Friday, 15 August 2008

Silent spring


Issue 21 of Cosmos, June 2008

by Lauren Monaghan

Deep in the radioactive bowels of the smashed Chernobyl reactor, a strange new lifeform is blooming.

Radioactive fungi

Credit: Nigel Buchanan

TWENTY-TWO YEARS AGO, on 26 April 1986, reactor No 4 at the Chernobyl Nuclear Power Plant, in Ukraine, blew apart, spewing radioactive dust and debris far and wide.

Ever since, a 30 km 'exclusion zone' has existed around the contaminated site, accessible to those with special clearance only. It's quite easy, then, to conjure an apocalyptic vision of the area; to imagine an eerily deserted wasteland, utterly devoid of life.

But the truth is quite the opposite. The exclusion zone is teeming with wildlife of all shapes and sizes, flourishing unhindered by human interference and seemingly unfazed by the ever-present radiation. Most remarkable, however, is not the life buzzing around the site, but what's blooming inside the perilous depths of the reactor.

Sitting at the centre of the exclusion zone, the damaged reactor unit is encased in a steel and cement sarcophagus. It's a deathly tomb that plays host to about 200 tonnes of melted radioactive fuel, and is swarming with radioactive dust.

But it's also the abode of some very hardy fungi which researchers believe aren't just tolerating the severe radiation, but actually harnessing its energy to thrive.

"Our findings suggest that [the fungi] can capture the energy from radiation and transform it into other forms of energy that can be used for growth," said microbiologist Arturo Casadevall from the Albert Einstein College of Medicine at Yeshiva University in New York, USA.

Fungi are weird, yes. They chow down on everything from decaying plant matter to the more exotic fare of asbestos and jet fuel. But being able to produce their own energy, independent of an actual food source, and use dangerous ionising radiation to boot? That's very new and very exciting, Casadevall says.

In 1999, a robot sent to map the inside of the reactor returned with samples of a particularly black fungi, indicating an abundance of the biological pigment melanin, which also colours your skin.

Though melanin is typically associated with 'protective' properties – absorbing and safely transforming different electromagnetic wavelengths, such as DNA-damaging ultraviolet light – the researchers had an inkling that a more extraordinary phenomenon was allowing the fungi to prosper; something still involving the combination of melanin and radiation, but beyond the bounds of radioactive protection.

After all, even without melanin, many fungi are intrinsically radiation-resistant.

Their hunch was bolstered by findings of melanised fungi, happily congregating in the cooling pools of functional nuclear reactors, and by studies of dark, 'radiation-seeking' fungi, purposefully growing towards radioactive particles in soil, particularly around Chernobyl.

The team looked to the example of photosynthesis as a model, said Casadevall. If plants can use the green pigment, chlorophyll, to absorb energy from the Sun and produce a usable form of chemical energy, they reasoned, fungi might be able to use their melanin pigment and radiation energy in a similar way. They even devised the snazzy moniker, 'radiosynthesis', for the process.

To test their idea, the group analysed three different types of fungi, including Cladosporium sphaerospermum, the species abundant in and around Chernobyl. Using ionising radiation from the radioactive isotope, caesium-137, they exposed the fungi to radiation doses similar to those inside the damaged reactor, and about 500 times greater than the Earth's normal background level.

Melanin-containing fungi exposed to the radiation – even when nutrient-starved on purpose – grew significantly larger and up to 2.5 times faster than fungi without melanin and those not exposed to radiation.

According to Yeshiva's Ekaterina Dadachova, the nuclear chemist who led the study, "the presence of melanin in the cells gives them a distinct advantage over non-melanised cells, in terms of better growth [with radiation]."

Dadachova's team also had a look at what melanin molecules were actually doing, searching for signs of active involvement in the growth process. They were able to show radiated melanin capable of boosting a type of reaction important in metabolism – called an oxidation-reduction reaction – four times faster when exposed to the influence of caesium-137.

They also saw a change in the pigment's electronic structure. This, Dadachova says, is evidence "that melanin transformed part of the ionising radiation energy into the energy of electrons, which represents the 'chemical' form of energy [that] fungi could potentially use in their metabolism."

Taken together, the researchers think their results do indeed hint that fungi can live off ionising radiation, harnessing its energy through melanin to somehow generate a new form of biologically usable growing power.

If they're right, then this is powerful stuff, said fungal biologist Dee Carter from the University of Sydney. The results will challenge fundamental assumptions we have about the very nature of fungi, she said.

It also raises the possibility that fungi might be using melanin to secretly harvest visible and ultraviolet light for growth, adds Casadevall. If confirmed, this will further complicate our understanding of these sneaky organisms and their role in ecosystems.

Radiation-loving fungi may also prove useful, according to Dadachova. Their melanin gene, she said, might eventually be popped into food crops and used to help growth in difficult regions. And astronauts on long spaceflights might one day find a useful, self-replenishing diet in black, melanin-rich fungi.

And because the fungi don't actually 'eat' radioactive material, but simply use the energy it radiates, Dadachova said, they're in no danger of becoming radioactive themselves.

But Fraser Torpy, a microbiologist at the University of Technology in Sydney, warns that we shouldn't get too excited – at least, not yet. He would prefer more definitive evidence that fungi use radiation for energy.

The researchers have been able to deduce this only "from the otherwise unexplained superior growth of the melanised organisms," he cautions. "Much more work will be required before a convincing case [for the cellular use of this energy] can be presented."

Casadevall himself agrees. "We have not ruled out all other explanations – science is always cautious. [But] our leading hypothesis continues to be that melanin captures the energy from radiation and transforms it into energy for growth."

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