PMC

Sample of the gypsum crust with clearly distinguishable coloured layers. (Online version in colour.)

(a) Soil crust community on gypsum soil dominated by the lichen Diploschistes diacapsis in a Mediterranean gypsum shrub community in Granada, Spain. (b) D. diacapsis growing on gypsum soil. (c and d) Thalli of D. diacapsis used in our experiments to test for the effect of various adhesives on thallus attachment to gypsum spoil; (c) in trays during the rainfall-simulation (see arrows), and (d) in one of the plots when the field experiment was set up in March 2014.

Microbe-mineral interactions—First column is the field photo of the hand sample. Second column is a photomicrograph of petrographic “wet” thin section, stained with crystal violet to highlight gram positive bacteria. Third and fourth column are SEM photos of critically dried samples. Orange/brown bulbous mat PBT from transition zone between Puquio 1 and 2 (a–d). (b) Photomicrograph of “wet” thin section under cross-polarized light showing Mg-silicates forming along cyanobacterial sheaths; (c,d) images showing microcrystalline gypsum crystals in an EPS matrix with cyanobacterial filaments. Orange gel mat PBT from transition zone between Puquio 1 and 2 (e–h). (e) Hand sample showing orange gel, green and purple microbial layers, and granular minerals; (g,h) images showing gypsum crystals in an EPS matrix (g) and acicular crystals coating a cyanobacterial filament (h). Spar crystals PBT sampled from a small lagoon near Puquio 2 (i–l). (j) coarse gypsum crystals, often with jagged outer surfaces with dark coatings shown in petrographic thin section; (k,l) images show large flat crystal faces coated with a thin crust, rich in manganese (EDS). Spar crystals PBT sampled from a small lagoon near Puquio 2 (m–p). (n) Cross sections of euhedral prismatic gypsum crystals with dark horizons; (o,p) images show large blocky, flat crystal faces, coated with granular aggregates rich in manganese (EDS). Pinnacles PBT sampled along the northern margin of Puquio 3 (q–t). (r) Gypsum is microcrystalline with an adjacent strip of lenticular sparry gypsum and blocky to prismatic gypsum; (s) images showing lenticular gypsum embedded in EPS and fine-grained gypsum (t) and filamentous cyanobacteria with gypsum crystals on sheaths. Botryoids PBT sampled from within Puquio 4 (u–x). (v) Crystalline matrix of prismatic gypsum; edges are coated with a fine microcrystalline crust; (w) images showing blocky crystals with planar faces and (x) fine cyanobacterial filaments with stellate crystals adhering to the sheaths.

Raman spectra taken at different layers of the gypsum crust (i) orange, (ii) green, (iii) red by the three instruments Delta Nu Advantage (a), ESA prototype (b) and Renishaw (c).