Radiotrophic fungi arefungi that can perform the biological process calledradiosynthesis, which means usingionizing radiation as a main energy source to drive metabolization. It has been claimed that radiotrophic fungi have been found in extreme environments such as in theChernobyl Nuclear Power Plant.[1][2]
Most radiotrophic fungi usemelanin in some capacity to survive.[3] The process of using radiation and melanin for energy has been termedradiosynthesis, and is thought to be analogous toanaerobic respiration.[4] However, it is not known if multi-step processes such asphotosynthesis orchemosynthesis are used in radiosynthesis.[citation needed]
Many fungi have been isolated from the area around the destroyedChernobyl Nuclear Power Plant, some of which have been observed directing their growth ofhyphae toward radioactive graphite from the disaster, a phenomenon called “radiotropism”.[5][1] Study has ruled out the presence of carbon as the resource attracting the fungal colonies, and in fact concluded that some fungi will preferentially grow in the direction of the source of beta and gamma ionizing radiation, but were not able to identify the biological mechanism behind this effect.[1] It has also been observed that othermelanin-rich fungi were discovered in the cooling water from some other working nuclear reactors. The light-absorbing compound in the fungus cell membranes had the effect of turning the water black.[6] While there are many cases ofextremophiles (organisms that can live in severe conditions such as that of the radioactive power plant), a hypothetical radiotrophic fungus would growbecause of the radiation, rather thanin spite of it.[7]
Further research conducted at theAlbert Einstein College of Medicine showed that three melanin-containing fungi—Cladosporium sphaerospermum,Wangiella dermatitidis, andCryptococcus neoformans—increased inbiomass and accumulatedacetate faster in an environment in which theradiation level was 500 times higher than in the normal environment.C. sphaerospermum in particular was chosen due to this species being found in the reactor at Chernobyl. Dadachova et al found that by exposingC. neoformanscells to these radiation levels, the organisms rapidly (within 20–40 minutes of exposure) altered the chemical properties of theirmelanin, and increased melanin-mediated rates ofelectron transfer (measured as reduction offerricyanide byNADH) three- to four-fold compared with unexposed organisms. However, each culture was performed with at least limited nutrients provided to each fungus. The increase in biomass and other effects could be caused either by the cells directly deriving energy from ionizing radiation, or by the radiation allowing the cells to utilize traditional nutrients either more efficiently or more rapidly.[7]
Outside of the laboratory studies, similar effects onmelanin electron-transport capability were observed by the authors after organism exposure tonon-ionizing radiation. The authors did not conclude whether light or heatradiation would have a similar effect on the fungi.[7]
Melanins are a family of dark-colored, naturally occurring pigments with radiation-shielding properties. These pigments can absorbelectromagnetic radiation due to their molecular structure, which results in their dark color; this quality suggests that melanin could help protect radiotropic fungi from ionizing radiation. It has been suggested that melanin's radiation-shielding properties are due to its ability to trapfree radicals formed duringradiolysis of water.[8] Melanin production is also advantageous to the fungus in that it can aid survival in many extreme environments. Examples of these environments include theChernobyl Nuclear Power Plant,[2] theInternational Space Station,[4] and theTransantarctic Mountains.[9][better source needed] Melanin may also be able to help the fungus metabolizeradiation, but more evidence and research is still needed.[3]
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 fungalcell wall or toxic intermediates formed inmelanin biosynthesis have been suggested to contribute to this phenomenon.[7] 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.[10] The exact biochemical processes in the suggested melanin-based synthesis of organic compounds or othermetabolites 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.[citation needed] Radiotrophic fungi is speculated to have been more widespread and numerous in theHadean withNatural nuclear fission reactor phenomena being more abundant. The pathway to radio-synthesis may exist dormant, as a sort genetic appendix in several other ancient lifeforms.[citation needed]
It is hypothesized that radiotrophic fungi could potentially be used as a shield to protect againstradiation,[4] specifically in relation to the use of astronauts in space or other atmospheres. An experiment taking place at theInternational Space Station in December 2018 through January 2019 was conducted in order to test whether radiotrophic fungi could provide protection fromionizing radiation in space, as part of research efforts preceding a possible trip toMars. This experiment used the radiotrophic strain of the fungusCladosporium sphaerospermum.[4] The growth of this fungus and its ability to deflect the effects of ionizing radiation were studied for 30 days aboard theInternational Space Station. This experimental trial yielded very promising results.
The amount of radiation deflected was found to directly correlate with the amount of fungus. There was no difference in the reduction of ionizing radiation between the experimental and control group within the first 24 hour period; however, once the fungus had reached an adequate maturation, and with a 180° protection radius, amounts of ionizing radiation were significantly reduced as compared to the control group. With a 1.7 mm thick shield of melanized radiotrophicCladosporium sphaerospermum, measurements of radiation nearing the end of the experimental trial were found to be 2.42% lower, demonstrating radiation deflecting capabilities five times that of the control group. Under circumstances in which the fungi would fully encompass an entity, radiation levels would be reduced by an estimated 4.34±0.7%.[4] Estimations indicate that approximately a 21 cm thick layer could significantly deflect the annual amount of radiation received on Mars’ surface. Limitations to the use of a radiotrophic fungi based shield include increased mass on missions. However as a viable substitute to reduce overall mass on potential Mars missions, a mixture with equal mole concentration ofMartian soil,melanin, and a layer of fungi roughly 9 cm thick, could be used.[4]
[1] "This black fungus might be healing chernobyl by "drinking radiation" a biologist explains.
