# Metabolic Potential of Halophilic Filamentous Fungi--Current Perspective
## 3 Molecular Adaptations to High Saline Conditions  
Osmotic and ionic stress related to high salinity prevents the survival of most microorganisms. Habitats with high salinity are also characterized by low water index and alkaline conditions and are a poor source of nutrients for the microorganisms living there [20]. To thrive in such harsh conditions, microorganisms create different adaptation mechanisms. One of the main ones is \"salt in\" strategy. It consists in accumulation by microorganisms of high concentrations of inorganic ions inside the cell to obtain osmotic balance [21]. Raising salt concentration in the cytoplasm is mostly a result of K\({}^{+}\) ions because organisms from every domain exclude Na\({}^{+}\) ions if possible due to their toxicity to several cell components, which include intracellular membranes and enzymes [22; 23]. This strategy requires all the intracellular proteins to remain active and stable in presence of potassium chloride and other salts. To achieve it, the proteome of those organisms adapts to such conditions and is mainly made up of the acidic residues (aspartic and glutamic acid), which are located on the surface of the protein. They coordinate water molecules to form a protecting barrier around a protein that prevents dehydration and the precipitation of molecules from the solution [5; 24]. Prokaryotes (bacteria and archaea) are the microorganisms that adapt to the \"salt in\" approach in environments where no fluctuating salinity conditions prevail [5]. Those organisms also are obligate halophiles with a demand to stay constantly in saline conditions [13]. Another strategy is known as \"organic osmolytes\" mechanism, \"low-salt\", \"salt-out\" or \"compatible solute\" strategy [5; 20; 22]. Rather than accumulating inorganic salts, microorganisms use organic solutes to maintain the osmotic balance inside the cell. The compatible solutes include polyols, sugars, glycerol, ectoine, and dimethylsulfonopropate which do not interfere with the activity of enzymes [24]. Organic osmolytes could also be divided into three chemical categories: zwitterionic solutes (e.g., betaine, ectoine), noncharged solutes (e.g., sucrose, trehalose) and anionic solutes (e.g., \(\beta\)-glutamate, hydroxybutyrate) [21]. Metabolites obtained in \"compatible solute\" strategy are mostly extracellular, and therefore, they could be easily extracted [25]. This strategy needs the use of more energy than the \"salt-in\" strategy and is widespread among all halophilic microorganisms but is always related to halophilic Eukarya such as fungi [5]. The membrane structure of halophilic microorganisms plays an important role in adapting to saline conditions, protecting the cell from the harmful effects of changing salt concentrations and maintaining the osmotic homeostasis inside the cell by proper membrane fluidity [26]. Changes in the plasma membrane and the cell wall include structural modifications and the presence of pigments and/or hydrophobins [27; 28; 29]. High salinity increases the cell wall thickness and influences the change in lipid composition,which includes the number of sterols, the type of fatty acyl chains and the nature of the polar phospholipid head-groups [30]. Another factor influencing the properties of cytoplasmic membranes in halophilic microorganisms is the presence of pigments such as carotenoids and melanins which additionally screen out UV radiation and protect them from the damaging sunlight effects [21; 31; 32]. The black yeasts are a group of fungi that produce melanin in order to protect them from environmental stress. They are common in saline habitats and represent up to 80% of halophilic fungi [22]. The characteristics of those polymorphic fungi are meristematic, filamentous, or yeast-like growth and very thick cell wall with a distinct melanin layer [31; 33]. Melanization of the cell wall helps to avoid water loss, leak intracellular compatible solutes, and maintain high fluidity of membrane, and therefore, fungi are able to survive in very high salinities [28].