Well for one, bacteria already exists that eats plastic: http://science.sciencemag.org/content/351/6278/1196 http://science.sciencemag.org/content/351/6278/1196

Well for one, bacteria already exists that eats plastic: http://science.sciencemag.org/content/351/6278/1196 http://science.sciencemag.org/content/351/6278/1196 It's not too far fetched to assume this already-existent bacteria could continue to exist and possibly mutate into something more efficient or proliferate to encompass the entire ocean.