Biomarkers of Endolithic Communities within Gypsum Crusts (Southern Tunisia)

Abstract

Evaporite accumulations produced by artesian waters in the arid zones of southern Tunisia led to the formation of subrounded, gypsiferous mounds consisting of irregular alternations of mineral precipitates and aeolian sand. The joint occurrence of gypsum crusts and plant colonization determined the stabilization of their top surface. Careful examination of the pigmented (green-brown) crusts revealed endolithic microbial communities just below the surface. In previous optical and scanning electron microscope studies cyanobacteria were the dominant component of these communities. Molecular diversity studies based on small subunit ribosomal RNA (SSU rRNA) gene analysis revealed that Flavobacteria, Actinobacteria, Deinococcales, Alpha- and Gamma- Proteobacteria are also important components of the microbial assemblage. Their pigment analyses, determined by high performance liquid chromatography (HPLC), detected the presence of carotenoids and chlorophyll (chl) a and b. Microbial communities that produce pigmentation and display an endolithic lifestyle typify the extreme environments as those found in arid/semiarid and hot desert regions.

Keywords:

• endoliths
• gypsum crusts
• Sahara Desert
• spring mounds

The authors acknowledge D. Moreira for the assistance provided with the sequence analysis and the two anonymous reviewers for constructive comments, which helped to improve the quality of the manuscript. Thanks to Maud Walsh, F. A. Martin, and Jackie Prudente for the hospitality in the Louisiana State University and LSU AgCenter School of Plant, Environmental and Soil Sciences during the preliminary HPLC analyses. The Italian MIUR Programme “Geomicrobiologia di depositi evaporitici continentali: analisi comparata di sistemi fossili e moderni e implicazioni astrobiologiche”, and the French CNRS Programme Interdisciplinaire “Origines des planètes et de la vie” financially supported this study.