TACK is a group of archaea acronym for Thaumarchaeota, Aigarchaeota, Crenarchaeota and Korarchaeota, the first groups discovered. They are found in different environments ranging from acidophilic thermophiles to mesophiles and psychrophiles and with different types of metabolism, predominantly anaerobic and chemosynthetic. TACK is a clade that is close to the branch that gave rise to the eukaryotes. It has been proposed that the TACK clade be classified as Crenarchaeota and that the traditional "Crenarchaeota" be classified as a class called "Sulfolobia", along with the other phyla with class rank or order.