Schematic of the Atlantis II Deep brine pool and hypothetical bacterial niche-specific evolution.

The figure briefly describes the vertical changes of the temperature and salinity in the Atlantis II Deep brine pool. Brine water and a sediment core were collected from the deep layer of the brine pool and the seafloor, respectively. Microbial samples were then obtained at depths of 12 cm and 222 cm from the core. Free microbe exchanges occur at the interface of the seawater and brine pool, all under natural selection; the microbial lineages in the deep layer (a closed ecosystem) heavily evolve during their adaptation to the sediment.