Paper: Lamb, JB, van de Water, JAJM, Bourne, DG, Altier, C, Hein, MY, Fiorenza, EA, Abu, N, Jompa, J, Harvell, CD (2017). Seagrass ecosystems reduce exposure to bacterial pathogens of humans, fishes and invertebrates. Science 355(6326): 731-733. doi: 10.1126/science.aal1956
Forget dogs, seagrasses are a human’s new best friend.
Okay, so maybe they don’t gleefully greet you at the door after a long day, tails wagging and saliva flying, but they provide several essential benefits. Seagrasses are the whales of the plant kingdom evolutionarily speaking—having originated in the ocean, evolved forms that colonized land, and then returned back to the sea. In fact, even though they look like algae, seagrasses are the only true flowering plants that live in the ocean!
Seagrasses can form lush underwater meadows that provide vital nursery areas for all kinds of life including commercially important species like fish, crustaceans, and more. They also play an important role in nutrient cycling, stabilizing sediment, protecting coastal communities by absorbing the impact of waves in extreme weather and absorb carbon dioxide and release oxygen via photosynthesis. A new study adds one more benefit to the list: seagrasses reduce the risk of disease outbreaks not only for humans, but also for fish and invertebrates like coral that we depend on for food, tourism and work.
Terrestrial plants have long been known for their abilities to fight disease-causing bacteria (aka pathogens) by producing natural pesticides, altering the chemistry of the soil and water, and outcompeting pathogens for nutrients. The marine plants or seagrasses that make up the most common coastal ecosystem on earth appear to have similar capabilities. Previous studies in the lab revealed that extracted chemicals from seagrasses have been shown to kill harmful pathogens. Now, scientists have found evidence that seagrasses have the ability to do this in the wild as well.
To study the pathogen-killing ability of seagrasses, scientists collected water samples from multiple paired sites (with and without seagrasses nearby) at different depths from four islands in Sulawesi, Indonesia (Figure 1). They chose sampling sites that shared similar characteristics such as human population, the sediment type, the absence of basic sewage treatment, thin soils that do not hold wastewater very well and mixed-species seagrass meadows.
After collecting over 400 water samples, Lamb and her team tested the water samples for the presence and abundance of Enterococcus, a lactic acid bacteria that can cause urinary tract infections, meningitis and other infections. Enterococcus was specifically chosen because it is often found in conjunction with other disease-causing bacterial pathogens in wastewater pollution thus serving as an indicator of human health risk.
Results showed that seawater samples from the seashore (closest to land) of all sites contained Enterococcus at levels 10 times above what was recommended for recreational water by the Environmental Protection Agency (EPA). The further out from shore, Enterococcus levels were lower at both sites with and without seagrasses. However, the samples taken from sites with seagrasses nearby were 3x lower compared with sites without seagrasses nearby (Figure 2). The trend continued for coral reef sites: the coral reef sites that had a seagrass meadow close by had 2x lower Enterococcus levels than the coral reef sites without seagrasses nearby.
The water samples from one of the sampling sites were tested for the presence (using 16s ribosomal sequencing, a similar technique to DNA sequencing) and abundance (by a technique called quantitative polymerase chain reaction) of pathogens that cause diseases in not only humans, but also fishes and invertebrates. Not surprisingly, sites with seagrasses nearby had half the number of bacterial pathogens when compared with sites without seagrasses. The community of pathogens were also different when comparing sites with and without segrasses. The sites without seagrasses had higher relative abundances of Flavobacterium (can cause disease in salmon, trout and freshwater fish), Corynebacterium (can cause throat infection in humans), Vibrio (can cause food poisoning), Rickettsia (can cause typhus) and Shewanella (may cause fish to spoil).
Another component of this study was to visually examine >8000 corals for signs of tissue damage associated with coral bleaching, coral disease (such as white syndrome and black band disease), and physical damage from moving debris at sites with and without seagrasses nearby. Again, not surprisingly, corals at sites with seagrasses nearby had half the amount of diseases than corals without seagrasses nearby.
The findings of this study highlight that healthy coral reefs need seagrasses like Batman needs Robin. Especially now as coral reefs face daunting challenges from climate change, ocean acidification, development and more. And in the end, it’s US that need seagrasses. For example, 275 million people live in coastal communities that depend on coral reefs. As for the rest of us, just as coming home to a happy, slobbering fuzzy face is awesome, so is not getting a disease or infection when we eat seafood or go swimming at the beach.
I’m a past oceanbites writer, occasional editor and guest poster. I graduated with a Masters of Coastal & Marine Management from the University of Akureyri in Iceland, and am currently working in marine conservation. In particular, I’m supporting an Indigenous-led initiative to safeguard the largest inland sea in the world (Hudson Bay & James Bay, Canada). I love weird ocean critters and *sigh…I really do enjoy long walks on the beach.