That fungus is called Cladosporium sphaerospermum, and some scientists think its dark pigment – melanin – may allow it to harness ionizing radiation through a process similar to the way plants harness light for photosynthesis. This proposed mechanism is even referred to as radiosynthesis.

Although scientists have shown that the fungus flourishes in the presence of ionizing radiation, no one has been able to pin down how or why. Radiosynthesis is a theory, one that's difficult to prove.

we still don't know what the fungus is actually doing.