Yes there are some bacterias,like the bactrium ideonella sakaiensis that can digest certain types of microplastics by having an enzyme that capable of breaking down plastics.
To the question, it is not clear. Much of the work that concludes biodegradation is conducted by ignorant workers - unaware "plastic" includes fillers and plasticizers from 20-50%. These are much more degradable than the accompanying plastic polymers - their effect leaving smaller particles of "plastic" not of necessity mineralized polymer.
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