![]() ![]() Hence, hotspots of biological activity were found at brine–seawater interfaces. In contrast to the overlying seawater, the brines are anoxic but electron acceptors other than oxygen are plentiful, most importantly sulfate. Seawater and brines differ strongly in their solute composition (in particular, in the concentrations of key electron donors and acceptors), so that a narrow (1–3 m) chemocline forms between the two media. Vertical mixing across the extreme density gradients between brine and overlying seawater is extremely slow. In contrast, the Kryos and the Discovery Basin contain athalassohaline brines, dominated by MgCl 2. The brines of most Mediterranean DHABs (l’Atalante, Bannock, Thetis, Tyro, and Urania basins ) are thalassohaline, i.e., the brine salts consist mainly of NaCl. Even though many DHABs in the Mediterranean are located in close proximity to each other, their respective geochemical composition can differ quite strongly, depending on the source evaporite. As hypersaline solutions have a higher density than seawater, brines accumulate in seafloor depressions and form lake-like features, which can be several kilometers long and more than hundred meters deep. ![]() Their exposure at the seafloor led to the formation of the present-day brine lakes. After sediment burial, tectonic activity mobilized relic brines and re-exposed some of these evaporite layers to seawater. Mediterranean brines originate from the dissolution of evaporites deposited during the Messinian salinity crisis (5–6 million years ago ). The deep-sea hypersaline anoxic basins (DHABs) in the Mediterranean are located at the Mediterranean Ridge at water depths >3000 m below sea level (mbsl). Marine brine basins have been discovered at the seafloor of the Gulf of Mexico, the Red Sea, the Black Sea, and the Eastern Mediterranean Sea (see ref. Hence, hypersaline environments on Earth may serve as analogs for potential niches for life in extraterrestrial environments. Furthermore, hypersaline settings were also discovered on extraterrestrial objects, for example on Mars, or on Saturn’s moon Enceladus (Hsu et al. Hypersaline environments exist in terrestrial settings (e.g., lakes, salt flats, salterns), and marine environment (e.g., mud volcanoes, sea-ice brine channels or brine basins ). Despite these constraints, microbial and eukaryotic life has been reported in many of these systems. Such environments are challenging for life due to the high energetic costs for osmotic adaptation and the limitation of available water. Our results demonstrate that a highly specialized microbial community in the Kryos Basin has adapted to the poly-extreme conditions of a DHAB with nearly saturated MgCl 2 brine, extending the known environmental range where microbial life can persist.Įnvironments characterized by salinities higher than 50 PSU are considered hypersaline. Sulfate reduction rates increased across the interface and were highest within the brine, providing first evidence that sulfate reducers (likely Desulfovermiculus and Desulfobacula) thrive in the Kryos Basin at a water activity of only ~0.4 A w. In the lower part of the interface, we also measured elevated rates of methane oxidation, probably mediated by aerobic methanotrophs under micro-oxic conditions. These likely originated from sulfide-oxidizing bacteria that fix carbon via the reverse tricarboxylic acid cycle. Within the interface, we detected fatty acids that were distinctly 13C-enriched when compared to other fatty acids. Here, we investigate the microbial community structure and activities across the seawater–brine interface using a combined biogeochemical, next-generation sequencing, and lipid biomarker approach. The strong density difference between the anoxic brine and the overlying oxic Mediterranean seawater impedes mixing, giving rise to a narrow chemocline. ![]() It is filled with brine of re-dissolved Messinian evaporites and is nearly saturated with MgCl 2-equivalents, which makes this habitat extremely challenging for life. The Kryos Basin is a deep-sea hypersaline anoxic basin (DHAB) located in the Eastern Mediterranean Sea (34.98°N 22.04☎). ![]()
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