The extremophiles have adapted their enzyme, called “extremozymes,” to cope with extreme habitats (very hot or cold conditions, a high concentration of salt, as well as acid and alkaline slots; or places with toxic waste, organic solvents, heavy metals, high pressure) in which their mesophilic counterparts could not survive. Because most bacteria are abundant in environments that are hostile to all other forms of life, this chapter focused the attention on the carbonic anhydrases (CAs, EC 4.2.1.1) from extremophilic microorganisms. The extreme CAs, catalyzing the hydration/dehydration reaction of carbon dioxide to bicarbonate and protons, , are considered multitasking enzymes able to fight the increase of CO2 in the atmosphere produced by the anthropogenic activities, and/or ameliorating the human health because of their use in biomedical applications (biosensors and artificial lungs). In particular, the biotechnological interest of SspCA identified in the genome of the bacterium Sulfurihydrogenibium yellowstonense is highlighted. SspCA is a highly active catalyst for the CO2 hydration reaction and extremely thermostable at high temperature, resulting in an excellent candidate to fight the increase of CO2 in the atmosphere and in improving the human health because it can be used in some biomedical application (artificial lungs).