Researchers have found an aquatic environment on Earth with complete absence of any forms of life, an advance that may lead to an improved understanding of the limits of habitability.

The study, published in the journal Nature Ecology and Evolution, revealed that any form of microbial life was absent in the hot, saline, hyperacid ponds of the Dallol geothermal field in Ethiopia.

The researchers, including those from the Spanish Foundation for Science and Technology (FECYT), said Dallol’s landscape extends over a volcanic crater full of salt, constantly releasing toxic gases with water boiling in the midst of the intense hydrothermal activity.

They said it is one of the most torrid environments on the planet with daily temperatures in winter exceeding 45 degrees Celsius.

The landscape, the researchers said, had abundant hypersaline and hyperacid pools, with pH - which is measured on a scale from 0 (very acidic) to 14 (very alkaline) - even hitting the negative mark.

Earlier studies had pointed that certain microorganisms can develop in this multi-extreme environment and researchers presented the place as an example of the limits of conditions that can support life.

The researchers said the place was even proposed as a terrestrial analogue of early Mars.

“After analysing many more samples than in previous works, with adequate controls so as not to contaminate them and a well-calibrated methodology, we have verified that there’s no microbial life in these salty, hot and hyperacid pools or in the adjacent magnesium-rich brine lakes,” said study co-author Purificacion Lopez Garcia from FECYT.

The researchers found great diversity of a type of primitive salt-loving microorganisms in the desert, and the saline canyons around the hydrothermal site but not in the hyperacid and hypersaline pools, nor in the Black and Yellow lakes of Dallol which are rich in magnesium.

“And all this despite the fact that microbial dispersion in this area, due to the wind and to human visitors, is intense,” Lopez Garcia said.

The researchers confirmed the findings with several other methods including a large-scale sequencing of genetic markers to detect and classify microorganisms, microbial culture attempts, using fluorescent probes to identify individual cells, chemical analysis of the hypersaline waters.

They also used scanning electron microscopy and X-ray spectroscopy to probe into the water samples looking for signs of life.

According to the researchers, the study helps in understanding the limits of habitability, and presents evidence that there are places even on the Earth’s surface that are sterile though they contain liquid water.

“Our study presents evidence that there are places on the Earth’s surface, such as the Dallol pools, which are sterile even though they contain liquid water,” said Lopez Garcia. This means that the presence of liquid water on a planet, which is often used as a habitability criterion, does not directly imply that it has life.

In this case, the researchers have found two physical-chemical barriers that prevent the presence of living organisms in ponds: the abundance of chaotropic magnesium salts (an agent that breaks hydrogen bridges and denatures biomolecules) and the simultaneous confluence of hypersaline, hyperacid and high-temperature conditions.

“We would not expect to find life forms in similar environments on other planets, at least not based on a biochemistry similar to terrestrial biochemistry,” points out Lopez Garcia, who insists on the need to have multiple indications, to analyse all types of alternatives and to be very prudent with interpretations before reaching any conclusions in astrobiology.

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