The role of microbial extracellular polymeric substances on formation of sulfate minerals and fibrous Mg-clays

A variety of Mg-, Na- and Ca-sulfates, including gypsum, and Mg-clays precipitate within the microbial mats that developed seasonally in the hypersaline playa lake El Longar (Central Spain). A multidisciplinary study of the microbial mats along a transect of the lake demonstrates that the degree of lithification of the mats increases from the soft and poorly lithified layers lying on the submerged ground to the highly crystalline mats present in the desiccated fringes. The community composition of the mats is diverse, dominated in terms of OTUs by Proteobacteria, Firmicutes and Cyanobacteria. The Proteobacteria include sulfur-cycling microbes, supporting biogeochemical cycling of this element. Cyanobacteria are the dominant EPS producers in microbial mats and Firmicutes and Proteobacteria have both been shown to play a role in EPS turnover. Several ions (e.g., Mg, 5, Si, Al, Fe, Ca, Na) are bound to functional groups of EPS. The abundance of sulfate groups and carboxylic acids suggested that the EPS have a strong binding capacity for Mg2+ and Ca2+, which were higher in the submerged microbial mats. Na, Mg, K, Ca, SO42- and Si are more concentrated in the EPS matrices than in the natural brines. The concentration of ions in the EPS is sufficient to precipitate a mixture of minerals through freeze-drying, that is similar to that found in intact mats. In the EPS of the submerged mats precipitate more types of minerals, including sulfates and Mg-clays, than in the EPS of the exposed areas. These results suggest that the mineral nucleation occurred during the degradation of the mats when the EPS was progressively desiccated and likely released high amounts of ions. The present study helps to understand intrasedimentary growth of sulfates and clays in microbial mats. The presence of these mineral assemblage can be related to a potential biosignature of microbial past life on Earth and possibly also on exoplanets, like Mars.