Seagrasses kill bad bacteria

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.

Cornell University's Drew Harvell and her colleagues took water samples from the shoreline, mudflats and reefs around 4 Indonesian Islands where the human inhabitants were discharging untreated sewage directly into the sea.

Apart from on the shoreline, compared with equivalent areas that lacked seagrasses, wherever seagrasses were present the density of pathogenic bacteria was 50% lower.

Surveys of reef-building corals located adjacent to seagrass meadows also showed a twofold reduction in disease rates compared with corals situated away from the seagrass.

"There are a range of factors that could account for this," says Harvell. "Seagrass meadows are home to a host of invertebrates like sponges and filter feeders, which could be taking up and neutralising the bugs. Also, the plants produce oxygen, which is directly toxic to many bacterial sewage pathogens."

Harvell also speculates that the plants' own microbes might be responsible.

"The plants have their own microbiome - bacterial community. This might be producing compounds that can neutralise pathogens."

The discovery, published this week in Science, is very exciting because it offers a way to help rejuvenate coral and fish stocks, by enhancing or promoting seagrasses to grow nearby, and it may even provide leads in the search for novel antibiotic compounds.

Seagrasses are interesting plants in themselves because they are unusual in being an example of a species that evolved in the water, invaded the land and have since returned to an aquatic enviroment. Yet they retain many of the features of their land-dwelling ancestors, including producing underwater flowers and setting seeds.

"They look very similar to the grasses we grow in meadows on land," says Harvell. "Some grow a foot tall, others, like the ones around the US coastline, grow a metre in length. But they grow everywhere, and that's why they are really valuable..."