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Ecology and molecular adaptations of the halophilic black yeast Hortaea werneckii

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Molecular studies on halophilic adaptations have focused on prokaryotic microorganisms due to a lack of known  appropriate eukaryotic halophilic microorganisms. However, the black yeast Hortaea werneckii has been identified
as the dominant fungal species in hypersaline waters on three continents. It represents a new model organism for studying the mechanisms of salt tolerance in eukaryotes. Ultrastructural studies of the H. werneckii cell wall have shown that it synthesizes dihydroxynaphthalene (DHN) melanin under both saline and non-saline growth conditions. However, melanin granules in the cell walls are organized in a salt-dependent way, implying the potential osmoprotectant role of melanin. At the level of membrane structure, H. werneckii maintains a sterol-to-phospholipid ratio signifi- cantly lower than the salt-sensitive Saccharomyces cerevisiae. Accordingly, membranes of H. werneckii are more fluid over a wide range of NaCl concentrations, indicating high intrinsic salt stress tolerance. Even H. werneckii grown in high NaCl concentrations maintains very low intracellular amounts of potassium and sodium, demonstrating the sodium-excluder character of this organism. The salt-dependent expressions of two HwENA genes suggest roles for them in the adaptation to changing salt concentrations. The high similarity of these ENA ATPases to other fungal ENA ATPases
involved in Na+/K+ transport indicates their potential importance in H. werneckii ion homeostasis. Glycerol is the main compatible solute which accumulates in the cytoplasm of H. werneckii at high salinity, although it seems that  ycosporines may also act as supplementary compatible solutes. Salt dependent increase in glycerol synthesis is  supported by the identification of two copies of a gene putatively coding for glycerol-3-phosphate-dehydrogenase.  Expression of only one of these genes is salt dependent.

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