Hyperacid, hypersalated and hot ponds in the geothermal field of Dallol (Ethiopia). Despite the presence of liquid water, this multi-extreme system does not allow the development of life, according to a new study. Credits: Puri López-García





Microbial cells (left) can be easily confused with silica-rich mineral precipitates (right). Credits: Karim Benzerara, Puri López-García et al

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Article: Hyperdiverse archaea near life limits at the geothermal polyextreme Dallol area



Authors: Jodie Belilla, David Moreira, Ludwig Jardillier, Guillaume Reboul, Karim Benzerara, Jose M. Lopez-Garcia, Paola Bertolino, Ana I. López-Archilla, Purification López-García



Magazine: Nature Ecology & Evolution

Vol .: 3, pages 1552-1561



DOI: 10.1038 / s41559-019-1005-0

"WHY A PLANET WITH LIQUID WATER IS NOT ENOUGH, Forms of life have been found everywhere: in Antarctica, at the bottom of the deepest mines and even in the alkaline waters of the so-called Dead Sea, micro-organisms of all kinds proliferate. But to Dallol, in the depression of Dancalia, in Ethiopia, nothing seems to survive, says research published in Nature Ecology & Evolution"A volcanic crater full of salt that gives off smoky toxic gases, where the water boils in intense hydrothermal activity and the daily temperatures in winter can exceed 45 ° C. A hostile and multi-extreme environment: very hot, very salty and very acidic at the same time. We have not just crossed the gates of hell: we are at Dallol , in the Danakil depression , in Ethiopia. It is in this place that a team of Franco-Spanish scientists, led by biologists Jodie Belilla and Purificación López-García of the French Cnrs, has discovered how it is impossible for forms of life to remain.A few months ago, another study - also conducted in Dallol and published in Scientific Reports - which highlighted an opposite result: the finding of nanobacteria . That territory, so apparently inhospitable, was described as a valid example for understanding the environmental limits of life, both on Earth and in other parts of the Solar System. And the geothermal area of ​​Dallol was proposed as a terrestrial analogue of a primitive Mars (as it was three billion years ago). The conclusions of López-García and colleagues, now published in Nature Ecology & Evolution, are of a different opinion . "After analyzing many more samples than the previous jobs - with appropriate controls to avoid contaminating them and with a well calibrated methodology - we verified that in these salty, hot and hyperacid pools the microbial life is absent. As it is absent in the adjacent salt lakes, rich in magnesium », emphasizes López-García.Yes, there is a great variety of halophilic archaea (primitive microorganisms that live in highly saline environments) in the desert and in the canyons around the hydrothermal site," adds the biologist, "but not in the hyperacid and hypersaline pools, nor in the so-called black and yellow lakes of Dallol, where magnesium abounds. And this despite the fact that the microbial dispersion, in this area, is intense, due to the wind and human visitors ".There are two obstacles to life that prevent micro-organisms from developing inside the ponds: the abundance of magnesium salts caotropic - capable of breaking hydrogen bonds and causing protein denaturation - and the simultaneous presence of conditions such as l hypersalinity, hyperacidity and high temperature.To confirm this, the team of scientists has used various research methods such as: massive sequencing of genetic markers to detect and classify microorganisms, chemical analysis of brines and scanning electron microscopy combined with X-ray spectroscopy , used to analyze silicon-rich mineral precipitates. «In other studies, in addition to the possible contamination of samples with archaea from adjacent lands, these mineral particles may have been interpreted as fossilized cells, but in reality they form spontaneously in brines even if there is no life, "observes López-García, pointing out that caution is needed in relying on the apparently cellular appearance - or "biological" - of a structure, because it could be non-living systems."We would never expect to find life in similar environments on other planets, at least not life that is not based on a biochemistry similar to that on earth," says López-García, insisting on the need to have more clues and analyze all possible alternatives before reaching a conclusion. "Our study shows that there are places on the earth's surface, such as the pools of Dallol, which are sterile even if they contain water in the liquid state," concludes the researcher, remarking as a criterion such as the presence of liquid water, often used to suggest the habitability of a planet, does not necessarily imply the presence of life.