May 14, 2021
Radiotrophic Fungi: The Fungus Among Us that Eat Radiation
A dosimeter in hands with a level of radioactivity near mushrooms growing in the city of Pripyat, Chernobyl Exclusion Zone.Source: Adobe Stock
Mushrooms: everybody’s favorite quirk of nature. From the psychedelic genus psilocybe and the Ophiocordyceps unilateralis that “mind control” ants, to the mushrooms that clean up oil spills and the mushrooms that may serve as planetary habitats, researchers have found yet another use for fungi: radiation protection.
“The greatest hazard for humans on deep-space exploration missions is radiation,” says a preliminary report in the bioRxiv journal. “Certain fungi thrive in high-radiation environments on Earth, such as the contamination radius of the Chernobyl Nuclear Power Plant… These organisms appear to perform radiosynthesis, using pigments known as melanin to convert gamma radiation into chemical energy. It is hypothesized that these organisms can be employed as a radiation shield to protect other lifeforms.”
Radiosynthesis runs parallel to photosynthesis — but instead of eating sunlight (UV radiation), these shrooms are eating gamma radiation. And it’s all possible through melanin, the same pigment that determines hair and skin color.
“Superheroes exist among us, but not in the forms we might traditionally expect. They aren’t cape-cladded, flying, crime-stopping superhumans we have come to know and love. They exist in more covert ways, for example, as microbial organisms.”— T. White (2017)
Now, the radiotrophic fungi in question aren’t “cape-cladded, crime-stopping superheroes,” nor are they adorned in red and white speckled caps, delicate gills, or dainty veils.
The fungi found thriving in the remnants of a Chernobyl Nuclear Reactor were actually single-celled organisms, explains Stanford University student Tyreke White. Lovingly nicknamed “Hulk bugs,” gamma rays seem to make these fungi stronger.
Radiation makes these fungi grow faster, and the fungi seem to grow in direction of radiation — the same way a houseplant begins stretching and growing towards the direction of the sun through your window.
“Scientists have since prescribed a name to this phenomenon: radiotropism,” White continues. “Radiotropism is defined as the directed growth of microfungi towards sources of ionizing radiation.”
A variety of fungi that live in extreme environments produce melanin, reports a Pigment Cell & Melanoma Research journal review. Such fungi are found in the walls and cooling pool water of nuclear reactors, growing at high altitudes, and in the Arctic as well as the Antarctic regions.
It’s believed melanin protects these fungal species from environmental threats, such as heavy metals, UV and gamma rays.
For context, most living organisms produce pigment for a variety of reasons, says a British Mycological Society review. Pigments allow living things to camouflage, they play an important role in makeup, and they allow organisms to photosynthesize (or radiosynthesize).
Melanins are unique; they’re hard to define and categorize because of how diverse and structurally complex they are. These pigments are prevalent throughout life on Earth, which implies their functional importance for the evolution of living species on our planet.
“Melanotic fungi appear to be phylogenetically diverse with a worldwide distribution, typically colonizing harsh environmental niches not suitable for most life forms,” the review says. “…Melanization allows these microorganisms to tolerate the various physical and chemical stresses from their surroundings.”
However, melanin doesn’t just allow fungi to grow in harsh environments. It also plays an important role in their virulence — how harmful these fungi are when they infect another species, like humans.
As the review explains, fungal melanin acts “as a non-specific armor during infection that protects the fungus” against the host’s immune system.
One melanized, radiotrophic fungus in particular, Cryptococcus neoformans, is known to infect humans. According to the Centers for Disease Control and Prevention, symptoms of a C. neoformans infection include:
- C. neoformans lung infection symptoms:
- Chest pain
- Shortness of breath
- C. neoformans brain infection symptoms:
- Behavioral changes
- Light sensitivity
- Neck pain
Cryptococcus neoformans, an encapsulated yeast, stained by Gram’s method. Preparation by Tina Demers.Source: Dr. Graham Beards on WikiMedia Commons
Although a C. neoformans infection is rare in otherwise healthy individuals, it can be deadly if left untreated. A Scientific Reports journal study found C. neoformans infections are one of the leading causes of death among adults living with HIV in sub-Saharan Africa.
However, the Cladosporium genus is quite common and usually isn’t harmful to humans (though it is a potential allergen for some). There’s a chance you may have even encountered Cladosporium growing in your basement, attic, or bathroom.
Radiotrophic Fungi in Space
In the aforementioned bioRxiv preliminary report, researchers sent Cladosporium sphaerospermum aboard the International Space Station for 30 days to observe their radiation attenuation.
Based on the results of their observations, they estimate it would only take about a 21 cm thick growth of C. sphaerospermum to neutralize the levels of radiation present on the surface of Mars.
“With concrete efforts to return humans to the Moon by 2024 under the Artemis program and establish a permanent foothold on the next rock from Earth by 2028, humankind reaches for Mars as the next big leap in space exploration,” the report beings. “In preparation for prolonged human exploration missions venturing past Earth-orbit and deeper into space, the required capabilities significantly increase.”
The average person is exposed to about 6.2 mSv of radiation annually on Earth, whereas astronauts on the International Space Station are exposed to roughly 144 mSv of radiation annually.
One year into a three-year trip to Mars, an astronaut could be exposed to over 400 mSv of radiation from Galactic Cosmic Radiation, the study notes. While researchers are unsure of just how this might affect astronauts’ health, we do know that radiation can damage living cells and their DNA. If the body is unable to repair these cells correctly, the damage can become cancerous.
Therefore, figuring out how to mitigate radiation and shield astronauts embarking on these near-future missions is of utmost importance.
In this case, radiotrophic fungi not only thrive in radiation, but they would act as a self-sustaining radiation shield scientists could simply grow, rather than have to manufacture.
The equipment necessary for this experiment, including an inoculated Petri dish of C. sphaerospermum, was sent to the ISS in cold storage on SpaceX mission CRS-16 in 2018.
These Petri dishes were photographed in bursts for the first five days of the trial, and in 30-minute intervals for the remainder of the study. The radiation in the environment was measured every 110 seconds or so, as well. After 30 days, they powered down the study and awaited the Petri dishes’ return to Earth in June 2019.
While the fungi didn’t grow much during their time in cold storage, they began to grow once they were at an ambient temperature. This demonstrates that the spores can withstand freezing temperatures without it having too much of an impact on the later stages of their life.
Typically, these types of fungi are characterized by their slow growth and must be incubated for 14 days for sufficient growth. However, the C. sphaerospermum on the ISS reached maximum growth after just 18 hours, and full maturity within 48 hours.
“Comparison to the preflight growth tests may indicate that the fungus could experience faster-than-average growth aboard the ISS, due to the utilization of radiation of the space environment as a metabolic support function, as has been reported for other high-radiation environments,” the study adds. “It has been previously shown that C. sphaerospermum can experience up to three times faster growth than normal with gamma-rays 500 times as intense as normal.”
So not only can C. sphaerospermum endure the radioactive conditions of space, they appear to enjoy it. Furthermore, the radiation directly beneath the Petri dishes with C. sphaerospermum decreased throughout the experiment, which shows they were certainly chowing down on those gamma rays.
The fungi among you and I are absolutely incredible. From their mind-expansive properties to their environmental cleaning capabilities, mushrooms are invaluable to all life on Earth.
Radiotrophic fungi have captured the hearts and minds of scientists since they were first observed thriving inside of a nuclear reactor at Chernobyl. Since then, researchers have begun studying some of these species for their potential to protect astronauts from Galactic Cosmic Radiation during interplanetary travels.
Although the bioRxiv study discussed throughout this article hasn’t been peer-reviewed yet, the preliminary results appear promising. As research on space exploration and biotechnology progresses, it’s intriguing to ponder the potential role radiotrophic fungi could play in facilitating life beyond Earth.
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