Accustomed as we are to microbial surprises, we were nonetheless taken aback by a report disclosing that certain fungi grow better when exposed to ionizing radiation. According to a paper from Albert Einstein Medical School, fungi can also use radiation as a source of energy – not exactly one's view of radiation as something malevolent and baneful. Had we paid attention to the news from Chernobyl, we would not have been surprised because the walls of the still hot reactor have become covered with mold. Not only that, but it has been known for some time that fungal species search out radioactive particles, i.e., manifest radiotropism.
Many fungi are relatively radioresistant, apparently due to their melanin content. These species are easy to spot because they turn mildewy walls black and form black colonies on agar plates. (These dark fungi even have a term of their own: dematiaceous.) There is, of course, a big difference between being resistant to radiation and actually using it as a source of energy as is proposed in this study. Significant growth enhancement by radiation was seen in Cryptococcus neoformans (which becomes melanized when grown on suitable precursors) and naturally melanized species such as Wangiella dermatitidis and Cladosporium sphaerospermum, but not in albino mutants. These results suggest that melanin is the antenna that captures radiation and converts it into usable biological energy.
The authors present physicochemical studies showing that melanin is indeed able to capture radiation. As shown by electron spin resonance, radiation alters melanin's electron structure. Irradiated melanin reduces NADH four times faster than non-irradiated melanin. Thus, radiation increases its electron-transferring properties. "Just as the pigment chlorophyll converts sunlight into chemical energy that allows green plants to live and grow, our research suggests that melanin can use a different portion of the electromagnetic spectrum-ionizing radiation – to benefit the fungi containing it," says Dr. Dadachova, one of the authors of the study.
There is a lot to melanin (or, more properly, to the melanins, as they come in quite a few varieties). They are unique polymers known to protect living organisms against extreme temperature as well as both UV and ionizing radiation. Melanin may well have played an important role in evolution. Many fungal fossils appear to be melanized – a point discussed in some detail in this paper.
A headline in the press release from the Albert Einstein College of Medicine modestly announces that their findings "...could trigger recalculation of Earth's energy balance and help feed astronauts."