Together, these studies suggested that Korarchaeota are exclusively thermophilic, expanded the geographical and geochemical range of the phylum, provided strong evidence of Korarchaeota endemism, and revealed extremely low phylogenetic Nilotinib diversity among Korarchaeota in terrestrial habitats. However, collectively, these studies incompletely identify the niche of Korarchaeota within geothermal habitats since relatively few geochemical measurements were made at the time and place of sampling. Here, we built on the work of Auchtung et al. and Reigstad et al. to define the habitat of Korarchaeota in terrestrial hot springs. To enhance our understanding of the precise geochemical habitats that support Korarchaeota, we expanded our sampling to a large number of geothermal features in two geographical regions, YNP and the U.S. Great Basin, and paired quantitative biological sampling with an extensive analysis of geochemistry. The resultant data set included 107 samples, over 5,000 measurements of individual geochemical analytes, and 90 new Korarchaeota 16S rRNA gene sequences. Subsequently, we applied a variety of statistical tests to determine which factors correlated with Korarchaeota habitability and used a classification support vector machine to develop models to predict whether a terrestrial geothermal habitat could support Korarchaeota based on geochemical data alone. The results described here provide a robust description of Korarchaeota habitat in terrestrial geothermal ecosystems, strengthen evidence of biogeographic structure, reveal new phylogenetic diversity, provide the first ecological niche models, and complement the genomic work by Elkins et al. in bringing the nature of Korarchaeota to light in the absence of axenic cultures. Springs were chosen to encompass a broad range of temperatures and pH. Temperature, pH and conductivity were measured with hand-held meters that were calibrated in the field prior to sampling. Measurements were taken immediately before sediment sampling as close as possible to the precise sampling location. Hydrothermal fluid was collected as close to the sampling site as possible prior to sediment sampling to avoid disrupting the sediment and altering the bulk water chemistry. Alkalinity, total ammonia, nitrate, nitrite, silica, total sulfide and dissolved oxygen were measured in the field colorimetrically. Some of these analyses are time LEE011 in vivo sensitive due to gas dissolution and chemical/biological redox reactions, while others are more temperature sensitive. Water samples for measurement of alkalinity, total ammonia, nitrate, nitrite and silica were allowed to cool to ambient temperature for analysis. Alkalinity was determined by titration to pH 4.5. Ammonia was determined by using Nesslerization or salicylate oxidation. Silica was determined by the measurement of molybdate-reactive silica with the heteropoly blue method in samples diluted with deionized water.