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2013

Comfortable in the Cold: Life Below Freezing in an Antarctic Lake
Nova Next, June 2013
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A big part of Ricardo Cavicchioli’s job as a biologist is finding new species. And Cavicchioli, a professor at the University of New South Wales in Australia, has had particular good fortune at a place called Organic Lake, in the Vestfold Hills. “We discovered things we were never even looking for,” Cavicchioli says.

If you actually tagged along with Cavicchioli on one of his trips to Organic Lake, however, you might be deeply skeptical that this was a place where anything could live. The Vestfold Hills are not a rolling tropical landscape. They are located in East Antarctica. Organic Lake gets as cold as -20˚ C. The only reason it doesn’t turn to ice is thanks to its staggering concentration of salt.

Despite these unbearably cold conditions, Organic Lake is home to all manner of microbial life, from fungi to bacteria to viruses. It even has viruses that infect other viruses. Though life in Organic Lake is in many ways the same as elsewhere on Earth, it is also profoundly different in some ways. If Cavicchioli—a warm-blooded mammal—were to fall into the lake, he might die of hypothermia in a matter of minutes. But for the species in Organic Lake, frigid temperatures are comfortable. Warm them up to room temperature, and many would die in the scorching heat.

Redefining Typical Temperature

Cavicchioli is among a growing group of scientists who are dedicating their careers to studying one of the least understood, yet profoundly important forms of life: organisms that thrive in the cold, or psychrophiles, also known as cryophiles. Traditionally, biologists have neglected chilly places, which, in hindsight, has been a huge oversight. Over 80 percent of the planet never gets above 5˚ C. “It’s the norm, not the extreme,” Cavicchioli says.

The “cold biosphere,” as Cavicchioli calls it, contains organisms with remarkable adaptations to let them grow at low temperatures. Scientists are seeking now to understand psychrophiles not just for their own sake, but for the lessons they can offer. Astrobiologists know that the most promising places for extraterrestrial life in the Solar System are also bitterly cold. If aliens exist, in other words, they are probably a lot more like the life in Organic Lake than in Pennsylvania. Meanwhile, biochemists are intrigued by the strange workings of cold-adapted proteins for an entirely different reason—they hope to adapt them as biotechnology, so that we can harness the power of the cold biosphere in our lukewarm lives.

The traditional neglect of the cold biosphere is understandable when you consider the tools that biologists used to look for life. For a long time, the only way they could study microbes was to take a sample from a place—say, the water of Organic Lake—and then try to grow the organisms in their lab.

This method works well for species that enjoy the same conditions we humans like. That’s why E. coli, which flourishes in warm temperatures and high levels of oxygen, became the favorite organism of thousands of scientists. But this method misses all the microbes that don’t thrive in those conditions.

In the early 2000s, Cavicchioli was among the first scientists to bring a new tool to the cold biosphere. Instead of trying to rear psychrophiles in the lab, they fished for their DNA. By sequencing stretches of DNA in water samples, the scientists could identify new species and find their closest relatives. This gene-based hunt is yielding a vast diversity of living things in even the most forbidding places, including and beyond Organic Lake. On mountaintops and chilly sea floors, in caves and polar caps, biologists would find bacteria, fungi, protozoans, and viruses. Some of these organisms got blown or washed away from warmer places and had become dormant. But many of them were alive—feeding, growing, and reproducing.

Cut Out for the Cold

As scientists examine the genes of these hidden psychrophiles, they’re also learning more about how these organisms manage to survive in places that would be fatally frigid for others. If you cool our cells down, for example, their membranes get rigid. Psychrophiles keep their membranes more pliable by adjusting the fatty acids that make them up. They use more polyunsaturated fats in their membranes than organisms at higher temperatures. To appreciate the difference, consider how much easier it is to spread cold margarine (which has lots of polyunsaturated fats) than it is to spread cold butter (which doesn’t).

Inside our cells, the cold also wreaks havoc on proteins. Proteins carry out thousands of different kinds of chemical reactions, such as cutting apart molecules, joining together others, and moving charged proteins out of cells. It takes energy for them to do these jobs. They get a lot of that energy from our body heat, which keeps them continually jiggling. A little burst of extra energy (from a molecule called ATP) is all it takes to launch them into action.

As the temperature drops, proteins get sluggish. A burst of ATP may not be strong enough to activate them. If you take a protein from our warm bodies and chill it down to the freezing point of water, its activity may drop by eighty-fold. Proteins also risk folding down into a deformed clump as they get cold, with the result that they react with other molecules in the wrong way, causing trouble inside the cell.

Yet the proteins in psychrophiles manage to work just fine even below the freezing point. Some of their success is due to so-called “cold-shock proteins,” which unfold proteins from their tangles and restore them to their proper shape. We humans and other warm-blooded organisms make cold-shock proteins when our cells sense that the temperature has dropped. Psychrophiles, by contrast, keep a steady supply of them on hand at all times.

Psychrophile proteins also avoid biochemical torpor by being much more flexible than typical proteins in warmer organisms. Even a little heat is enough to make them jitter. They’re so floppy, in fact, that warming them up to room temperature can tear them apart.

Strategies like these let psychrophiles survive where we would freeze to death. So far, the current record-holder for surviving in the cold is a microbe called Planococcus halocryophilus. Scientists from McGill University discovered it in the permafrost of northern Canada, and recently they demonstrated that it can grow at -15˚ C and stay metabolically active at -25˚ C.

But the adaptations that allow psychrophiles to endure in these extreme habitats don’t exactly leave them breeding like rabbits. In many cold ecosystems, life unfolds in awesomely slow motion. In Organic Lake, for example, Cavicchioli studies a species of microbe that divides only six times per year. (E. coli, in contrast, needs as little as 20 minutes to divide in the lab.) “This is incredibly slow—they’re basically doing nothing,” Cavicchioli says. Yet these microbes are one of the dominant species in the lake, in part because nothing else grows faster than they do. “From their point of view,” asks Cavicchioli, “who cares?”

Clues to Life Elsewhere

The fact that life can endure in a place like Organic Lake doesn’t just have implications for biology on Earth. It also raises our hopes about the possibility of life elsewhere in the Solar System. When astrobiologists contemplate extraterrestrial life, they first think about water. All life as we know it here on Earth has to have water to survive. But Earth is not the only place in the Solar System with the molecule in abundance. Europa, a moon of Jupiter, may have an ocean of liquid water hidden under a thick sheet of ice. Other moons of Jupiter and Saturn are known to have ice. The white at the North Pole of Mars is an ice cap made of water, too, and there are many clues that Mars was once a watery planet, and vestiges of those ancient seas may still be underground.

But these planets aren’t just wet. They’re also cold—brutally so. The surface of Europa can get down to -223˚ C, for example. On a balmy day, some parts of Mars can get as warm as 20˚ C, but for the most part it’s a far colder place than Earth, getting down to as cold as -140˚ C.

If we only knew about life based on warm-adapted biology, these temperatures might seem like a show-stopper. Life on Europa ought to just grind to a halt. But Earth’s psychrophiles are living proof that organisms can evolve many adaptations for withstanding cold.

Harnessing the Cold

Meanwhile, here on Earth, some scientists are interested in psychrophiles for much more practical reasons. We use proteins in our everyday lives for doing everything from baking bread to washing clothes to cleaning up oil spills. We typically use proteins that scientists isolated from organisms that live with us in the warm biosphere. These proteins require a lot of extra heat to work as well as possible.

Millions of years of life in the cold have adapted psychrophile proteins to work with very little heat. That raises the possibility that they could carry out many jobs far more efficiently than current technologies allow. “There’s a huge scope there, and companies are realizing it more and more,” Cavicchioli says.

A few psychrophile proteins have already entered the marketplace. A Belgian company called Puratos sells a cold-adapted protein for bread-baking, for example. It makes dough rise faster by breaking down cell walls quickly. Researchers are looking into detergents for washing machines that will work effectively in even colder water than current “cold-water” formulations. Other possible applications of psychrophile proteins include enzymes for purifying water.

Cavicchioli worries that we may not be able to appreciate the lessons psychrophiles have to offer if we don’t protect fragile places like Organic Lake from the threats they now face—everything from tourism to climate change. Global warming is hitting polar regions the hardest; species adapted to warmer temperatures may outcompete the cold-adapted species there now.

“There’s very good reason to protect as many of these natural environments as we can,” Cavicchioli said.

Copyright 2013 Carl Zimmer
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