Soap Lake is a haloalkaline lake located in central Washington. This lake is a remnant of the Missoula flood events that
created the landscape of western Montana, the southeastern portion of Washington state, and much of Oregon. It is
15,000 - 20,000 years old, and has maintained a stable meromixis for the last 10,000 years. This carbonate lake is
characterized by a brackish mixolimnion, and a monimolimnion with a salinity of ~14%. The pH of both layers of the
lake is approximately 10. Both layers also have a high concentration of dissolved sulfate, with the mineral mirabilite
(Na2SO4•10H2O) found in the monimolimnion sediments. Sulfide concentrations in the monimolimnion exceed 100 mM.
As part of the mission of the NSF Soap Lake Microbial Observatory, microorganisms involved in the sulfur cycle in this
lake were studied in terms of their diversity and function. High rates of sulfate reduction were measured in both layers of
the lake, with new species of sulfate-reducing bacteria seen in both areas. A particularly novel psychrophilic sulfur
oxidizer was isolated from the monimolimnion. This organism has the ability to induce the formation of mirabilite,
which was assumed to be an abiotically deposited evaporite mineral. This is the first evidence for a biogenic origin of
this mineral. This leads to the possibility that related sulfate minerals, such as those reported on the Mars surface, may
have a biogenic origin.