Mechanism of water extraction from gypsum rock by desert colonizing microorganisms

Abstract

Microorganisms, in the most hyperarid deserts around the world, inhabit the inside of rocks as a survival strategy. Water is essential for life, and the ability of a rock substrate to retain water is essential for its habitability. Here we report the mechanism by which gypsum rocks from the Atacama Desert, Chile, provide water for its colonizing microorganisms. We show that the microorganisms can extract water of crystallization (i.e., structurally ordered) from the rock, inducing a phase transformation from gypsum (CaSO4·2H2O) to anhydrite (CaSO4). To investigate and validate the water extraction and phase transformation mechanisms found in the natural geological environment, we cultivated a cyanobacterium isolate on gypsum rock samples under controlled conditions. We found that the cyanobacteria attached onto high surface energy crystal planes ({011}) of gypsum samples generate a thin biofilm that induced mineral dissolution accompanied by water extraction. This process led to a phase transformation to an anhydrous calcium sulfate, anhydrite, which was formed via reprecipitation and subsequent attachment and alignment of nanocrystals. Results in this work not only shed light on how microorganisms can obtain water under severe xeric conditions but also provide insights into potential life in even more extreme environments, such as Mars, as well as offering strategies for advanced water storage methods.

Crystalline water in gypsum is unavailable for cyanobacteria in laboratory experiments and in natural desert endolithic habitats

“…1), and the results of previous works surprisingly not cited in ref. 1 in the context of their hypothesis (4-10), definitively confirm that the transformation of gypsum to anhydrite and the liberation of crystalline water in gypsum do not occur in the natural interface between gypsum and endolithic cyanobacteria. As an additional consequence, the authors (1) do not provide insights into potential adaptations of life on Mars.…”

“…Huang et al (1) describe a supposed mechanism of water extraction from gypsum by cyanobacteria sampled from endoliths inhabiting Ca sulfates in the Atacama Desert, and cultivated in the laboratory. The authors claim that the phase transformation from gypsum (CaSO 4 •2H 2 O) to anhydrite (CaSO 4 ) (G→A) occurred under "dry conditions" in the contact zone between a "dry biofilm" and the gypsum, where only {011} planes of gypsum are transformed to anhydrite, supposedly providing water for cyanobacteria.…”

“…Selected area electron diffraction (SAED) data in ref. 1 do not allow any mineral identification, due to their differences with referenced reciprocal distances data (Table 1) for gypsum and anhydrite. Consequently, the authors neglect the fact that both gypsum and anhydrite phases were already included as natural components in the gypsum samples from the Tarapacá region (refs.…”

“…Obviously, these theoretical conditions are not met in the experiments described in ref. 1, and are unachievable within the dry (or moist) biofilm. Fourth, any dissociation and liberation of H + is only possible in liquid water and not under "dry conditions."…”

Metabolomics of Dry Versus Reanimated Antarctic Lichen-Dominated Endolithic Communities

“…Ertekin and coworkers [ 10 ] found that, regardless of water availability, substrate architecture was the driving factor that constrained microbial community diversity and function in gypsum endoliths of Atacama Desert. More recently, Huang et al [ 11 ] showed that the microorganisms can extract water of crystallization (i.e., structurally ordered) from the rock, inducing a phase transformation from gypsum (CaSO 4 ·2H 2 O) to anhydrite (CaSO 4 ).…”