Put some sand grains in a round bowl of water, then twirl the water neatly, so that the water rotates cleanly around the inside of the bowl. You will observe most of the sand settle out of the water in the center of the bowl. Do the same thing with tiny bits of floating plastic, and you will get the same result. Now do the same thing to the Pacific Ocean with all the floating trash we throw into it, and you get the Great Pacific Garbage Patch. A large portion of the North Pacific naturally rotates in a current called the North Pacific Gyre. Take a boat to the center of rotation, and you will find enourous quantities of floating junk, especially plastic. Wave action and UV gradually break this plastic down into microscopic bits, which become part of the local ecosystem along with the water, plankton, fish and so forth. But nothing eats plastic, right?
This is where my current pondering comes in. Imagine that one of the hundreds of billions, maybe trillions, of plankton in the Great Pacific Garbage Patch had a mutation that allowed it to gain some slight advantage from plastic. It would surely find some tiny scrap of plastic, and would have a slight advantage over its competitors. If that helps it reproduce successfully, and its offspring carry that same mutation, then we have more plankton who gain a competative advantage from plastic. At the rates plankton reproduce, it is not long before the GPGP is full of plankton who can make some use of plastic. Now suppose another mutation in one of those plastic-loving plankton makes it even better at using plastic. Able to slowly digest some component of it or incorporate it into its protective covering. The larger plankton that eat those little plankton will also have to evolve to deal with plastic. Sooner or later, in the crazy fast generations of open ocean plankton, someone is going to start evolving down a path that leads to the ability to digest plastic. And when that happens, when the plastic in the North Pacific Gyre starts myseriously disapearing, don't expect the little plastic eaters to stay there. There are far to many ecosystems with far to much plastic available for plasticovores to be contained. Plastic is, after all, an organic substance with a high energy content. Sure it isn't biodegrable. Not yet.