Getting a drink in the Atacama Desert

It’s not quite life on Mars, but it may be a pointer.

US researchers have shown that, as some had suspected, microorganisms can survive in the harshest of conditions by extracting water from the rocks they colonise.

A team from the University of California (UC) and Johns Hopkins University (JHU) studied interactions between Chroococcidiopsis – a cyanobacteria commonly found in deserts – and gypsum in Chile’s Atacama Desert. 

Or below it, to be precise. The Chroococcidiopsis exist beneath a thin layer of rock that gives them a measure of protection against the high solar irradiance, extreme dryness and battering winds in what is the world’s driest non-polar region.

When gypsum samples were studied back in the lab, the most striking discovery was that the microorganisms change the very nature of the rock. By extracting water, they cause a phase transformation of the material – from gypsum to anhydrite, a dehydrated mineral.

Intrigued, the researchers ran some experiments, allowing the organisms to colonise half-millimetre cubes of rock, called coupons, under two different conditions: one in the presence of water, to mimic a high-humidity environment, and the other completely dry. 

Amid moisture, they found, the gypsum did not transform to the anhydrite phase.

The cyanobacteria “didn’t need water from the rock, they got it from their surroundings,” says David Kisailus, from UC Irvine. “But when they were put under stressed conditions, the microbes had no alternative but to extract water from the gypsum, inducing this phase transformation in the material.”

Kisailus’s team used a combination of advanced microscopy and spectroscopy to examine the interactions between the biological and geological counterparts, finding that the organisms bore into the rock by excreting a biofilm containing organic acids.

UCI’s Wei Huang then used a modified electron microscope equipped with a Raman spectrometer to discover that the cyanobacteria used the acid to penetrate the gypsum in specific crystallographic directions – only along certain planes where they could more easily access the water existing between faces of calcium and sulfate ions.

“Researchers have suspected for a long time that microorganisms might be able to extract water from minerals, but this is the first demonstration of it,” says JHU biologist Jocelyne DiRuggiero 

“This is an amazing survival strategy for microorganisms living at the dry limit for life, and it will guide our search for life elsewhere.”

The findings are reported in a paper in the journal Proceedings of the National Academy of Science.