I’m vaguely qualified (biochem degree) to say it’s probably possible

that being said there’s just a lot of stuff in nature easier to eat than plastic so if some kind of plasticphagic microbe starts causing issues it’ll likely be somewhere otherwise very inhospitable, like near the poles or in space, where there’s not much else in the way of metabolizable carbon sources

which, lol. imagine going on an Antarctic or space mission and your fucking PPE starts fermenting lmao good luck

or the ocean might become so horribly poisinous that everything dies and after microbes eat all the dead biomass and then each other, then they start eating the plastic 😬