A new experiment reveals that Earth's microbes can grow in harsh conditions similar to what may exist just below the surface of Mars, suggesting new directions in the search for potential cellular life on the red planet.
The microbes grew over months in just a fraction of Earth's normal atmosphere, in a solution 7 times saltier than seawater and breathing chemicals similar to bleach.
The study, led by University of Florida researcher Adam Robinson tested whether a salt-loving microorganism could do more than survive several of the stresses associated with Mars. This includes stresses that would exist on the surface and potentially in the subsurface. The microbe could reproduce and use chemicals in its surroundings to sustain its metabolism, although growth was substantially slower than in comparable cultures kept at Earth's atmospheric pressure.
The scientists focused on Haloferax volcanii, a microbe known as an extremophile for its ability to tolerate extreme conditions here on Earth. They chose Haloferax because of its known preference for high-salt environments, similar to those found in the shallow Martian subsurface.
Past experiments have attempted to answer similar questions. Researchers have grown Earth microbes under individual stresses associated with Mars, including high salt concentrations or low pressure. Other studies have tested whether organisms remain alive after exposure to several harsh conditions at once, often for only hours or days.
"Far fewer have been able to combine these conditions together and show that microorganisms can actively grow," Robinson said.
For this experiment, the team kept cultures of Haloferax in oxygen-free, carbon dioxide-rich conditions for 160 days. The microbes were given nitrate or perchlorate, compounds relevant to Mars that they could use in place of oxygen during respiration. Cultures supplied with neither chemical remained viable but showed no measurable growth, helping the scientists distinguish survival from reproduction.
Observing microbial processes under these conditions can help scientists decipher between potential biosignatures from extinct cells and biological processes from living/extant cells.
The researchers also observed dense structures called biofilms in the growing low-pressure cultures. The films may have helped hold water in the salty solution: the cultures lost little of their water over the experiment.
The team, which was co-led by Scott Perl, Ph.D., of the Los Angeles Natural History Museum and the Blue Marble Space Institute of Science, published their findings Nov. 5 in the journal Applied and Environmental Microbiology, a journal of the American Society for Microbiology.
The laboratory did not reproduce every condition the organisms would encounter on Mars. The cultures were held at 70 degrees Fahrenheit, while most of the Martian subsurface is far colder. They also received nutrients supplied by the researchers. And the liquid-water pockets they were designed to resemble have not been confirmed on Mars.
"The environment that we're mimicking in this experiment has not been observed on Mars, but is theorized to exist," Robinson said.
That distinction matters. The results do not establish that life exists on Mars today. They do suggest that future missions searching beneath its surface should consider whether some environments could support active life, rather than looking only for evidence that life existed billions of years ago.
Perl said scientists also need to reconsider their assumptions about what makes an environment hospitable to life. Any microorganisms that evolved on Mars would have had billions of years to adapt to the planet's changing conditions.
"They've evolved within the Martian environment that we consider hostile. So what is hostile to us isn't necessarily going to be hostile to those microbes," he said.
The work also offers a way to test how microorganisms might behave in such environments, information that could help scientists design life-detection experiments and reduce the risk of carrying Earth microbes to Mars aboard spacecraft.
"We are adding to the growing body of evidence suggesting that we should consider the possibility of life on Mars today," Robinson said.
