Radiotrophic fungus

Radiotrophic fungi are a recent discovery, first seen as black molds growing inside and around the defunct nuclear reactor in Chernobyl, Ukraine. These fungi use the pigment melanin to capture gamma rays that excites electrons inside melanin which may be used by the fungus to convert radiation energy into chemical energy for growth. This proposed mechanism may be similar to anabolic pathways for the synthesis of reduced organic carbon (e.g., carbohydrates) in phototrophic organisms, which capture photons from visible light with pigments such as chlorophyll whose energy is then used in photolysis of water to generate usable chemical energy (as ATP) in photophosphorylation of photosynthesis. However, whether melanin-containing fungi employ a similar multi-step pathway as photosynthesis, or some chemosynthesis pathways, is unknown.

Research at the Albert Einstein College of Medicine showed that three melanin-containing fungi, Cladosporium sphaerospermum, Wangiella dermatitidis, and Cryptococcus neoformans increased in biomass and accumulated acetate faster in an environment in which the radiation level was 500 times higher than in the normal environment. Exposure of C. neoformans cells to these radiation levels rapidly (within 20-40 minutes of exposure) altered the chemical properties of its melanin and increased melanin-mediated rates of electron transfer (measured as reduction of ferricyanide by NADH) 3 to 4-fold compared with unexposed cells. Similar effects on melanin electron-transport capability were observed by the authors after exposure to non-ionizing radiation, suggesting that melanotic fungi might also be able to use light or heat radiation for growth.

However, melanization may come at some metabolic cost to the fungal cells: in the absence of radiation, some non-melanized fungi (that had been mutated in the melanin pathway) grew faster than their melanized counterparts. Limited uptake of nutrients due to the melanin molecules in the fungal cell wall or toxic intermediates formed in melanin biosynthesis have been suggested to contribute to this phenomenon. It is consistent with the observation that despite being capable of producing melanin, many fungi do not synthesize melanin constitutively (i.e., all the time), but often only in response to external stimuli or at different stages of their development. The exact biochemical processes in the suggested melanin-based synthesis of organic compounds or other metabolites for fungal growth, including the chemical intermediates (such as native electron donor and acceptor molecules) in the fungal cell and the location and chemical products of this process, are unknown.