A UNIQUE BACTERIAL MINERALIZING ENVIRONMENT AT MICKEY HOT SPRINGS, SOUTHEASTERN OREGON

Mickey Hot Springs is a cluster of geothermal springs located in southeastern Oregon. This is a unique site that allows us to study the mineralization process of thermophilic organisms growing in a hydrothermal system that is in its waning stages of existence. The alkaline fluid of the hot springs contains dissolved minerals, and forms a steep thermal gradient along the outflow channel. Thermophilic bacteria living within the hot spring fluid form distinct communities distributed on the basis of temperature. Similar to other geothermal systems, the organisms become mineralized; but unlike other geothermal systems studied elsewhere the mineralization process at Mickey is no longer as robust as it was in the past as indicated by older sinter deposits. In order to understand the present mineralization process, samples of the thermophilic organisms were collected along the temperature gradient of the active hot spring system in two ways: on glass slides and by coring microbial mats. Theses samples were analyzed using the optical light microscope, the transmission electron micrscope and the scanning electron microscope.

This analysis revealed that the modern mineralization process is dominated an evaporation process that primarily mineralizes organisms when the fluid level drops causing the silica to precipitate out onto the organisms; this occurs mostly on the margins of the system. The mats that do become silicified mostly contain grains of sediment that become entrained within the matrix of the mat and contain little evidence of mineralized cells. There is also mineralization occurring within the extracellular matrix of the microbial mats and biofilms, but there is little mineralization accumulating on the individual cells. These mineralization processes result in very transient mineralized structures that do not form stable siliceous sinter along the margins and floors of the hydrothermal system. There appears to be no enduring mineralized biofacies within the active system.

Older siliceous sinter deposits in the Mickey Hot Spring area do contain biosignatures that are indicative of specific niches along the hydrothermal systems, which are similar to other geothermal areas. These deposits probably formed under different mineralizing circumstances than the modern deposits.