The presence of plants in urban areas has been shown on numerous occasions to improve human health, and a variety of ecological systems have proven to be a useful part of sewage treatment systems, including Reed beds and other wetlands, Mangrove forests and Bivalve reefs. Seagrass meadows are the world’s most common coastal ecosystem, and chemicals derived from Seagrasses have been demonstrated to inhibit the growth of a range of Bacterial pathogens in the lab, but the role of Seagrasses in reducing Bacterial pathogens in coastal waters has never been assessed.
In a paper published in the journal Science on 17 February 2017, Joleah Lamb of the Department of Ecology and Evolutionary Biology at Cornell University, Jeroen van de Water of the Australian Institute of Marine Science and the Department of Marine Biology at the Centre Scientifique de Monaco, David Bourne, also of the Australian Institute of Marine Science and of the College of Science and Engineering at James Cook University, Craig Altier of the Department of Population Medicine and Diagnostic Sciences at Cornell University, Margaux Hein, also of the College of Science and Engineering at James Cook University, Evan Fiorenza, also of the Department of Ecology and Evolutionary Biology at Cornell University, Nur Abu and Jamaluddin Jompa of the Faculty of Marine Science and Fisheries at Hasanuddin University and Drew Harvell, again of the Department of Ecology and Evolutionary Biology at Cornell University, describe the results of a study of pathogenic Bacterial in coastal ecosystems both with and without Seagrasses in the islands of the Spermonde Archipelago in Indonesia.
The Spermonde Islands are densely populated and lack basic sanitation systems. The soils of the islands are thin, and do not retain wastewater well, with the effect that this quickly enters coastal waters. Lamb et al. sampled a variety of coastal sites both with and without Seagrass meadows, from the both in the in tidal flats where Seagrass grows and the Coral reefs that surround the islands, for the pathogenic Bacterium Enterococcus.
All waters were found to be heavily contaminated by the Bacterium, with levels averaging 1123 Bacteria per 100 ml of water – more than 10 times the level that the U.S. Environmental Protection Agency deems unsafe in recreational water. Open water sampled between the island had Enterococcus levels of 0-12 Bacteria per 100 ml of water, indicating that the Bacteria were derived from the islands. While the Bacteria were present at high levels in all coastal waters, water collected from tidal flats where Seagrass was present contained only about a third as many Bacteria as tidal flats where Seagrass was absent, and water collected from Coral reefs downslope of Seagrass beds contained only about a third as many Bacteria as water collected from Coral reefs where Seagrass beds were absent.
Diagrammatic representation of the inshore environments around the Spermonde Islands. Lamb et al. (2017).
Enterococcus Bacteria are often found in association with other pathogenic Bacteria derived from wastewater. Lamb et al. determined 42 genera of Bacteria harmful to Humans, Fish and Marine Invertebrates that had been found in association with Enterococcus in other studies, and sampled the waters around Bonetambung Island for these, determining that at least 18 of these were present in coastal waters around the island. These other pathogenic Bacteria were found to be present at twice the level in waters on tidal flats where Seagrass was absent and the Coral reefs below them compared to flats and reefs where Seagrass was present.
Next Lamb et al. examined Corals growing around the island for five commonly reported pathogenic symptoms, Black Band, White Syndrome, Growth Anomalies, Skeletal Eroding Band and Brown Band. They found that instances of disease in Corals were twice as prevalent in areas with Seagrass, and two conditions, Black Band and White Syndrome, were particularly associated with the absence of Seagrass.
This suggests that Seagrasses play a significant role in reducing the levels of Bacteria in seawater harmful to Corals as well as Humans. This is significant as Corals around the world have suffered significant levels of decline (as much as 80% loss in parts of the Caribbean and Indo-Pacific regions), with at least part of this due to disease. Pathogens affecting Corals are poorly understood, but at least one sewage-derived Bacterium, Serratia marcescens, has been shown to cause disease in Corals.
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