20Temperature is one of the defining parameters of an ecological niche. Most organisms thrive within a 21 temperature range that rarely exceeds ~ 30°C, but the deep subsurface bacterium Kosmotoga olearia 22 can grow over a temperature range of ~60°C (20°C -79°C). To identify genes correlated with this 23 flexible phenotype, we compared transcriptomes of K. olearia cultures grown at its optimal 65°C to 24 those at 30°C, 40°C, and 77°C. The temperature treatments affected expression of 573 of 2,224 K. 25 olearia genes. Notably, this transcriptional response elicits re-modeling of the cellular membrane and 26 changes in metabolism, with increased expression of genes involved in energy and carbohydrate 27 metabolism at high temperatures and up-regulation of amino acid metabolism at lower temperatures. 28