Seagrasses clean up polluted waters

For years we’ve abused the oceans and used them as dumping grounds for everything from plastics and chemicals to industrial waste and even raw sewage. This is having a devastating effect in some areas by causing bacteria to thrive, which is killing off fish and corals and also increasing the odds of humans becoming unwell. Now scientists have discovered that the sea may have its own solution in the shape of seagrasses, which come with their own inbuilt water cleansing system. Drew Harvell, from Cornell University spoke to Chris Smith about these mysterious plants...

Drew - Seagrasses are flowering plants so just like our grasses on land they have little flowers and they produce seeds which are then sexually produced and start new plants. They also can clonally produce so vast meadows can spread really quite quickly. Seagrass beds are the base of the food chain in the ocean. They provide critical nursery areas for baby fishes, and forage fishes which are big fishes like salmon and even killer whales eat. They also do a lot of filtering of coastal pollution and even absorb carbon dioxide. So seagrass meadows are considered a source of what we call blue carbon in a sense they can absorb a lot of carbon dioxide and potentially mitigate ocean warming impacts.

Chris - What have you been asking in this present study about seagrasses?

Drew - In this study we wanted to focus on the filtration services of seagrass and their capability to reduce bacteria in the water in areas where there was very high sewage pollution. We worked on these four islands in Indonesia that were very large sources of sewage pollution and we examined how the levels of bacteria changed as the water passed over the seagrass beds. This was done by my post-doc, Joleah Lamb, in collaboration with Indonesian scientists that we’d been working with and they both studied particular species of bacteria which is the one that we in the United States use for testing the quality of our waters (the enterococcus). And then when they found a large impact of the Enterococcus, they went and completely sequenced all of the bacteria in the water and showed large reductions in multiple pathogens that could affect humans as well as fishes and invertebrates.

Chris - So, in a nutshell, where you see seagrass you see fewer nasty bacteria?

Drew - Exactly. There’s a big difference in the levels of bacteria that have passed over the seagrasses. So we think of them as a possible mitigation for sewage pollution or a way to make the water cleaner and healthier.

Chris - How much of a difference was there in terms of the bacterial burden in the water when you compare areas that have got a lot of seagrass with those that haven’t?

Drew - At some of these sites there were levels of potentially pathogenic marine bacteria that impact human, fishes, and invertebrates that were reduced by 50 percent when the seagrass meadows were present compared to sites without seagrass meadows.

Chris - How do you think the seagrass is doing that then?

Drew - that’s really one of the exciting frontiers for our future work. We’re very excited to work out what are called the mechanisms by which seagrass are removing bacteria. One possibility is that a lot of the animals that live in the seagrass, such as the clams, the bivalves, the sponges are filter feeders and they’re directly sucking up bacteria for themselves. But we also think that the seagrasses can detoxify or remove the bacteria. For example, seagrasses produce oxygen and a lot of these bacteria can’t live with oxygen and so that’s an important mechanism of detoxification. And then finally, there are the microbiome of the seagrass, the bacteria that live on the surface of the seagrass, can also kill the bacteria. So there are a range of these different mechanisms that we’re investigating.

Chris - Do you think then, that it might be worth probing seagrass to see how they’re doing this and whether there might be some potential untapped resources in there that we could use for making human’s healthier? We know that we’re facing an antibiotic apocalypse where there are just not drugs for some of these infections any more. Do you think seagrass might have lurking in it a solution?

Drew - Oh that’s such a great question. There’s so much potential for innovation in the way that we deal with our pathogen bacteria in our society and, of course, we should be looking to natural sources like the oceans and seagrasses. So yes, for example, there’s a bacterium that lives on seagrasses that kills harmful algal blooms so likely there are some bacteria that live on seagrasses that could also kill other bacteria. So that’s a very exciting frontier for discovery.