MICROBIAL SIGNATURES IN HYPERSALINE ENVIRONMENTS: A CASE STUDY FROM THE SABKHA DEPOSITS OF THE SOUTHERN TUNISIAN CHOTTS

Evaporite environments are good sites for testing the preservation potential of biologically induced structures because of the rapid sealing provided by precipitating minerals. Moreover, as extreme (hypersaline) environments, they allow an evaluation of the fossilization conditions for specific groups of extremophiles, such as moderate and extreme halophiles.

The inland, ephemeral saline lakes (sabkhas) of southern Tunisia (Chott el Gharsa and Chott el Jerid) provided an ideal area of study because of the variety of evaporite precipitations, including fossil, sub-fossil, and modern sulfate (gypsum and anhydrite) deposits and halite crusts. Artesian spring mounds, located SE of Chott el Jerid, have also provided gypsum and other minerals surface precipitations.

The in situ and laboratory observations of these evaporite products have allowed the recognition of structures produced by the concurrence of organic and sedimentary/mineral compounds. In addition, mineralogical examinations were centered on the interactions between microbes and mineral precipitations, because microbes may produce the reactants leading to the supersaturated conditions that are necessary for the mineral precipitation. In absence of direct evidences of microbes, the presence of bio-induced minerals in the environment can be useful for identifying some microbial pre-existence. In summary, the structures that we have recognized include the following: i) dark, organic laminae, which directly adhere to well-formed crystal beds, from finely bedded, fossil gypsum deposits with a cyclic sedimentation. These laminae represent a distinct feature of the (centimeter-scale) sedimentary cycles, in which laminated, microcrystalline gypsum alternate with macrocrystalline gypsum, clayey and organic-rich laminae; ii) wrinkled surfaces and domes likely caused by complex interaction between activity of microorganism and crystallization processes; iii) irregular laminae of brownish and green color from quasi-surface evaporite deposits of spring mounds likely due to the presence of cyanobacterial groups; iv) pinkish coloration in halite crusts likely due to pigments produced by halophilic green algae and halobacteria.