Cyanobacteria drink from a stone

Chile’s Atacama Desert is one of the driest places on Earth. Among the most common minerals found on the surface is gypsum, a hydrated form of calcium sulfate. The mineral is crystalline and usually embedded in rock, with 21% of its mass held in water molecules bound by the lattice. It’s also porous, and photosynthetic microbes readily colonize the rock’s inner pores and grain boundaries. The habitat offers distinct advantages: By filtering UV radiation, the gypsum rock protects multiple microbial taxa. By storing water, it offers nourishment.

Just how microbes extract water from gypsum crystals has long eluded explanation. Wei Huang, a postdoctoral researcher at the University of California, Riverside, his adviser David Kisailus , and their colleagues have now discovered that the microbes effectively force gypsum to undergo a phase transition from a hydrous structure to an anhydrous one. In doing so, the microbes free water molecules from the lattice. To make their discovery, Huang and his coworkers performed microscopic and spectroscopic analysis of rock samples. Microcomputed tomography revealed that microorganisms are pervasive on gypsum, as shown in the series of insets in the figure above (yellow and red represent microorganism colonies). Scanning electron microscopy (SEM), shown in the figure below, revealed that the microbes prefer to attach themselves to specific crystal facets—primarily the (011) planes. Those faces likely have a rougher surface, which enables stronger adhesion of microbes to it, and they offer easier access to bonded water molecules. According to SEM and IR spectroscopy, the microbes exude carboxylic acid when they attach. The acid dissolves the mineral surface, and water is released from the lattice structure as liquid. That sequence steadily transforms gypsum to anhydrite, the waterless form of the mineral.

The analysis was primarily based on measurements of field samples collected by team member Jocelyne DiRuggiero of Johns Hopkins University. She expanded it by culturing cyanobacteria in the laboratory—under both dry and wet conditions. A distinct phase transformation of gypsum to anhydrite accompanied only the dry culture and was directly correlated to the culture’s number of microbial cells. Those results imply that cyanobacteria extract water from gypsum only in environments where water is scarce. (W. Huang et al., Proc. Natl. Acad. Sci. USA, 2020, doi:10.1073/pnas.2001613117 .)