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X-treme Microbes — Text-only | Flash Special Report
Radiation Eaters

Overview
Not so long ago, everyone believed that the primary source of energy for all life was sunlight. Even for carnivores: After all, they eat herbivores that eat vegetation produced by photosynthesis. Ditto for bacteria stuck in the perpetual dark of the human gut, or for lightless ocean-bottom ecosystems that utilize oxygen dissolved in seawater—oxygen created by sunshine in plants and algae above. Ultimately, it seemed, everything depended on the Sun.

But in the 1960s, scientists began discovering exotic organisms that play by astonishingly different rules, such as microbes living in near-boiling water or high-acidity environments. Now, a team searching deep in a South African gold mine has found one that redefines the very limits of life: Bacteria that subsist in rock at huge pressure for thousands of years by ’eating‘ by-products of radioactivity, completely isolated from any organic matter or effects of photosynthesis.

Tullis Onstott of Princeton University, Lisa Pratt of Indiana University and colleagues think the microbes may have first trickled down there between three and 25 million years ago. The microbes were forced to survive on the leftovers that result when radioactivity from uranium, thorium and potassium in the native rock breaks down molecules of water, prompting a sequence of chemical reactions that produce hydrogen peroxide, break down pyrite, and form sulfates.

They developed a way of taking metabolic advantage of these reactions that is very different from the processes used by their conventional topside cousins.

These rock-dwellers may be some of above-ground life’s oldest relatives. Pratt and Onstott suspect that they’re probably not much different now than when they were separated from the surface, because they grow very slowly to conserve scarce nutrients.

In fact, ’very slowly‘ is an understatement: Whereas E. coli, like those found in the intestines of mammals, divide every day or so, the subsurface microbes reproduce once a year at most, and possibly only every 300 years … or more!   

So far, researchers haven’t been able to grow them in the lab under the microbes’ natural conditions. But they are working on genomic sequencing to evaluate how closely related the newly discovered bacteria are to other extremophiles and surface organisms.

In the future, those studies may change the way instruments look for life on Mars. And they may even begin to answer the question: Did life on Earth begin underground?

Caption/Credit:
1. Illustration of "radiation-eating" microbes. Credit: Nicolle Rager Fuller, National Science Foundation
2. The rod-shaped organism in this scanning tunneling microscope image was found two miles below the surface, living in rock and obtaining energy from the breakdown products of radioactivity. Credit: G. Wanger and G. Southam, The University of Western Ontario
3. Lisa Pratt (left) of Indiana University and Tullis Onstott (right) of Princeton University led a multi-institutional team that discovered the "radiation-eating" microbes. Credit: Courtesy of Lisa M. Pratt, The Trustees of Indiana University, NASA, National Science Foundation