Plastics break down to tiny polymers in the seas and are carried by currents and concentrated in certain areas.
Now scientists have discovered a diverse range of microbes thriving on the plastic flecks and say it represents a 'novel ecological habitat' in the oceans.
On one fragment, the size of a pin head, more than 1000 different bacterial cells were found - many unknown to science.
In journal Environmental Science & Technology, they question how it will change environmental conditions for marine microbes and the ocean ecosystem as a whole.
A team of scientists - Erik Zettler from Sea Education Association (SEA), Tracy Mincer from Woods Hole Oceanographic Institution (WHOI), and Linda Amaral-Zettler from the Marine Biological Laboratory (MBL), analyzed marine plastic debris that was skimmed with fine-scale nets from the north Atlantic.
Dr Amaral-Zettler said: "We're not just interested in who's there. We're interested in their function, how they're functioning in this ecosystem, how they're altering this ecosystem, and what's the ultimate fate of these particles in the ocean.
"Are they sinking to the bottom of the ocean? Are they being ingested? If they're being ingested, what impact does that have?"
They found at least 1,000 different types of bacterial cells on the plastic samples, including many individual species yet to be identified.
These included plants, algae, and bacteria that manufacture their own food (autotrophs), animals and bacteria that feed on them (heterotrophs), predators that feed on these, and other organisms that establish synergistic relationships (symbionts).
These complex communities exist on plastic bits hardly bigger than the head of a pin, and they have arisen with the explosion of plastics in the oceans in the last 60 years.
Dr Mincer said: "The organisms inhabiting the plastisphere were different from those in surrounding seawater, indicating that plastic debris acts as artificial 'microbial reefs.
"They supply a place that selects for and supports distinct microbes to settle and succeed."
Because plastics last longer than normal flotsam and jetsam, and offer other different conditions, scientists suspect the communities are unlike those seen before.
But the microbes may also play a role in degrading plastics.
The researchers saw microscopic cracks and pits in the plastic surfaces that they suspect were made by microbes embedded in them, as well as microbes possibly capable of degrading hydrocarbons.
Dr Zettler said: "When we first saw the 'pit formers' we were very excited, especially when they showed up on multiple pieces of plastic of different types of resins.
"Now we have to figure out what they are by [genetically] sequencing them and hopefully getting them into culture so we can do experiments."
But they warn that the plastic also provides a new mode of transportation, acting as rafts that can convey harmful microbes, including disease-causing pathogens and harmful algal species.
One plastic sampled they analysed was dominated by members of the genus Vibrio, which includes bacteria that cause cholera and gastrointestinal maladies.