Abstract: The largest number of deep anoxic brine pools known so far on Earth are in the Red Sea. Such brine pools are very saline water bodies originating from the dissolution of ancient evaporites and laying at the bottom of the sea. They do not mix with the overlaying water column because of the high density of the brines. The conditions of these deep lakes are anoxic, very saline, with a relatively high hydrostatic pressure and in some cases very hot, far above the already high temperature of the Red Sea. Under such conditions and due to the physical separation from the normal seawater column, unique microbial assemblages have been selected over time. The most productive compartment of the brine pools is the transition zone between the normal seawater column and the brine, because gradients of redox couples exist that feed specialized microbial taxa finely stratified across thin seawater-brine interfaces. In this talk the unique features of such microbial assemblages are discussed with reference to the stratification of the microorganisms within the water column of the lakes. The data indicate that the deep anoxic brine pools in the Red Sea are home of unique microbial types specifically adapted to such polyextreme environments.