Polystyrene is the solid form of a clear liquid - styrene - which is a made from petroleum and natural gas by-products. The chemical bonds that make up polystyrene are very strong and stable; they do not break apart easily in water the way that the bonds in starch do. And while many common bacteria produce enzymes to break the chemical bonds in starch, you'll be hardpressed to find any that make enzymes that can break apart polystyrene.