Zach Oestreicher Master’s Thesis 2004 Title: Geomicrobiology Investigation of Mickey Hot Springs, Southeastern Oregon Mickey Hot Springs is a cluster of active hot springs and extinct hot spring vents located north of the Alvord Desert in southeastern Oregon. The extinct vents contain deposits of siliceous sinter. The active hot springs are not depositing sinter, but they do posses actively growing communities of thermophilic bacteria. Hot spring sinter is known to contain silicified microfossils of thermophilic bacteria that live in hot springs that are depositing sinter. The goals of this study are to identify the microfossils in the sinter samples from the extinct vents by comparing them to the living thermophilic bacteria of one of the modern active hot spring. This information can be used to identify the paleoenvironmental conditions of the extinct hot springs. Four sites along the thermal gradient of one of the active hot springs were sampled to identify the thermophilic bacteria living in the hot spring. These samples were analyzed with the optical light microscope, the scanning electron microscope, and the transmission electron microscope. Five different sinter samples were obtained from 5 different extinct vents to sample for microfossils. The samples were analyzed in the scanning electron microscope. Six thermophilic bacteria were identified from one of the active hot springs based on their morphology and ultrastructure: Synechococcus, Chloroflexus, Spirochetes, Geitlerinema/Leptolyngbya, Isosphaera, and Spirulina. Several microfossils were found in the sinter samples based on their morphological characteristics. The dominant type of microfossil was silica encrusted filaments that were similar to cyanobacteria. Two of the microfossils were identified based on their morphological similarity to living bacteria—Microcoleus and Fischerella. Based on these lines of evidence the microfossils were probably a community of cyanobacteria that grew along the edge of a pool. The hydrothermal fluid around the community had a temperature between 40-60C and was supersaturated with respect to amorphous silica, which precipitated out onto the community of organisms causing them to silicify.