Radiotrophic fungi were first discovered in 2007 as black moulds growing inside and around the damaged Chernobyl Nuclear Power Plant in Ukraine. Subsequent research at the Albert Einstein College of Medicine showed that three melanin-containing fungi, Cladosporium sphaerospermum, Wangiella dermatitidis, and Cryptococcus neoformans, were able to increase biomass and accumulated acetate faster in an environment in which the radiation level was 500 times higher than in the normal environment.
The fungi appear to have a biochemical mechanism which is able to use gamma radiation (high energy photons) to synthesise organic compounds or other metabolites. The mechanism is unknown.
If it's confirmed that the fungi are actually using the radiation - rather than tolerating it - there could be profound implications for the evolution of life in hostile environments.
More info : Wikipedia
Note: A recent paper in BioXiv suggests that such fungi might be useful as a protective shield in spacecraft.