Chernobyl's black moulds grow toward radiation. No one knows why.
The black moulds that seem to feed on gamma rays could protect astronauts—if we ever understand how they work
Every biology textbook teaches the same fundamental lesson: life on Earth runs on two energy sources. Plants capture sunlight through photosynthesis. Everything else either eats plants, eats something that ate plants, or, in the case of certain bacteria discovered at hydrothermal vents in 1977, harvests chemical energy from minerals in a process called chemosynthesis. Sunlight or chemistry. Those are the options.
Except something strange was growing on the walls of the exploded Chernobyl nuclear reactor. Something black, something alive, and something that appeared to be doing neither.
In 1991, five years after the worst nuclear accident in history, Ukrainian mycologist Nelli Zhdanova and her colleagues made a discovery that should have rewritten the textbooks. Crawling robots sent into the intensely radioactive ruins of Reactor 4 had returned with samples of dark, mould-like fungi thriving in conditions lethal to virtually all other life. The fungi were not merely surviving. They were growing toward the radiation source.
The growth that should not be
Zhdanova, working at the Institute of Microbiology and Virology in Kyiv, would spend the next two decades documenting this phenomenon. Her surveys eventually identified more than 200 fungal species in and around the Chernobyl site, many of them heavily melanised—packed with the same dark pigment that gives human skin its colour. But the most striking finding was behavioural. When exposed to directional radiation sources in laboratory conditions, certain species extended their thread-like hyphae preferentially toward the radioactive particles.
She called this radiotropism, by analogy to phototropism in plants. Just as a sunflower turns to follow the sun, these fungi appeared to seek out gamma rays.
The observation was peculiar enough to attract international attention, though not the kind that transforms scientific understanding. Radiotropism remained a curiosity, an unexplained footnote in the literature. Part of the problem was practical: working at Chernobyl required accepting dangerous radiation exposures, limiting what could be studied and by whom. Part was conceptual: the finding simply did not fit existing frameworks for understanding how life relates to energy.
Tatiana Tugai, who had worked alongside Zhdanova on many of these expeditions, continued the research into the following decades. On 24 February 2022, as Russian shells began falling on Kyiv, her thoughts turned to the Chernobyl site, now occupied by invading troops. The fungi, of course, continued their strange business regardless.
Melanin's hidden talents
What makes these fungi black is melanin, a pigment whose properties extend far beyond colouration. In human skin, melanin absorbs ultraviolet radiation, dissipating its energy as heat and protecting underlying cells from damage. It functions not as a reflective shield but as a molecular sponge, soaking up photons that would otherwise shred DNA.
The Chernobyl fungi suggested melanin might do something similar with far more energetic radiation—gamma rays, whose photons carry roughly a million times more energy than visible light. But mere protection could not explain radiotropism. Why would a fungus grow toward something it needed to defend against?
In 2007, Ekaterina Dadachova and Arturo Casadevall at the Albert Einstein College of Medicine in New York provided a possible answer. Their experiments showed that ionising radiation altered the electronic properties of melanin, increasing by fourfold its capacity to participate in a key metabolic reaction. When they exposed melanised fungi to radiation levels 500 times higher than normal background, the organisms grew up to three times faster than unexposed controls.
Dadachova proposed that melanin was not just absorbing radiation but transducing it—converting gamma ray energy into chemical energy the fungus could use for metabolism. She called the process radiosynthesis, a term deliberately echoing photosynthesis. If correct, these fungi had discovered a third way for life to harvest energy from its environment.
The sceptic's case
Extraordinary claims demand extraordinary evidence, and radiosynthesis faced immediate scrutiny. Eric Walberg, in a 2015 analysis at the University of Wisconsin, identified several problems with the published work. The energy deposited by radiation in the experimental systems, he calculated, was insufficient to account for the observed growth enhancement. The culture media contained carbon sources that could have supported the measured proliferation regardless of radiation effects. Some results, he argued, did not show what they were claimed to show.
His alternative hypothesis was less dramatic but more parsimonious: the fungi might be responding to radiation as a signal rather than an energy source, triggering gene expression changes that happened to promote growth. The radiation could be a stimulus without being fuel.
The distinction matters enormously. Radiotropism—growing toward radiation—appears robust across multiple independent studies. Radiosynthesis—metabolically harvesting radiation energy—remains unproven. The two phenomena have become conflated in popular accounts, but separating them is essential to understanding what these fungi are actually doing.
John Dighton of Rutgers University, who collaborated with the Ukrainian researchers on several studies, has urged caution about the metabolism claims while acknowledging that radiotropism itself is remarkable and unexplained. The fungi are doing something genuinely strange. What exactly remains unclear.
A parallel from the ocean depths
The resistance to radiosynthesis follows a pattern familiar from the history of biology. In 1977—the same year the Chernobyl reactor was under construction—scientists in the submersible Alvin discovered something impossible at the bottom of the Pacific Ocean.
Near hydrothermal vents on the Galápagos Rift, at depths where no sunlight penetrates, they found thriving communities of giant tube worms, clams, and crabs. The organisms were living, feeding, and reproducing in complete darkness, supported by bacteria that harvested energy from hydrogen sulphide gushing from the vents. The food chain had no photosynthesis at its base.
Chemosynthesis, as this process came to be called, challenged a fundamental assumption about life on Earth. For more than a century, biology had held that all ecosystems ultimately depended on the sun. Now here was an entire community sustained by heat and minerals from the planet's interior.
The discovery changed everything. It suggested that life might exist wherever chemical gradients provide energy—on icy moons with subsurface oceans, in Martian aquifers, anywhere the thermodynamics work out. Astrobiologists today routinely discuss chemosynthetic possibilities on Europa and Enceladus.
Yet acceptance came slowly. Initial reports were met with scepticism. The tube worms had no mouths or digestive systems; how could they be eating? Only gradually did researchers work out that the worms housed symbiotic bacteria inside their bodies, feeding on the chemical energy the bacteria captured. The full picture took years to assemble.
Radiosynthesis, if real, would be a similar revolution—a third primary energy source for life, joining sunlight and chemistry. Perhaps the resistance it faces reflects not only the weakness of current evidence but the enormity of what is being claimed.
Building houses from spores
While biologists debate mechanism, engineers are pressing ahead with applications.
Lynn Rothschild, an astrobiologist at NASA's Ames Research Centre, has spent years developing what she calls myco-architecture: using fungal mycelium to grow structures rather than building them from transported materials. Her NIAC-funded project envisions astronauts arriving on Mars with lightweight bags of dormant fungal spores. Add water extracted from Martian ice, and the fungi grow into habitat walls, self-assembling structures that could shelter humans from the radiation that makes the Martian surface hostile to unprotected life.
The concept exploits multiple fungal properties. Mycelium forms strong, lightweight composites when dried. It can be grown on regolith and organic waste. It self-repairs. But Rothschild's team has also incorporated melanin-producing strains into their materials specifically for radiation protection.
Tests at Brookhaven National Laboratory have shown myco-composite materials reducing simulated solar radiation by more than 99 per cent with just three inches of material—dramatically better than the four to ten feet of lunar regolith otherwise required. Whether this effect depends on melanin specifically, on the water content of biological materials, or on some combination remains under investigation.
In December 2018, samples of Cladosporium sphaerospermum—the same species Zhdanova found dominating Chernobyl's reactor—were sent to the International Space Station. Over 30 days, researchers monitored radiation levels beneath a thin layer of growing fungus. The results showed a modest but measurable effect: a 1.7-millimetre-thick fungal film reduced radiation by about 2 per cent compared to controls.
Nils Averesch, a biochemist at the University of Florida and co-author of the ISS study, remains appropriately cautious about interpreting the results. The enhanced growth observed in space could have been caused by microgravity rather than radiation. The shielding effect might be attributable to water content rather than melanin. He is conducting follow-up experiments to disentangle these variables.
"What we showed is that it grows better in space," Averesch has said. What exactly that means remains open.
An ancient capacity
One intriguing piece of the puzzle comes from deep time. Melanised fungal spores appear abundantly in sediment layers from the early Cretaceous period, roughly 100 million years ago. This timeframe coincides with a geomagnetic reversal—a period when Earth's magnetic field weakened dramatically, stripping away much of the planet's protection against cosmic radiation.
The correlation is suggestive rather than definitive. But it hints that melanin's relationship with radiation may be ancient, not a recent adaptation to nuclear accidents but a capacity that has existed for geological ages. Early Earth was far more radioactive than today, bathed in cosmic rays that our atmosphere and magnetic field now largely block. Life that emerged under those conditions would have needed radiation tolerance as a baseline requirement.
If melanin originally evolved to manage radiation exposure, radiosynthesis might be an elaboration of a very old biological trick rather than something entirely new. The fungi of Chernobyl would not be freaks but inheritors of an ancestral capability, one that happens to shine conspicuously in the artificial radiation environment of a nuclear disaster.
What we actually know
Thirty years after Zhdanova's initial observations, the honest summary runs something like this: fungi grow toward radiation in ways that remain unexplained. Whether they metabolically harvest radiation energy is unproven. Their melanin provides radiation protection through mechanisms that are at least partially understood. Practical applications in radiation shielding are advancing regardless of theoretical resolution.
This uncertainty is itself scientifically interesting. We have discovered something genuinely strange, documented it across multiple studies and conditions, and cannot yet explain it. The gap between observation and explanation is not a failure but an invitation.
The pattern recalls Thomas Kuhn's observations about scientific revolutions. Anomalies accumulate. Existing frameworks cannot accommodate them. Eventually, someone proposes a new paradigm that makes sense of what was previously inexplicable. The transition can take decades.
Chemosynthesis was such a revolution. Whether radiosynthesis will be another depends on evidence not yet gathered, experiments not yet designed, and perhaps questions not yet asked. The black fungi of Chernobyl have been waiting patiently since 1986. They can wait a while longer.
What cannot wait, perhaps, is the acknowledgment that our assumptions about energy and life may be too narrow. The textbooks list two primary energy sources. The ruins of Reactor 4 suggest there might be a third. Proving it will require more than observation—it will require understanding the mechanism well enough to predict and manipulate it. Until then, the fungi will continue doing whatever it is they do, indifferent to whether we have explained it.
Life, as the hydrothermal vents taught us, is stranger than our theories allow. Chernobyl's black moulds may be teaching the same lesson again.