Plastic gets consumed by oceanic microorganisms, though slowly. Take a look at the fascinating microscopic images of plastic partially eaten by microorganisms in this 2013 research article, "Life in the Plastisphere: Microbial Communities on Plastic Marine Debris":

http://onemoregeneration.org/wp-content/uploads/2012/07/Life...

Plastics also break down to simpler hydrocarbons and carbon dioxide with prolonged exposure to sunlight. Unfortunately, these gaseous small molecules are greenhouse gases, but they don't persist for as long.

"Production of methane and ethylene from plastic in the environment"

https://journals.plos.org/plosone/article?id=10.1371/journal...