The chemical composition of the Bannock basin has been studied in some detail 1,2 . We recently showed that unusual microbial populations, including a new division of Archaea (MSBL1) 3 , inhabit the NaCl-rich hypersaline brine. High salinities tend to reduce biodiversity 4 , but when brines come into contact with fresher water the natural haloclines formed frequently contain gradients of other chemicals, including permutations of electron donors and acceptors, that may enhance microbial diversity, activity and biogeochemical cycling 5,6 . Here we report a 2.5-mthick chemocline with a steep NaCl gradient at 3.3 km within the water column betweeen Bannock anoxic hypersaline brine 7 and overlying sea water. The chemocline supports some of the most biomass-rich and active microbial communities in the deep sea, dominated by Bacteria rather than Archaea, and including four major new divisions of Bacteria. Significantly higher metabolic activities were measured in the chemocline than in the overlying sea water and underlying brine; functional analyses indicate that a range of biological processes is likely to occur in the chemocline. Many prokaryotic taxa, including the phylogenetically new groups, were confined to defined salinities, and collectively formed a diverse, sharply stratified, deep-sea ecosystem with sufficient biomass to potentially contribute to organic geological deposits.High-precision sampling was conducted during cruises of the research vessel Urania equipped with the Modus-Scipack system (http://www.geo.unimib.it/BioDeep/Project.html; Fig. 1a). The vehicle Modus, connected by cable to the research vessel, held a second instrument, the Scipack, with a 10-m data transmission cable. The Scipack, consisting of a Rosette sampler equipped with a CTD (conductivity-temperature-depth probe) and a series of Niskin bottles, was connected to the Modus through the Sciskid, a module equipped with a pressure sensor for recording the pressure at which the Niskin bottles were closed (Fig. 1c). A camera on the Modus could provide an image of the Scipack entering the brine lake (Fig. 1b, and Supplementary Fig. S1). Immediately after sampling, the Modus-Scipack was raised, the Niskin bottles were retrieved and their contents were carefully fractionated on board ship by slowly recovering 0.5-litre, 1-litre or 2-litre fractions from the bottom tap. These were then immediately analysed for salinity (Fig. 1d). The reconstructed interface salinity profile was strongly positively correlated (r ¼ 0.98, P , 0.001) with the CTD conductivity profile recorded in independent non-sampling casts (Fig. 2d), indicating that little or no mixing had occurred.The interface halocline was about 2.5 m deep, in agreement with previous estimates that employed alternative sampling strategies 1 . Although biomass values fluctuated along the halocline, there were significantly greater numbers of microbial cells in the interface (about 10 6 cells ml 21 ) than in either the deep sea water or the underlying hypersaline brine, both of which had about...
