The photosynthetic microbial mat in the effluent channel of an alkaline hot spring (Mushroom Spring) in Yellowstone National Park experiences extreme diel fluctuations in physicochemical parameters. During the day, photosynthesis causes the oxygen concentration within the mat to rise to highly supersaturating levels, and the intense photosynthetic activity of the cyanobacteria causes a pH >9.5 in the euphotic zone of the mat. During the night, the mat rapidly becomes anoxic, and intense respiration, reoxidation of reduced solutes and fermentation acidifies the mat to pH ~7.5. High temperature (55-70 ^{0C) itself imposes a constraint on the availability of Ci for photosynthesis because of the low solubility of CO_{2 at elevated temperatures; this is exacerbated by the relatively high O2-concentration and high pH in the mat during the day. Therefore, it is likely important for the cyanobacteria in the mat to develop the capacity to accumulate Ci over the course of the day through the induction of genes associated with the carbon concentrating mechanism (CCM). The genomes of two Synechococcus isolates from the mat have revealed the presence of two CO2 hydration systems, two putative HCO3- transporters and a ß-type carboxysome [1] that are likely associated with the CCM. In this study we have performed experiments with axenic Synechococcus isolates in the laboratory and are attempting analogous experiments in situ to elucidate the function and diel regulation of putative CCM genes.}}