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Plastishpere: A Hotbed for Vibrio, No Threat of Cholera

A dense flotilla of plastic trash rides on the ocean tide around the barrier islands off of Belize. [Photo Laurie Penland. Smithsonian] – click to see Laurie’s video of the dive through this trash float

Last year, scientists discovered that huge patches of garbage in the middle of the ocean were creating a completely new kind of ecosystem made primarily of microbes thriving on plastic. Of the 1,000 species of microbes the scientists found living on ocean plastic, one of the frequent inhabitants was vibrio. That specific bit of news was often included in media reports followed by the clarification that certain forms of vibrio cause cholera, a potentially fatal human pathogen.

I started wondering what that meant exactly. The mention of cholera certainly makes it sound as if this new ecosystem in the middle of ocean poses a potential threat to human health. It doesn’t seem like too big a stretch to imagine floating plastic carrying cholera across the ocean, or to assume that beachgoers may be in danger of becoming infected with cholera from debris washed ashore. But how much of a threat is there, and what are the scientists really concerned about?

I thought I’d find out for myself what the presence of vibrio on floating plastic really means, so I called Dr. Tracy Mincer, a marine chemist from Woods Hole Oceanographic Institution. He was one of the three scientists who discovered this new plastic ecosystem and named it the plastisphere.

Dr. Mincer said that the type of vibrio he and his colleagues found in the plastisphere isn’t harmful to humans, and he doesn’t expect to find any bacteria that is. “It’s unlikely to find human pathogens because these things live out in the open ocean where they don’t come into contact with humans,” he said. He also added that things may be different in coastal waters, so he’s collaborating with colleagues in India who are collecting plastic debris from coastal areas to see what kind of bacteria are growing there.

For now, his concern is the potential impact of plastisphere vibrio on fish in the open ocean. The bacteria they found “certainly do have the toolset that is making us think that they are fish pathogens,” he said. “They have these somewhat uncommon genes that allow them to attach specifically to intestinal lining [in fish] and make specific toxins to thrive as a pathogen and get as much nutrients out of that gut environment as possible.”

“They don’t align with vibrio cholera which is a known human pathogen,” he said, “because they’re missing some of those human-specific toxins.”

But because they align with fish-specific toxins, their presence in the open ocean could spell trouble. Normally, the open ocean is somewhat of a desert environment with respect to nutrients, but the plastisphere is providing the perfect home for these bacteria, which serve as a rich new source of food for small crustaceans and fish. What’s more, these microbes seem to be advertising their presence to hungry passersby.

“The surprising thing that we found is that much of the plastic debris we harvested out in the ocean is bioluminescent,” Mincer said. “There’s a lot of bioluminescence in the open ocean, especially in the tropics, but one of the things we think is that there are microbes that are actually luminescing for the purpose of attracting fish.”

Mincer believes that if the microbes thrive and multiply in fish guts, it makes sense for them to light up like a big “buffet” sign to attract fish that will eat them. The problem, of course, is they’ll either kill the fish that eat them, or work their way up the food web to kill larger predators. Either way, in large enough quantities, they could have the potential to cause big shifts in the mid-ocean ecosystem that will have unknown ripple effects.

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