Life on the edge: active microbial communities in the Kryos MgCl(2-)brine basin at very low water activity

The Kryos Basin is a deep-sea hypersaline anoxic basin (DHAB) located in the Eastern Mediterranean Sea (34.98°N 22.04° E). It is fi lled with brine of re-dissolved Messinian evaporites and is nearly saturated with MgCl 2 -equivalents, which makes this habitat extremely challenging for life. The strong density difference between the anoxic brine and the overlying oxic Mediterranean seawater impedes mixing, giving rise to a narrow chemocline. 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. Within the interface, we detected fatty acids that were distinctly 13 C-enriched when compared to other fatty acids. These likely originated from sul fi de-oxidizing bacteria that fi x carbon via the reverse tricarboxylic acid cycle. In the lower part of the interface, we also measured elevated rates of methane oxidation, probably mediated by aerobic methanotrophs under micro-oxic conditions. Sulfate reduction rates increased across the interface and were highest within the brine, providing fi rst evidence that sulfate reducers (likely Desulfovermiculus and Desulfobacula ) thrive in the Kryos Basin at a water activity of only ~0.4 A w . 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.