People can live on every continent, but our planet really belongs to the microbes. Some even thrive in particularly pernicious environments, from deep-sea vents cooking at 251 degrees Fahrenheit to highly radioactive mine shafts.
These extremophiles can also be found in highly salty or highly acidic environments. But to date, microbes haven’t been unequivocally found inhabiting the combination — a simultaneously hot, hypersaline and hyper-acidic realm. If an organism could survive in such an unforgiving place, scientists would need to expand their search parameters for life in more unwelcoming corners of other worlds.
Attempting to ascertain life’s outer limits, scientists headed to Ethiopia’s Danakil Depression. A scorching, arid 155-mile-long lowland, it also contains the volcanic Dallol dome and its polychromatic geothermal field, which features some of the world’s saltiest, most acidic bodies of superheated water. The team scoured the landscape, looking for something that called one of these pools home.
But contrary to previous claims, no unambiguous evidence for any pioneering extremophiles was found. This was a disappointment — but also a crucial revelation.
Life needs a watery environment, both inside and outside the cell, to function, said Louisa Preston, an astrobiologist at London’s Natural History Museum who was not involved in the work. That is why astrobiologists are so keen to identify liquid water on other planets.
But this study is a reminder that “the presence of liquid water on the surface of a planet is not enough to have life,” said Purificación López-García, a microbial diversity expert at the French National Center for Scientific Research and an author of the study, which was published Monday in Nature Ecology & Evolution. The team also found several duplicitous lifelike objects, which served as a different reminder: There are things in extreme environments that an observer could be fooled into thinking are biological.
Many of Earth’s extremophiles belong to the Archaea domain, a superkingdom on the tree of life that is separate from both bacteria as well as the eukaryotes, which encompass all plants, fungi and animals, including us. Archaea were discovered in the 1970s, and since then an ever-increasing variety have been found in horrific environments.
Dr. López-García and her colleagues, on the hunt for the planet’s most irrepressible archaea, made several visits to Dallol from 2016 to 2018. They gathered myriad samples from the pools — of varying temperatures, salinities and acidities — and used a range of techniques to painstakingly search for biosignatures.
The team found that the combination of severe salinity and abundant acidity was too extreme for any extremophiles. High concentrations of magnesium salts, which fatally disrupt cell membranes and large molecules, appeared to be another life-limiting factor.
They did find plenty of impostors, in the form of nanoscale, silica-rich grains that closely resemble tiny cells. These mimics, known as biomorphs, serve as a warning, Dr. López-García said. If they appeared in rock samples gathered by robots exploring other worlds, they could hoodwink overzealous scientists into thinking they’d found traces of life.
That does not rule out the existence of a polymathic extremophile, Dr. Preston said: “Life seems to always find a way of surviving in places we think it can’t.”
The expedition did make some discoveries in the hypersaline-only waters near Dallol: some remarkable extremophiles, including several new archaea.
These microbes represent many limbs of the archaean evolutionary tree, indicating that salt-loving adaptations emerged many times. That may be because their common ancestor used potassium to adapt to high temperatures, Dr. López-García said. This chemical shield was co-opted by many descendants to help them thrive in hypersaline conditions.
Some scientists think those kinds of adaptations are a reason that fingerprints of extremophilic life — extinct or extant — could be be found in groundwater or briny lakes possibly locked beneath Mars’ harsh surface.
The limits to alien life may differ from those imposed on our own world’s microorganisms. But, said Dr. López-García, “if the conditions are permissive for life as we know it on Earth, they should be permissive for life as we don’t know it on another planet.”