Life in extreme environments: Survival strategy of the endolithic desert lichen Verrucaria rubrocincta

Verrucaria rubrocincta Breuss is an endolithic lichen that inhabits caliche plates exposed on the surface of the Sonoran Desert. Caliche surface temperatures are regularly in excess of 60°C during the summer and approach 0°C in the winter. Incident light intensities are high, with photosynthetically active radiation levels typically to 2,600 μmol/m ² s^-1 during the summer. A cross-section of rock inhabited by V. rubrocincta shows an anatomical zonation comprising an upper micrite layer, a photobiont layer containing clusters of algal cells, and a pseudomedulla embedded in the caliche. Hyphae of the pseudomedulla become less numerous with depth below the rock surface. Stable carbon and oxygen isotopic data for the caliche and micrite fall into two sloping, well-separated arrays on a δ¹³C-δ¹⁸O plot. The δ ¹³C_PDB of the micrite ranges from 2.1 to 8.1 and δ¹⁸O_SMOW from 25.4 to 28.9, whereas δ¹³C_PDB of the caliche ranges from -4.7 to 0.7 and δ¹⁸O_SMOW from 23.7 to 29.2. The isotopic data of the micrite can be explained by preferential fixing of ¹²C into the alga, leaving local ¹³C enrichment and evaporative enrichment of ¹⁸O in the water. The ¹⁴C dates of the micrite range from recent to 884 years b.p., indicating that "dead" carbon from the caliche is not a significant source for the lichen-precipitated micrite. The endolithic growth is an adaptation to the environmental extremes of exposed rock surfaces in the hot desert. The micrite layer is highly reflective and reduces light intensity to the algae below and acts as an efficient sunscreen that blocks harmful UV radiation. The micrite also acts as a cap to the lichen and helps trap moisture. The lichen survives by the combined effects of biodeterioration and biomineralization. Biodeterioration of the caliche concomitant with biomineralization of a protective surface coating of micrite results in the distinctive anatomy of V. rubrocincta.