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

摘要

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/m2 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 δ13C_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.