The fungi are likely oxidizing the plastic to CO2, probably via many metabolic intermediates. This is likely driven by the fact that plastics are chemically reduced - a rich source of chemical potential energy. Accessing that energy requires enzymatic conversion to a less reduced state, culminating in the fully oxidized CO2 molecule.

So, a byproduct of this process is, potentially, greenhouse gases? Yay.

I've heard living organisms tend to output carbondioxide