’t want to spend time in areas where there is sewage,…So I think we are lagging behind in terms of research and understanding.”

“A lot of people said, ‘that’s crazy’,” said Lamb. Her peers assumed that seagrasses play a passive role in water quality and expressed doubt that the team would find results.

But wetlands, mangroves and bivalve reefs are all known to filter sediments and other particulates. Clams can take up avian influenza virus and deactivate it,11 Lamb said, so why not seagrasses? Combing through the literature, she found pharmaceutical studies showing that seagrass extracts kill pathogens in laboratory experiments,12 and also confirmed that there were no existing studies of their relationship to pathogens in situ—or, where they live in the ocean.

The study by Lamb et al. not only includes evidence that seagrasses reduce pathogen exposure for humans and marine organisms; it was bolstered by a second step the researchers took by examining the prevalence of disease in corals. The team visually inspected over 8,000 coral reefs throughout the study area, looking for patches of dead tissue, bleaching, and other signs of disease. They found significantly less disease in reefs adjacent to seagrass meadows, compared to reefs without seagrasses. Coral reefs are also an ecosystem in global decline,13 and the correlation of seagrasses with healthier reefs in waters with sewage contamination could have significant implications for coral protection efforts. For instance, in Florida, sewage is a major driver of coral reef loss,14 which has contributed to two main species being listed as threatened under the Endangered Species Act,15,16 Lamb said.

But exactly how seagrasses are reducing bacterial pathogens in the water is still an open question. One mechanism could be that when the plants photosynthesize, they release large amounts of oxygen, to the extent that they are sometimes referred to as “the lungs of the sea.” This is significant because “gut pathogens are anaerobic, which means they do not like oxygen,” said Dr. Lamb. “When they face that oxygenated environment, they are really susceptible to mortality.” For this reason, wastewater treatment plants often use pulses of oxygen to treat sewage.

But there are other possibilities. “It also could be that the seagrasses are releasing some type of biocide that’s killing pathogens,” Lamb said. “There are also a lot of clams, sponges and other marine organisms that cohabitate with seagrasses that could be filtering the water. It could be the ecosystem as a whole.”

The study’s results imply that the role of seagrasses in the health of coral reef and fisheries is much more significant than what was previously understood. One reason that the pathogen-controlling aspects of seagrasses may have been previously overlooked is that many researchers do not want to spend time in sewage-polluted water, said Dr. Amelia Wenger, Research Fellow at the University and Queensland and an Associate Conservation Scientist with the Wildlife Conservation Society. Wenger was not involved in this study but is a frequent collaborator with Dr. Lamb.

“It’s sort of an out-of-sight, out-of-mind problem,” Wenger said, “and I think that speaks more broadly to why this is such a pervasive problem globally. People don’t like to think about sewage. People don’t want to spend time in areas where there is sewage,” Wenger said. “So I think we are lagging behind in terms of research and understanding.”

Expanding research efforts and raising awareness about the problems of sewage in the ocean—and the potential solutions inherent in functioning intact ecosystems—have the potential to both motivate conservation and improve human health at the same time.

“There are a lot of remote communities where putting in wastewater infrastructure is just not feasible for a lot of reasons,” Wenger said. “In countries where you have coral reefs, mangroves, and seagrasses, you already have these natural wastewater systems. It provides a viable option for managing this problem in places where you don’t have built gray infrastructure.”

The movement to ‘green’ cities has become popular for solving several urban problems at once, and Lamb would like to see a similar movement take hold, to advance ‘blue solutions’ through conservation and protection of the marine environment.

“If you’re developing a city or a village, you can start to plan which ecosystems you keep,” Lamb said. “These seagrass meadows are worth our investment for protection and restoration; they reduce hospitalizations and improve the length of our lives. The more services that nature provides that improve our health, the more likely we are to be able to conserve them.”

“There are a lot of remote communities where putting in wastewater infrastructure is just not feasible for a lot of reasons,…In countries where you have coral reefs, mangroves, and seagrasses, you already have these natural wastewater systems. It provides a viable option for managing this problem in places where you don’t have built gray infrastructure.”

Notes

Report citation: Lamb, Joleah B., Jeroen A. J. M. van de Water, David G. Bourne, Craig Altier, Margaux Y. Hein, Evan A. Fiorenza, Nur Abu, Jamaluddin Jompa, C. Drew Harvell. Seagrass ecosystems reduce exposure to bacterial pathogens of humans, fishes, and invertebrates. Science (2017) 355: 731-733. doi: 10.1126/science.aal1956.

  1. United Nations, World Water Development Report, Wastewater: The Untapped Resource, 2017.
  2. Hayley T. Olds, Steven R. Corsi, Deborah K. Dila, Katherine M. Halmo, Melinda J. Bootsma, Sandra L. McLellan. “High Levels of Sewage Contamination Released from Urban Areas After Storm Events: A Quantitative Survey with Sewage Specific Bacterial Indicators, Plos Medicine (2018). https://doi.org/10.1371/journal.pmed.1002614.
  3. Alison Green & P.J. Mous. Delineating the Coral Triangle, its Ecoregions and Functional Seascapes (The Nature Conservancy, TNC Coral Triangle Program, Report 1, Version 5 2008): 4. https://www.conservationgateway.org/Documents/Green%20and%20Mous%202008%20CT%20Delineation%20v5%200.pdf.
  4. M.D. Spalding, R.D. Brumbaugh, and E. Landis, Atlas of Ocean Wealth (The Nature Conservancy, 2016). https://oceanwealth.org/wpcontent/uploads/2016/07/Atlas_of_Ocean_Wealth.pdf.
  5. R.K. Unsworth, L.M. Nordlund, and L.C. Cullen‐Unsworth. “Seagrass Meadows Support Global Fisheries Production,” Conservation Letters 12 no. 1 (March 2018) doi:10.1111/conl.12566.
  6. James W. Fourqurean, “Seagrass Ecosystems as a Globally Significant Carbon Stock,” Nature Geoscience. 5 no. 7 (2012): 505–509.  doi:10.1038/ngeo1477.
  7. Ibid.
  8. Pamela L. Reynolds, “Seagrass and Seagrass Beds,” The Smithsonian Institution, April 2018, accessed August 11, 2020. https://ocean.si.edu/ocean-life/plants-algae/seagrass-and-seagrass-beds.
  9. Michelle Waycott, Carlos M. Duarte, Tim J. B. Carruthers, Robert J. Orth, William C. Dennison, Suzanne Olyarnik, Ainsley Calladine, James W. Fourqurean, Kenneth L. Heck Jr, A. Randall Hughes, Gary A. Kendrick, W. Judson Kenworthy, Frederick T. Short, Susan L. Williams, “Accelerating Loss of Seagrasses Across the Globe Threatens Coastal Ecosystems,” Proceedings of the National Academy of Sciences 106 no. 30 (Jul 28, 2009), 12377-81. doi: 10.1073/pnas.0905620106.
  10. MacGregor Campbell, “Meadows of the sea in ‘shocking’ decline,” New Scientist, July 3, 2009. https://www.newscientist.com/article/dn17412-meadows-of-the-sea-in-shocking-decline/.
  11. Christina Faust, David Stallknecht, David Swayne, and Justin Brown,Filter-feeding Bivalves Can Remove Avian Influenza viruses from Water and Reduce Infectivity,” Proceedings of the Royal Society Biological Sciences 276 no. 1673 (Oct 22, 2009): 3727–3735. doi: 10.1098/rspb.2009.0572
  12. Rengasamy Ragupathi Raja Kannan, Radjassegarin Arumugam, Palanisamy Iyapparaj, Thirunavukarasu Thangaradjou, Perumal Anantharaman  “In Vitro Antibacterial, Cytotoxicity and Haemolytic Activities and Phytochemical Analysis of Seagrasses from the Gulf of Mannar, South India,” Food Chemistry 136 no. 3-4 (2013): 1484-1489. https://doi.org/10.1016/j.foodchem.2012.09.006.
  13. Stephanie Wear, “Missing the Boat: Critical Threats to Coral Reefs are Neglected at Global Scale,” Marine Policy  no. 74 (2016): 153-157.
  14. Stephanie Wear and Rebecca Vega Thurber, “Sewage Pollution: Mitigation is Key for Coral Reef Stewardship,” Annals of the New York Academy of Science 1355, no. 1 (2015) https://doi.org/10.1111/nyas.12785.
  15. National Oceanic and Atmospheric Administration, “Endangered and Threatened Species: Final Listing Determinations for Elkhorn Coral and Staghorn Coral, ” Federal Register, May 9, 2006, accessed August 17, 2020. https://www.federalregister.gov/documents/2006/05/09/06-4321/endangered-and-threatened-species-final-listing-determinations-for-elkhorn-coral-and-staghorn-coral.
  16. Kathryn Patterson Sutherland, Sameera Shaban, Jessica L. Joyner, James W. Porter and Erin K. Lipp, “Human Pathogen Shown to Cause Disease in the Threatened Eklhorn Coral Acropora palmata,” PLOS One, (, 2011) https://doi.org/10.1371/journal.pone.0023468.

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