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Great Salt Lake (GSL), Utah, U.S.A. is the largest lake in the western United States and the fourth largest terminal lake in the world. A railroad causeway across Great Salt Lake, Utah (GSL) has restricted water flow since its construction in 1959, resulting in a more saline North Arm (NA; 24%-31% salinity) and a less saline South Arm (SA; 11-14% salinity) providing a unique opportunity to study how salinity gradients influence microbial communities and their activities. Importantly, ~30% of the lake bottom is covered with carbonaceous microbialites, or organo-sedimentary structures that develop between interacting microbial mat communities and their environments.  The study of microbialites and their communities across salinity gradients presents an opportunity to identify the populations involved in their formation and whether their activity leaves recognizable biosignatures. This work is timely, considering recent evidence (in the form of a microbialite fossil) for the presence of life at 3.8 Ga.

Microbialites were sampled from the SA and the NA of the GSL for use in evaluating the hypothesis that NA microbialites were remnant structures. After showing this to likely be the case, we compared the microstructure, mineralogy, community composition, function, and abundance of these microbialites to identify putative architects of these communities and their potential to leave biosignatures.  In August 2016, we sampled an additional 8 microbialite morphologies from across the GSL taking note of variations in the hydrology, geochemistry, and geology of the locations.  With these samples we wished to evaluate whether variation in the microfabrics (biosignatures) of microbialites were associated with differences in the composition of the communities.  Comparison of thin sections by transmission electron microscopy and microbial communities will provide unprecedented insight into whether microorganisms influence the microfabric structure of carbonate microbialites.