Microbial community of the deep‐sea brine L ake K ryos seawater–brine interface is active below the chaotropicity limit of life as revealed by recovery of mRNA

Within the complex of deep, hypersaline anoxic lakes ( DHALs ) of the Mediterranean Ridge, we identified a new, unexplored DHAL and named it ‘ L ake K ryos ’ after a nearby depression. This lake is filled with magnesium chloride ( MgCl 2 )‐rich, athalassohaline brine (salinity > 470 practical salinity units), presumably formed by the dissolution of Messinian bischofite. Compared with the DHAL Discovery , it contains elevated concentrations of kosmotropic sodium and sulfate ions, which are capable of reducing the net chaotropicily of MgCl 2 ‐rich solutions. The brine of L ake K ryos may therefore be biologically permissive at MgCl 2 concentrations previously considered incompatible with life. We characterized the microbiology of the seawater– K ryos brine interface and managed to recover mRNA from the 2.27–3.03 M MgCl 2 layer (equivalent to 0.747–0.631 water activity), thereby expanding the established chaotropicity window‐for‐life. The primary bacterial taxa present there were Kebrit Deep Bacteria 1 candidate division and DHAL ‐specific group of organisms, distantly related to D esulfohalobium . Two euryarchaeal candidate divisions, Mediterranean Sea Brine Lakes group 1 and halophilic cluster 1, accounted for > 85% of the rRNA ‐containing archaeal clones derived from the 2.27–3.03 M MgCl 2 layer, but were minority community‐members in the overlying interface‐layers. These findings shed light on the plausibility of life in highly chaotropic environments, geochemical windows for microbial extremophiles, and have implications for habitability elsewhere in the Solar System.