The Dead Sea is not all dead. Sure, it is one of the most extreme ecosystems on our planet, with a salinity so high that tourists can easily float atop its dense, briny brew. And with no plants, fish or other visible life, swimmers can be excused for assuming that nothing stirs in the deep. But long ago scientists discovered single-celled microorganisms called archaea living in the lake’s waters—causing many to wonder whether other simple life could also survive within the sediments below despite the absence of oxygen, light or nutrients.
Now Camille Thomas, a geomicrobiologist at the University of Geneva, and his colleagues have unearthed molecular fossils in Dead Sea sediments that suggest bacteria lived there as recently as 12,000 years ago. It is the first time scientists have discovered a life-form other than archaea in this ecosystem—which hints that such life might exist (or have existed in the past) in similar places across the globe and elsewhere in the solar system, including Mars. The results were published in March in Geology.
Thomas and his colleagues were part of an international collaboration that in 2010 drilled 430 meters below the lake bed in an unprecedented opportunity to better assess our climate’s past. After several years of analyzing the samples, Thomas’s team found archaea buried within the sediment. It was proof that these organisms could survive both within the lake itself and in the sediment below, where conditions are even more hostile. But Thomas still thought it was unlikely that anything other than archaea could survive there. “I was thinking, ‘It’s an extreme environment, and it’s only for the extreme guys,’” he says.
The team’s most recent finding upends that notion. Thomas and his colleagues analyzed layers of gypsum (a mineral left behind when saltwater evaporates) that were deposited 12,000, 85,000 and 120,000 years ago. Entombed within them, they discovered wax esters—energy-rich molecules that small organisms create and store when food becomes scarce. Because archaea cannot produce these molecules, and multicellular organisms are very unlikely to survive such hostile conditions, the team concludes that ancient bacteria must have produced the compounds.
But how did these bacteria survive? The wax esters carried traces of archaea cell membranes, so the researchers hypothesize that the bacteria scavenged remains of archaea. That survival mechanism would explain how the community managed to thrive in such seemingly desolate conditions. “Although we know there’s a ton of diversity in the microbial biomass, it’s always exciting to see what strategies these microbial communities use to survive in different environments,” says Yuki Weber, a biochemist at Harvard University, who was not involved in the study. “There’s still a lot that has to be learned about the microbial metabolism.”
Furthermore, Thomas and his colleagues found tantalizing hints that bacterial life may exist in the Dead Sea ecosystem even today. When they first opened a large vial of contemporary sediments, for example, they smelled rotten eggs—a telltale sign of hydrogen sulfide gas, which is often produced by bacteria. But the gas can also have a nonbiological origin, such as geothermal activity (for which Yellowstone National Park is famous), so the researchers are not certain that bacteria continue to reside below the salty lake.
Even if they do not, bacteria most likely live in similar conditions across Earth’s vast underground biosphere, Weber argues. And as scientists continue charting the extreme environments in which life can survive, they will better understand how and where it arises on Earth and other planets, he says.
Take Mars—in 2011 NASA’s Opportunity rover stumbled on gypsum, the same mineral that Thomas found in the Dead Sea sediments. Its presence suggests that as the Red Planet warmed, its oceans and lakes evaporated. But before they did, these bodies of water probably would have looked a lot like the Dead Sea—maybe even down to the biological processes, says Tomaso Bontognali, a scientist at the Space Exploration Institute in Switzerland, who was not involved in the Dead Sea study. Bontognali works on the European Space Agency’s ExoMars rover, which is set to land in 2021 in an ancient ocean bed on Mars. It will analyze sediment cores with a simplified version of the method used by Thomas’s team. The Dead Sea evidence “makes the hypothesis that life may have existed on Mars more plausible,” Bontognali says.