Microbial communities and processes within a hypersaline gypsum crust in a saltern evaporation pond (Eilat, Israel)

Gypsum crusts containing multicolored, stratified microbial communities develop in the evaporation ponds of a commercial saltern in Eilat, Israel at salt concentrations between 190 and 240 g l^-1. The upper 0.5-2 cm of the crust is densely populated by orange-brown unicellular cyanobacteria. Below, a layer of green-colored filamentous cyanobacteria is found. Underneath, a bright purple layer of anoxygenic phototrophs is present, below which a reduced black layer is found. We have investigated the biological properties of this crust using a wide variety of techniques, and we here review the results of these interdisciplinary studies. The tests performed included microscopic examination of the biota, phylogenetic analyses based on 16S rRNA gene clone libraries and denaturing gradient gel electrophoresis, fatty acid analysis, light intensity and light quality measurements, microelectrode studies of oxygen profiles and oxygen evolution, determination of sulfate reduction using radioisotope methods, and measurement of methane evolution. The stable vertical stratification in the system enabled separate analyses of the different layers with a high spatial resolution. It was therefore possible to combine the different approaches and obtain information on the activities of the different types of oxygenic and anoxygenic phototrophs, dissimilatory sulfate reducers and methanogens in the different layers, as well as phylogenetic information on the nature of the microorganisms responsible for these processes. The gypsum crust thus becomes a paradigm for the study of a wide variety of microbial processes and their interrelationships in the presence of high salt concentrations.