The advent of molecular tools in microbial ecology paved the way to exploit the diversity of microbes in extreme environments. Here, we review these tools as applied in one of the most polyextreme habitats known on our planet, namely, deep hypersaline anoxic basins (DHABs), located at ca. 3000–3500？m depth in the Eastern Mediterranean Sea. Molecular gene signatures amplified from environmental DHAB samples identified a high degree of genetic novelty, as well as distinct communities in the DHABs. Canonical correspondence analyses provided strong evidence that salinity, ion composition, and anoxia were the strongest selection factors shaping protistan community structures, largely preventing cross-colonization among the individual basins. Thus, each investigated basin represents a unique habitat (“isolated islands of evolution”), making DHABs ideal model sites to test evolutionary hypotheses. Fluorescence in situ hybridization assays using specifically designed probes revealed that the obtained genetic signatures indeed originated from indigenous polyextremophiles. Electron microscopy imaging revealed unknown ciliates densely covered with prokaryote ectosymbionts, which may enable adaptations of eukaryotes to DHAB conditions. The research reviewed here significantly advanced our knowledge on polyextremophile eukaryotes, which are excellent models for a number of biological research areas, including ecology, diversity, biotechnology, evolutionary research, physiology, and astrobiology. 1. Introduction Ocean bottom surveys in the early 1980s observed abyssal depressions at a depth of more than 3000？m in the Eastern Mediterranean Sea showing unusual reflection profiles and backscatter images [1, 2]. Subsequent hydrochemical analyses of the water trapped in these depressions identified the respective environments as deep hypersaline anoxic basins (DHABs) [1, 3, 4]. With the most recent discovery , there are eight known DHABs in the Eastern Mediterranean Sea, distributed in the Strabo Trench (Tyro), the Mediterranean Ridge (Bannock, Kryos, Medee, and Thetis), and the Medriff Corridor (L’Atalante, Discovery, and Urania) (, Figure 1). The formation of DHAB brines is reviewed in Cita . They originated by submarine dissolution of Messinian evaporites (late Miocene, ca. 6 MYA) thought to originate primarily from the dissolution of evaporites ~2000–176,000 years ago. Due to the fact that different minerals deposit in different orders depending on evaporation conditions, evaporites may contain different concentrations of halite (NaCl-mineral), kieserite
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