Microbial biosignatures in evaporite deposits: Evidence from Death Valley, California

We report on a microstratigraphic layering of mineral types that correlates with layering of the microbial community. Environmental Scanning Electron Microscopy–Energy Dispersive X-ray Spectroscopy analysis showed that in the upper layers of the community, dominated by cyanobacteria, gypsum (CaSO4·2H2O) and bassanite (CaSO4·H2O) give way to rosickyite, a rare form of sulfur (γ-sulfur; monoclinic symmetry), which is not thermodynamically predicted to be stable at Earth surface temperatures, reverting to the more common α-sulfur form (orthorhombic symmetry) and yet exists and is stably maintained within the endoevaporitic microbial community implying a biogenic role in its formation. This mineral may be formed by a cycle of microbial dissolution of gypsum to sulfide and re-oxidation of the sulfide to elemental sulfur (rosickyite) within a stable oxygen–sulfide gradient maintained by the organisms and appears specifically in association with the cyanobacterial layer. Cells in the lower region of the pigmented phototroph-dominated layers (i.e., purple and green bacteria) have associated strontianite (SrCO3) and calcite in the form of fine-grained minerals deposited on the cell surfaces and within the extracellular polymers surrounding them. Knowledge of how microbial communities can affect the mineralogy of evaporite deposits on Earth can help us to identify potential markers of the past or present existence of life on extraterrestrial bodies bearing evidence of ancient seas or lakes.