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...

Urania basin in the deep Mediterranean Sea houses a lake that is >100 m deep, devoid of oxygen, 6 times more saline than seawater, and has very high levels of methane and particularly sulfide (up to 16 mM), making it among the most sulfidic water bodies on Earth. Along the depth profile there are 2 chemoclines, a steep one with the overlying oxic seawater, and another between anoxic brines of different density, where gradients of salinity, electron donors and acceptors occur. To identify and differentiate the microbes and processes contributing to the turnover of organic matter and sulfide along the water column, these chemoclines were sampled at a high resolution. Bacterial cell numbers increased up to a hundredfold in the chemoclines as a consequence of elevated nutrient availability, with higher numbers in the upper interface where redox gradient was steeper. Bacterial and archaeal communities, analyzed by DNA fingerprinting, 16S rRNA gene libraries, activity measurements, and cultivation, were highly stratified and metabolically more active along the chemoclines compared with seawater or the uniformly hypersaline brines. Detailed analysis of 16S rRNA gene sequences revealed that in both chemoclines ␦-and -Proteobacteria, predominantly sulfate reducers and sulfur oxidizers, respectively, were the dominant bacteria. In the deepest layers of the basin MSBL1, putatively responsible for methanogenesis, dominated among archaea. The data suggest that the complex microbial community is adapted to the basin's extreme chemistry, and the elevated biomass is driven largely by sulfur cycling and methanogenesis.deep anoxic hypersaline lake ͉ element cycling ͉ geosphere-biosphere interaction ͉ Mediterranean Sea ͉ microbial diversity T he Urania basin is one of the deep-sea hypersaline anoxic basins (DHABs) located in the eastern Mediterranean Sea. DHABs are far below the photic zone (3,200-3,600 m deep) and contain brines, the origin of which has been attributed to dissolution of 5.9-to 5.3-million-year-old Messinian evaporites (1). Urania is less saline than the other Mediterranean DHABs, with NaCl concentrations 5.4-7 times higher than normal seawater, but has higher concentrations of methane (5.56 mM) and exceptionally high levels of sulfide (up to 16 mM), making Urania basin among the most sulfidic marine water bodies on Earth (2-4).Interfaces are considered to be hot spots for biological activity (2, 5), and environmental gradients represent an important part of the biosphere that must be accounted for in models of global biogeochemical cycles, especially in otherwise oligotrophic environments like the Eastern Mediterranean (6).In the present study, we discovered 2 different environmental chemoclines within the Urania basin. We finely dissected the gradients and compared the oxic/anoxic upper interface of Urania basin with those found in chemically different DHABs. We concluded that the lower overall salinity but higher sulfide and methane concentrations in Urania DHAB are the primary factors determining the observed ...

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