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Temperature is one of the defining parameters of an ecological niche. Most organisms thrive within a temperature range that rarely exceeds ~30 Â°C, but the deep subsurface bacterium Kosmotoga olearia can grow over a temperature range of 59 Â°C (20-79 Â°C). To identify genes correlated with this flexible phenotype, we compared transcriptomes of K. olearia cultures grown at its optimal 65 Â°C to those at 30, 40, and 77 Â°C. The temperature treatments affected expression of 573 of 2224 K. olearia genes. Notably, this transcriptional response elicits re-modeling of the cellular membrane and changes in metabolism, with increased expression of genes involved in energy and carbohydrate metabolism at high temperatures and up-regulation of amino acid metabolism at lower temperatures. At sub-optimal temperatures, many transcriptional changes were similar to those observed in mesophilic bacteria at physiologically low temperatures, including up-regulation of typical cold stress genes and ribosomal proteins. Comparative genomic analysis of additional Thermotogae genomes indicates that one of K. olearia's strategies for low-temperature growth is increased copy number of some typical cold response genes through duplication and/or lateral acquisition. At 77 Â°C one-third of the up-regulated genes are of hypothetical function, indicating that many features of high-temperature growth are unknown.