Salt-loving archaea grew for 160 days under laboratory conditions designed to simulate hypothetical underground saline environments on Mars, researchers report. The findings suggest that some Earth microbes could remain metabolically active under certain low-pressure, oxygen-free conditions, but they do not show that life exists on Mars.
The study focused on Haloferax volcanii, an archaeon adapted to extremely salty environments. Dr. Adam Robinson of the University of Florida and colleagues published their findings in Applied and Environmental Microbiology.
Simulating a hypothetical underground Mars environment
“The current surface of Mars is generally considered inhospitable due to low atmospheric pressure, intense surface radiation, global aridity, and oxidative chemistry that has increased since the late Noachian,” Robinson and his colleagues wrote.
“However, shallow underground regions on Mars, where mineralogy indicates groundwater movement, may contain local hypersaline environments that can hold liquid water and support microbial metabolism.”
The researchers placed cultures in a vacuum chamber filled with carbon dioxide and stripped of oxygen. The pressure was maintained at 24 millibars, approximately 2% of Earth’s sea-level pressure, while the temperature was held at 21 degrees Celsius (approximately 70 degrees Fahrenheit).
The archaea were grown in highly salty liquid media containing either nitrate or perchlorate. Both chemicals have been detected on Mars and can potentially serve as alternative electron acceptors in microbial metabolism, taking the place of oxygen in some forms of respiration.
The pressure used in the experiment could occur within sealed pockets of salty fluid in mineral deposits approximately 20 centimeters (8 inches) below the Martian surface.
“The environment we are mimicking in this experiment has not been observed on Mars, but is theorized to exist,” Robinson said.
Growth and chemical changes
All low-pressure treatments containing nitrate or perchlorate showed growth, based on increases in optical density, which indicates that the cultures became more turbid.
The researchers also measured decreases in nitrate and perchlorate that did not occur in cell-free controls, suggesting that the microbes were metabolizing the chemicals. Under the simulated Martian pressure, nitrate levels fell by approximately 15% to 29%, while perchlorate levels declined by about 16% to 29%.
Liquid loss remained below 4% in all treatments over the 160-day experiment. This was important because water generally evaporates rapidly at such low pressures, making long-term experiments difficult.
Electron microscopy revealed dense, biofilm-like structures in actively growing low-pressure cultures. These structures were not observed in soil-pressure controls or in low-pressure cultures that did not grow.
The chemical signature of carotenoid pigments was also still detectable, although it was weaker than in the oxygenated control. Carotenoid signatures can be identified with Raman spectroscopy and have been proposed as potential biosignatures.
Important limitations
Growth at 24 millibars was significantly lower than growth in the same cultures under soil pressure. The experiment also did not reproduce the freeze-thaw cycles, radiation environment or complete chemical conditions of Mars. In addition, the nutrient-rich growth medium was not representative of the organic matter thought to be available on the planet.
The cultures were measured only at the beginning and end of the experiment because opening the chamber during the study would have allowed oxygen to enter.
The researchers emphasize that their results do not demonstrate that Mars hosts life. Instead, they show that a salt-tolerant Earth microorganism can grow and form biofilm-like structures under one set of hypothetical subsurface conditions.
“There is growing evidence to suggest that we should consider the possibility of life on Mars today,” Robinson concluded.
Adam Robinson et al., “Cultivation of halophilic archaea in the shallow underground environment of Mars has implications for extant life on Mars,” Applied and Environmental Microbiology, published online November 5, 2026. doi: 10.1128/aem.01749-26.
Source: www.sci.news
