There’s life on polar ice sheets, in scalding hot springs, and kilometers underground. But in hot briny lakes in Africa’s Rift Valley and the cold, dry soil of Antarctica’s Shackleton Glacier Valley, life reaches its limits. In both places, teams of researchers have pinpointed conditions too hostile for even the toughest microbes.
Together the two projects “contradict the current wisdom that life is really everywhere,” says Nathan Smith, a paleontologist at the Natural History Museum of Los Angeles County in California who was not involved with either project. “Both, in their own ways, in different types of environments, are homing in on what might be real barriers to life.”
In the Rift Valley lakes, in Ethiopia, volcanic gases venting from below acidify the water, which is also rich in salts from brines created by the evaporation of ancient and modern bodies of water. Add the heating effect of the volcanic activity, and the lakes represent an environment more extreme than any found in Yellowstone National Park or even in the Atacama Desert.
Purificación López-García, a biologist at with the French national research agency CNRS and the University of Paris-Sud, and her team used microscopy, cell sorting, genomics, and other techniques to look for signs of life in the water. In some ponds, microbes known as archaea thrived, the researchers report today in Nature Ecology & Evolution. But a few kilometers away, in isolated canyons and lakes where salt concentrations top 50% and acidity was high, they could detect no signs of permanent life at all.
They did notice balls of minerals that others have found elsewhere and interpreted as “microfossils.” But they argue that the structures are unlikely to have biological origins. The high salinity along with the acidity and temperature are a deadly combination, especially because the salts there are magnesium-based. Too much magnesium causes cell membranes to dissolve, López-García says.
Very different conditions define life’s limits in the Shackleton Glacier Valley. In some parts of the ice-carved landscape, the soil has been exposed for hundreds of thousands of years to frigid, bone-dry winds. Nicholas Dragone, a graduate student at the University of Colorado in Boulder, took soil samples from those sites and from low-lying spots that were ice-covered until recently. He tried for 2 months or longer to get microbes from each sample to grow in the lab, under more than a dozen types of conditions. He also tested for DNA. Finally, he and his colleagues added glucose with labeled carbon to the soil samples and watched for any labeled carbon dioxide—a sign that a living organism was processing the glucose.
Multiple types of bacteria and fungi turned up in the lowland soils, Dragone reported 14 October at the Interdisciplinary Antarctic Earth Sciences meeting in Julian, California. But some of the high-elevation, older soils were devoid of life. “To be able to identify sites that have nothing in them is unique,” Dragone said. He has not done the extensive chemical analyses that López-García’s team did to help define life’s limits, but he suspects these high-elevation soils are inhospitable because they lack any liquid water.
With the two studies, “We’ve hit that boundary [for life] on both sides,” says Dragone’s adviser, Noah Fierer, a microbial ecologist. Knowing those boundaries should prove useful in searches for life at Earth’s extremes—and on other planets. “This gives us a lot more tools for exploring the potential of life existing elsewhere,” Smith says.
Source: Science Mag