Drinkable rocks: Plants can use crystallization water from gypsum

November 25th, 2014

Dr. Sara Palacio, José Azorín, Gabriel Montserrat & Dr. Juan Pedro Ferrio

Some minerals hold water in their crystalline structure. Such is the case of gypsum (CaSO42H2O), a rock-forming mineral present in five continents that is particularly prevalent in the arid and semi-arid regions of Africa and Western and Central Asia1. Crystallization water can account for up to 20.8% of gypsum’s weight2, and these water molecules can be released under natural conditions; when temperature, pressure, or dissolved electrolytes or organics pass a certain threshold3. Indeed, the activation energy of the dehydration reaction of gypsum is quite low4 and the conversion of gypsum to bassanite (i.e. hemihydrate: CaSO4½H2O) or anhydrite (CaSO4) may take place in ambient conditions 5.

It has been suggested that gypsum crystallization water could constitute a significant water source for organisms growing on gypsum soils, particularly during dry periods6,7. This idea is consistent with the phenology observed in some shallow-rooted plants growing on gypsum, which remain active when drought is intense6, and with the increased soil moisture recorded in gypsum soils during summer as compared to surrounding non-gypsum soils8. In a recent study9, we explored this possibility and found evidence that gypsum crystallization water may account for a significant proportion of the water used by plants during summer.

In our study, we analyzed the hydrogen (?2H) and oxygen (?18O) isotope composition of the xylem water of the gypsum-specialist plant Helianthemum squamatum (L.) Pers. and compared it to the isotopic composition of the free and crystallization water of the gypsum soils where it grows. Our target species, H. squamatum, is a shallow-rooted species that does not reach the water table10, and remains phenologically active during summer11. Nevertheless, to explore the generality of our results, we analysed another three shallow-rooted species that coexist with H. squamatum in the study area. These included: the gypsum specialist Lepidium subulatum L., and the non-specialists: Linum suffruticosum L. and Helianthemum syriacum (Jacq.) Dum. Cours. These species are all small woody sub-shrubs with a similar architecture to H. squamatum and shallow root systems10. Consequently, the main sources of water available to them are free soil water and potentially also gypsum crystallization water.

We compared the isotope composition of the xylem sap of plants with that of free and crystallization water of gypsum soils, both in spring (May) and summer (late August). This was done by harvesting plants and their surrounding soil in the field and extracting the xylem sap and free and crystallization gypsum water by cryogenic vacuum distillation12. Xylem sap was distilled directly at 120ºC. For the soils, we performed a stepwise distillation, first at 35ºC, then at 120ºC, to separate the free soil water from crystallization water, taking advantage of the thermal response of gypsum dehydration3. ?2H and ?18O compositions were further determined by cavity ring-down spectroscopy at the Serveis Científico-Tècnics of the University of Lleida (Lleida, Spain), using a Picarro L2120-i, coupled to a high-precision vaporizer A0211. The relative contribution of different water sources to the composition of the xylem sap of plants was estimated using Bayesian stable isotope mixing models.

According to our results, the isotopic composition of the xylem sap of H. squamatum plants was closer to gypsum crystallization water than to free soil water, particularly during summer. The analysis through Bayesian stable isotope mixing models indicated that gypsum crystallization water accounted for up to 90% of the water used by this species during summer. Plants could also take up gypsum crystallization water during spring, when it accounted for up to 30% of the xylem sap of plants. The isotopic composition of H. squamatum plants was similar to that of other shallow-rooted plants coexisting in the same community, indicating that the use of crystallization water is a common mechanism for shallow-rooted plants growing on gypsum.

Our findings open up exciting questions about the mechanisms used by plants to access gypsum crystallization water. We suggest two complementary mechanisms: passive uptake by soil heating and active extraction through changes in soil chemistry. In the first case, water molecules would be released from the crystalline structure of gypsum simply by the soil heating, particularly during summer. Previous studies indicate that the conversion of gypsum to anhydrite is initiated at 42-60°C3. However, this temperature threshold may be lower in non-pure gypsum, as the activation energy for the dehydration of gypsum is lower in soils than in pure mineral4. Temperatures of the top centimeters of gypsum soils frequently exceed 40°C during summer, with values of 51°C being reported at 3 cm depth in some regions7. Under such circumstances, gypsum from the most superficial layers of the soil can be easily dehydrated, releasing water molecules profitable to plants and potentially also other organisms.

In the second case, plants and/or their associated microorganisms could actively force the release of water molecules from the crystalline structure of gypsum by excreting organic acids or electrolytes. The chemical bonds that link water to the crystalline structure of gypsum are sensitive to the presence of organics and electrolytes. Plants and their associated microorganisms can modify the chemistry of the soil in contact with roots, leading to rock weathering13. Endolythic microorganisms can grow in gypsum by dissolving rocks, including lichens, free-living algae, fungal hyphae, cyanobacteria and non-photosynthetic bacteria14. Some gypsum plants like H. squamatum also show the ability to dissolve gypsum, being able to grow their roots into hard Petrogypsic soil horizons 9. The ability of plants and their associated microorganisms to dehydrate gypsum would also explain the use of crystallization water during spring, when gypsum is thermodynamically stable.

Although the underlying mechanisms require further research, our results provide the first experimental evidence in support of the role of gypsum crystallization water as a water source for life. These results significantly modify the current paradigm on water use by plants, where water held in the crystalline structure of mineral rocks is not regarded as a potential source. Given the existence of gypsum on the surface of Mars15 and its widespread occurrence on arid and semi-arid regions worldwide1,16, our results have important implications for exobiology, the study of life under extreme conditions and arid land reclamation.

References:

Watson, A. Gypsum crusts in deserts. 1979. Journal of Arid Environments 2, 3-20. Bock, E. 1962. On the solubility of anhydrous calcium sulphate and of gypsum in concentrated solutions of sodium chloride at 25° C, 30° C, 40° C, and 50° C. Canadian Journal of Chemistry 39, 1746-1751. Freyer, D. and Voigt, W. 2003. Crystallization and phase stability of CaSO 4 and CaSO 4 -based salts. Monatshefte Fur Chemie 134, 693-719. Hudson-Lamb, D. L., Strydom, C. A. and Potgieter, J. H. 1996. The thermal dehydration of natural gypsum and pure calcium sulphate dihydrate (gypsum). Thermochimica Acta 282-283, 483-492. Van Driessche, A. E. S. et al. 2012. The role and implications of bassanite as a stable precursor phase to gypsum precipitation. Science 335, 69-72. Escudero, A., Palacio, S., Maestre, F. T. and Luzuriaga, A. L. 2014. Plant life on gypsum: an overview of its multiple facets. Biological Reviews, in press. doi:DOI: 10.1111/brv.12092. Herrero, J. and Porta, J. 2000. The terminology and the concepts of gypsum-rich soils. Geoderma 96, 47-61. Meyer, S. E., García-Moya, E. and Lagunes-Espinoza, L. C. 1992. Topographic and soil surface effects on gypsophile plant community patterns in central Mexico. Journal of Vegetation Science 3, 429-438. Palacio, S., Azorín, J., Montserrat-Martí, G. and Ferrio, J. P. 2014. The crystallization water of gypsum rocks is a relevant water source for plants. Nature Communications 5. Guerrero-Campo, J., Palacio, S., Pérez Rontomé, C. and Montserrat-Martí, G. 2006. Effect of root system morphology on root-sprouting and shoot-rooting abilities in 123 plant species from eroded lands in North-east Spain. Annals of Botany 98, 439-447. Aragón, C. F., Albert, M. J., Giménez-Benavides, L., Luzuriaga, A. L. and Escudero, A. 2007. Environmental scales on the reproduction of a gypsophyte: A hierarchical approach. Annals of Botany 99, 519-527. Ehleringer, J. R. and Dawson, T. E. 1992. Water uptake by plants: perspectives from stable isotope composition. Plant, Cell and Environment 15, 1073-1082. Puente, M. E., Bashan, Y., Li, C. Y. and Lebsky, V. K. 2004. Microbial populations and activities in the rhizoplane of rock-weathering desert plants. I. Root colonization and weathering of igneous rocks. Plant Biology 6, 629-642. Wierzchos, J. et al. 2011. Microbial colonization of Ca-sulfate crusts in the hyperarid core of the Atacama Desert: Implications for the search for life on Mars. Geobiology 9, 44-60. Langevin, Y., Poulet, F., Bibring, J. P. and Gondet, B. 2005. Sulfates in the north polar region of Mars detected by OMEGA/Mars express. Science 307, 1584-1586. Eswaran, H. and Gong, Z. T. 1991. Properties, genesis, classification, and distribution of soils with gypsum. In: Occurrence, characteristics, and genesis of carbonate, gypsum, and silica accumulations in soils, W.D. Nettleton (ed.). Soil Science Society of America: 89-119.

Sara Palacio is a postdoctoral researcher at the Pyrenean Institute of Ecology (CSIC) in Jaca (Spain), José Azorín is a laboratory technician at the Pyrenean Institute of Ecology (CSIC) in Jaca (Spain), Gabriel Montserrat is a researcher at the Pyrenean Institute of Ecology (CSIC) in Zaragoza (Spain), and Juan Pedro Ferrio is a “Ramon y Cajal” postdoctoral researcher at the University of Lleida (Spain).

The views expressed in this article belong to the individual authors and do not represent the views of the Global Water Forum, the UNESCO Chair in Water Economics and Transboundary Water Governance, UNESCO, the Australian National University, or any of the institutions to which the authors are associated. Please see the Global Water Forum terms and conditions here.