Hot, dry, and with dangerous levels of ultraviolet (UV) light, deserts are a tough place to exist. Now, researchers have discovered how one desert-living microbe solves that triple whammy of challenges—by mining water from within protective gypsum rocks.
Organisms that live inside rocks, or endolithic microbes, were already known to exist in Chile’s Atacama Desert, one of the oldest, driest deserts on Earth. But how the microbes managed to survive there was unclear.
Now, a multidisciplinary team may have found the key to how microbes survive: making their own water source by transforming the rocks in which they live. The puzzle was cracked by a team led by David Kisailus, who studies biomimicry and nanostructured materials at the University of California, Irvine, and Jocelyne DiRuggiero, a microbiologist at Johns Hopkins University in Baltimore. Their results are part of a U.S. Army–funded study and were published in the Proceedings of the National Academy of Sciences of the United States of America last month.
In 2018, DiRuggiero traveled to the Atacama to collect rock samples, which she wanted to examine for any resident microorganisms. The high Chilean desert, with its temperature extremes, high UV radiation, and a lack of water, is often used as an Earth analogue for inhospitable extraterrestrial habitats like Mars.
Once back in her lab, DiRuggiero cut the rock samples into tiny square “coupons” for microscopic and spectroscopic analysis. Kisailus’s postdoctoral fellow Wei Huang used electron microscopy for the first analysis of the rock coupons, which revealed the cyanobacterium Chroococcidiopsis tucked between rock layers.
Using X-ray diffraction, which shows how atoms are organized inside rocks, the team could see the rock coupons were mostly gypsum, which is calcium sulfate dihydrate, or CaSO4 plus two molecules of water. “The water itself is part of the [gypsum] crystal—it’s not like pools of water are trapped in there,” Kisailus explained. They could also see traces of anhydrite, which is calcium sulfate, or CaSO4—gypsum—without water. This “dry” mineral was found only in tandem with microbes in the rock samples, which piqued the researchers’ curiosity.
The team decided to test the connection between the microbes and the minerals. DiRuggiero placed some of the sample coupons in two different environments (one wet and one dry) and allowed the cyanobacteria to colonize the samples. Back in Kisailus’s lab, X-ray diffraction revealed that with microbial colonies grown in the dry environment, gypsum changed to anhydrite. In the wet environment, nothing changed.
The microbes must have been transforming the gypsum to anhydrite, the researchers reasoned, but how they were doing so remained unclear. The team used electron microscopy to take a closer look at biofilms the microbes formed on the gypsum coupons. Fourier transform infrared spectroscopy revealed that these biofilms were rich in carboxylic acid. The team hypothesized that the microbes in the biofilm exuded this acid to locally dissolve the rock, “drilling” into it to extract available water. Once the water was extracted, the mineral components left behind precipitated into new nanosized crystals of anhydrite.
The team used rigorous methodology, producing intriguing results, commented Patricia Craig, who studies Mars mineralogy at the Planetary Science Institute in Tucson, Ariz., and was not involved in the project. Craig, a collaborator on the Chemistry and Mineralogy instrument on the Mars Curiosity rover, cautioned that conducting similar tests on Martian rocks with the same level of sophistication might be challenging. Until researchers can bring samples home from Mars, explorers are limited by what they can analyze using a remotely controlled robot communicating from off planet.
Microbes that can survive on Mars may be living inside rocks, where they must get nutrients and water to survive. This new research, said Craig, suggests that on any planetary surface, gypsum sitting adjacent to anhydrite “might be a biosignature” of microbes mining for water.
—Lesley Evans Ogden (@ljevanso), Science Writer