Comment by scythe

Polyethylene terephthalate is a little unique. Part of its popularity comes from recycling, because it is "easy" to break down. For other polymers like polystyrene or PVC it's not so easy.

But any plastic is going to be harder to break down than cellulose because life depends on water and plastics are usually hydrophobic. So non-porous things will always break down pretty slowly. Plenty of plants grow in the water, after all, and aren't immediately consumed by bacteria. Microplastics should, in principle, be the first things to go.