How extreme bacteria squeeze water from a stone

Chile’s Atacama Desert is the driest place on Earth between the poles. Yet photosynthetic microbes called cyanobacteria somehow thrive on the region’s gypsum-rich rocks. Johns Hopkins University microbiologist Jocelyne DiRuggiero wanted to find out how it’s possible for a microbe to carry out photosynthesis in an environment where there is essentially no water available. So she collaborated with University of California, Irvine, materials scientist David Kisailus, who probed the biogeochemical relationship between microbe and rock using a battery of analytical tools usually reserved for studying nonliving materials. Using X-ray imaging, electron microscopy, and spectroscopy to examine Atacama rocks, Kisailus and DiRuggiero got a close look at what the bacteria are doing (Proc. Natl. Acad. Sci. USA 2020, DOI: 10.1073/pnas.2001613117). Gypsum is a crystalline material made up of calcium and sulfate ions layered with water molecules. Where the bacteria grow, there’s much more anhydrite, the water-free version of the mineral. Where there are no bacteria, gypsum remains. The researchers found that the cyanobacteria surround themselves with a film made of carboxylic acids, which seems to help them dissolve the rock in order to extract the water, leaving behind anhydrite. And they found that cyanobacteria prefer to live on the high-energy, rougher plane of the gypsum crystal, where their acids are more effective. Kisailus says he hopes to develop a cyanobacteria-inspired water-mining extraction technology for humans living in arid environments.