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  1. Life on Ice

    From Arctic sea ice to Antarctic lakes and dry valleys, scientists study microbes that tolerate freezing temperatures on Earth to learn where to look for life on other worlds. Among the possibilities are fossils in ancient Martian lakebeds and bacteria wrapped in mucus and ice on Jupiter’s moon Europa.

    “It’s terribly important that we learn more about cold-adapted microbes because all the environments we even contemplate for supporting life elsewhere [in our solar system] are cold,” says microbiologist Jody W. Deming, an oceanography professor at the University of Washington in Seattle.

    “We need to know how all forms of life are managing at supercold temperatures in our search for extraterrestrial life. The surfaces of Mars and Europa are both very, very cold, so any samples we might ever obtain from Mars or Europa are going to be frozen.”

    Cold-loving or psychrophilic organisms – psychro is the Greek word for cold – are microbes that can grow and replicate at temperatures between 15 degrees Celsius (59 degrees Fahrenheit) to about minus 15 C (5 F), and can survive at even frostier temperatures, Deming says. She says researchers routinely store bacteria in the lab at minus 80 C (minus 112 F), and some survive.

    True psychrophiles die when it gets warmer than about room temperature (20 C, or 68 F), so most of those on Earth live in aquatic environments such as the ocean, where temperatures remain stable, says biologist and NASA Astrobiology Institute member Imre Friedmann, of NASA’s Ames Research Center in Mountain View, CA.

    Because land temperatures fluctuate, he explains, “in terrestrial environments, there are practically no psychrophiles,” only “psychrotolerant” organisms that can survive either cold or warm temperatures.

    Earth’s psychrophilic and psychrotolerant organisms include archaea – the most primitive bacteria-like life forms – bacteria, algae, cyanobacteria (nicknamed blue-green algae) and fungi.

    Most thermophiles, or heat-loving organisms, are primitive and evolved soon after the origin of life on Earth, perhaps near seafloor volcanic vents. But cold-adapted organisms occur in different groups on the evolutionary tree, so Friedmann believes the ability to adapt to cold evolved independently in different organisms.

    On Earth, many cold-loving and cold-tolerant microbes live in large masses in the oceans, some forming blooms, such as toxic cyanobacteria responsible for killing seals in the North Sea, says microbiologist David Wynn-Williams, astrobiology project leader for the British Antarctic Survey.

    Cold-tolerant algae are responsible for red-tinged snow in glaciers and polar ice. Other psychrotolerant microbes live within the fabric of rocks in the Antarctic desert and at the edge of the Antarctic ice sheet, as well as and on the floors bottoms of ice-covered Antarctic lakes.

    Deming believes “sea ice is a seed bed for cold-adapted microbes that appear everywhere else in the ocean.”

    She and her colleagues use snowmobiles and sleds to sample Arctic sea ice off Barrow, Alaska, in winter. Deming says Karen Junge, a doctoral student at Washington, found evidence of bacteria surviving – and even metabolically active – at minus 20 C (minus 24 F) in winter sea ice.

    Survival Strategies

    Deming says some bacteria survive in sea ice by secreting antifreeze chemicals called she calls “exopolymers” or “organic goop.”

    “A layman’s word is mucus,” she says. “The bacteria just surround themselves with mucus. It keeps them in a fluid environment. It protects them against freezing, against actual ice crystal damage to their cell walls,” and against high salt concentrations.

    “We think this mucus also keeps the pore spaces inside the ice large enough so the bacteria aren’t crushed by the ice,” she adds.

    If mucus-coated microbes live within the upper layers of the sea ice believed to cover Europa, Deming speculates, the exopolymers might change reflectance of the ice in a way orbiting spacecraft could detect – a possibility that requires more research.

    Many terrestrial organisms, including some yeasts and nematode worms, survive extreme cold by producing trehalose, a sugar that replaces water and prevents vital proteins such as enzymes from collapsing, enabling them to survive when conditions are cold and dry, Wynn-Williams says.

    Deming says Junge’s research has uncovered another survival trick: “Something like 99 percent of the active bacteria in wintertime sea ice are attached to some surface,” either pore walls within ice, mineral grains or salt crystals.

    “This becomes very exciting in searching for life on Europa because the surface of Europa is peppered with dark red-brown areas where planetologists think salt has been deposited,” Deming says.

    She suspects cold-loving bacteria adhere to surfaces for the same reason microbes grow on the walls of sewage pipes: Liquid flowing through ice pores delivers nutrients and carries away wastes.

    Wynn-Williams, a member of the NASA Astrobiology Institute (NAI), notes that cold-tolerant microbes also survive because their cell membranes “remain relatively fluid at low temperatures.”

    Yet another survival technique is suspended animation. Wynn-Williams says a pond in Antarctica’s Taylor Dry Valley is saturated with so much calcium chloride that it does not freeze until minus 53 C (minus 63 F), yet bacteria survive there – barely.

    “They are not doing anything,” says Wynn-Williams. “Microbes that live inside rock ridges in the Antarctic dry valleys take 10,000 years to metabolize a single molecule of carbon dioxide.”

    It’s Cold Out There

    Antarctica’s Lake Vostok, some 23 kilometers (14 miles) long, is permanently covered by a 3.7-kilometer (2.3-mile) layer of ice. Bacteria have been found in this overlying ice, but Wynn-Williams wants to look for microbes in lake-floor sediments, which are at least 500,000 years old and free from human contamination.

    He says Vostok “is a Europa analog. You’ve got to go through an ice sheet, through a water column and down to the bottom sediments.”

    Wynn-Williams expects that on Europa heat-loving microbes living near seafloor hot-water vents are more likely than cold-tolerant organisms. Yet he says cold-tolerant microbes might have evolved if any heat-loving bugs near Europa’s seafloor were carried upward to the ice. Friedmann is less optimistic. He doubts there is volcanism or adequate convection for either heat- or cold-loving life on Europa. He thinks that even if conditions for life are present, it’s difficult to imagine how life could have originated on Europa or transported there from Earth or from Mars.

    Antarctica also offers environments similar to habitats on Mars in which cold-adapted organisms once lived – or might still be living.

    If life evolved on Mars, it may have done so earlier than life on Earth, because Mars cooled earlier to a temperature that could support a biosphere. Earth was remelted 4.55 billion years ago, after it first cooled, by a massive collision with another object. (That collision formed the Moon.)

    But because Mars is so much smaller than Earth, it continued rapidly cooling and eventually froze, dying volcanically and biologically while Earth remained alive, says Chris McKay, a planetary scientist at Ames Research Center and an NAI member. The last surviving organisms on Mars likely were adapted to low temperatures.

    “Mars would have gotten cold,” McKay says. “It would have become like the Antarctic dry valleys. Organisms we find in the cold desert regions of Earth are the best models for what life may have been like on Mars early in its history.” Antarctica’s dry valleys “are so dry because there is very little precipitation and the mountains block the flow of ice from elsewhere,” he says. Yet annual summertime glacial runoff maintains ice-covered lakes that harbor algae and bacteria, which collect in lakebed sediments.

    Wynn-Williams says that as Mars cooled and dried up, “the abundant water available originally would have receded into ice-covered lakes like [those that exist today] in the dry valleys.”

    Then, after these Martian lakes evaporated, “it is conceivable that organisms were alive in rocks, like in Antarctic rocks,” which harbor algae, fungi, bacteria and cyanobacteria, Friedmann says.

    But “one of the best places too look for evidence of life on Mars is to look for ancient lakebeds that would have been ice-covered lakes billions of years ago,” says McKay, noting 20 such lakebeds have been identified so far. “We should land on these lakebeds and dig, dig, dig.”