Normally, salt should be used in moderation—unless, that is, you’re a bacterium.
In a new paper, Jacob Heinz and colleagues (including me) from the Technical University Berlin and Tufts University in Massachusetts report that bacteria have shown increased survival rates when exposed to large amounts of salt. The permafrost bacterium Planococcus halocryophilus had been previously described by Nadia Mykytczuk and Lyle Whyte from McGill University in Canada, who watched it grow at temperatures down to -15 degrees Celsius. In the new study, the German-American team exposed the bacteria to various chloride and perchlorate salts, and found that they had a higher survival rate in chloride salts than in perchlorate-containing samples, even during freeze-thaw cycles. These alternating periods of freezing and thawing are common in Martian sediments. Astonishingly, the survival rate of Planococcus increased with lower temperatures, indicating that the salts somehow protect the bacteria from the cold.
Sodium chloride is the most common salt on Earth, and it’s common on Mars, particularly in the planet’s Southern Highlands. Perchlorates have also been detected at the Phoenix Lander site and the Curiosity Rover sites on Mars. Since both types of salts are highly hygroscopic, meaning that they can absorb water directly from the atmosphere, microbes can use them to their advantage, just as salt-loving microbes do in extremely dry deserts on Earth, and may do on Mars.
The survival of these microbes at salt concentrations and in environmental conditions similar to what we see on Mars, particularly in temperatures well below freezing, has broad implications for the potential habitability of the Red Planet. When searching for life there, we should look not only for liquid water, but also for the presence of salt, particularly chloride salts. If we follow that trail, it will lead us to the Southern Highlands of Mars as a prime landing site for future life-seeking missions.