To investigate the influence of ionizing radiation on the electron-transfer properties of melanin and on the growth of melanized fungi, we performed multiple physico-chemical tests and in vivo experiments with 3 genetically diverse fungi. HPLC results which reveal the chemical structure of melanin from different fungi are important for understanding the electronic properties of melanin. The number of electrons per gram is an important contributor to the attenuation properties of a material at the energy levels where the Compton effect predominates 
. Compton scattering predominates for chemical elements with low atomic numbers such as C, N, O and S 
, which constitute melanin. In Compton scattering, transfer of a photon energy to matter occurs via a cascade of interactions, where the energy of the incident photon is transferred to high-energy electrons, and to secondary photons of progressively lower energy until the photoelectric effect takes place. Thus, the existence of structures composed of electron-rich covalently linked aromatic motifs could explain radiation scattering properties of melanins. Furthermore, the higher number of electrons in oligomers of pheomelanin relative to eumelanin – 388 versus 287 – could result in better scattering properties of pheomelanin.
The high-energy electrons generated by Compton scattering are ultimately responsible for the radiobiologic effects caused by gamma radiation by either direct interaction with DNA or through radiolysis of water in the cells, a process that results in the formation of reactive short-lived free radicals capable of damaging DNA. Stable free radicals in melanin may interact with these high-energy electrons and prevent them from entering a cell, thus enabling melanin to function as a radioprotector. The Compton electrons may then undergo secondary interactions with melanin molecules with their energy gradually lowered by melanin.
When performing experiments on measuring the growth and incorporation of 14
C-acetate into irradiated and non-irradiated melanized C. neoformans
cells and its non-melanized Lac(-) mutant, we noted that non-irradiated Lac(-) cells grew better and incorporated more 14
C-acetate than non-irradiated melanized cells (). Although the basis for this difference is not understood it may be related to melanin limited porosity in the wild type melanized strain, since pore size decreases with culture age 
and might reduce the availability of nutrients. Also, the process of melanization involves an oxidation reaction with generation of toxic intermediates which may impose a certain metabolic cost that could translate into slower in growth relative to Lac(-) cells. The slight increase in the CFU's and 14
C-acetate incorporation of irradiated Lac(-) cells at 23 and 30 hr is probably due to the well documented phenomenon that very low doses of ionizing radiation can stimulate cell proliferation 
. However, the crucial difference between the wild type H99 and Lac(-) cells is that the exposure to ionizing radiation resulted in 2.5 times more CFUs and almost 3 times more incorporation of 14
C-acetate in irradiated melanized cells than in non-irradiated melanized controls, while irradiation of Lac(-cells resulted only in a 1.1-fold increase in CFUs and 14
C-acetate incorporation ().
Melanins are unique biopolymers that protect living organisms against UV and ionizing radiation and extreme temperatures. The electronic complexity of melanins allows them to scatter/trap photons and electrons, which was evidenced in this study by the following observations: 1) changes the electronic structure of melanin post radiation exposure as measured by amplitude changes in the ESR signal; 2) electron transfer properties of melanin in the NADH oxidation/reduction reaction increased 4-fold after melanin irradiation. The ability of radiation to preferentially enhance the growth of melanized fungi is implied by the following observations made in this study: melanized C. neoformans and W. dermatitidis cells exposed to levels of radiation approximately 500 times higher than background grew significantly faster as indicated by the presence of more CFUs, greater biomass as shown by dry weight measurements and/or relative incorporation of more 14C-acetate than non-irradiated melanized cells. Furthermore, comparative analysis of MTT/XTT reduction assays revealed that radiation-induced effects on the electron transfer properties of melanin were localized to the extracellular space thus establishing a spatial relationship between the site for electron-transfer events and the location of the melanin pigment. In addition, we recorded radiation-induced effects on the growth of melanized C. sphaerospermum cells under limited nutrients conditions. Hence, we observed that radiation increased the growth of melanized cells relative to non-melanized cells using three fungal species and four measures of cell growth.
The literature already contains some indirect evidence for the notion that radiation can enhance the growth of melanized microorganisms. For example, the melanotic fungus C. cladosporioides
manifests radiotropism by growing in the direction of radioactive particles and this organism has become widely distributed in the areas surrounding Chernobyl since the nuclear accident in 1986 
. Both in the laboratory and in the field several other species of melanized fungi grew towards soil particles contaminated with different radionuclides, gradually engulfing and destroying those particles 
. In addition, there are recent reports that certain life forms can utilize non-conventional forms of energy - microbes in geothermal vents at the bottom of the ocean can harvest thermal radiation as an energy source 
while some microorganisms living in mines exploit energy from radiolysis of water 
. On the basis of these precedents and the results of this study we cautiously suggest that the ability of melanin to capture electromagnetic radiation combined with its remarkable oxidation-reduction properties may confer upon melanotic organisms the ability to harness radiation for metabolic energy. The enhanced growth of melanotic fungi in conditions of radiation fluxes suggests the need for additional investigation to ascertain the mechanism for this effect.