Radioactive Fungi—A New Extreme

© Kabir Peay
Lab
Original publication: Mycena News, April 2008

For those of us that collect mushrooms, fungi can seem fairly fragile and ephemeral. Most fungi certainly do seem to thrive in relatively benign conditions (think wet, dark, and warm). As a kingdom though, fungi have displayed an incredible adaptive flexibility that allows them to end up in some fairly unlikely places. This flexibility is based primarily on a biochemical and metabolic arsenal that allows fungi to make food out of almost any substrate. Combine this with the ability of some fungi to withstand incredibly harsh environmental conditions, and you have the fungal equivalent of the X Games. The most common fungal extremophiles are probably lichens, the symbiotic association between fungi and algae. Lichens are among the first colonists of the harsh landscapes left after large disturbances, such as volcanic eruption or glacial retreat, where they often colonize bare rock and withstand extreme temperatures, desiccation, and exposure to ultraviolet radiation. In recent decades, however, fungi have been discovered growing in a panoply of extreme or odd environments. Some of the more spectacular examples include fungi that grow actively beneath snow (Schadt et al. 2003), fungi that use jet fuel as a food source (Edmonds & Cooney 1967), underwater mushrooms (Desjardin et al. 1995), and space fungi found devouring metal polymers on the Russian space station Mir (Alekhova et al. 2005). However, one recent study has been particularly noteworthy for expanding the list of fungi in extreme environments and our understanding of how fungi make their living (Dadachova et al. 2007).

Cryptococcus

Albino (left) and melanized (right) strains of Cryptococcus yeast used in the Dadachova et al. study. The melanized strains were able to grow up to 3 times faster when exposed to ionizing radiation 500 times background levels, suggesting that the melanin (seen in the dark cell wall in the yeast on the right) may allow these fungi to convert normally harmful radiation into energy for growth.Courtesy Kate Dadachova.

The meltdown of the Chernobyl nuclear reactor in 1983 was one of the greatest cautionary tales of the nuclear age. The explosion of the reactor released a radioactive cloud that spread 1,000s of kilometers and left the soil, water, and vegetation in Chernobyl contaminated to this day. Despite the damaging biological effects of nuclear radiation, the fungus Cladosporium sphaerospermum has been found growing abundantly in and around the Chernobyl nuclear reactor. Researchers interested in this fungus’ ability to thrive in the contaminated environment of Chernobyl noted that it was darkly pigmented with melanin—the same pigment responsible for skin coloration in humans.

Melanin is known to protect organisms against exposure to harmful radiation; however, researchers Kate Dadachova, Arturo Casadevall, and colleagues became interested in the idea that melanin might play a more complex role in the success of fungi exposed to ionizing radiation (i.e. radiation that can be biologically harmful). To do so, the researchers studied the effects of ionizing radiation on both fungal melanin itself and the effects of such radiation on the growth of melanized and albino strains of common microfungi such as Cryptococcus and Wangiella (see photo on page 1). By examining the electron spin resonance of melanin exposed to ionizing radiation, Dadachova and colleagues surprised the scientific community by showing that the exposed melanin had a fourfold increase in its capacity to do biological work (specifically the ability to reduce NADH, a key process in cellular metabolism). This is much the same as the way in which the primary plant pigment, chlorophyll, is able to capture energy from radiation in the visible spectrum (i.e. sunlight) and translate this into biological work.

Taking their research a step further, the team showed that melanized fungi exposed to radiation levels 500 times background grew approximately 3 times faster than under ambient levels of exposure. Non-melanized, albino mutants, however, did not show the same growth increases when exposed to the same levels of radiation. This suggests that melanin was indeed the critical factor. So, not only does melanin protect these fungi from negative effects of radiation, it also appeared to allow them to turn it into a positive source of energy for growth.

Looking at the bigger picture, these two pieces of evidence lead to a provocative conclusion: melanin may be the basis of a widespread, yet relatively unknown, form of energy capture that is in many ways analogous to photosynthesis. If true, this would have major implications for our understanding of biology. For example, does melanin in our skin cells capture energy from ultraviolet radiation? Could this explain why fungi are able to grow so well in space, where exposure to ionizing radiation is much higher? Any provocative finding is bound to be controversial, and the work of Dadachova et al. has not convinced everyone in the scientific community of its generality. However, this research has certainly expanded our understanding of how fungi are able to thrive in extreme environments and will likely lead to greater insight into the biological world in general.

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