The wonderful halophiles: microorganisms producing useful compounds.

The halophilic microorganisms are present in all three domains of life, namely Archaea, Bacteria and Eukarya. They have the unique property of thriving in environments characterized by high salt concentration, and are widespread in hypersaline lakes, solar salterns, saline soil, evaporation ponds, and marine environments (Ventosa et al. 1999, Deepa et al. 2015, Boyadzhieva et al. 2018). Halophiles can be distinguished in moderately halophilic and extremely halophilic microorganisms. Moderate halophiles include both Gram-positive and Gram-negative microorganisms growing optimally in media with NaCl concentration ranging from 3 to 15%; however, they can also grow below and above these values (Ventosa et al. 1998). Extremely halophilic microorganisms show optimal growth in media containing from 15 to 30% NaCl, and the main representatives are halophilic aerobic Archaea (also known as Haloarchaea), although some extremely halophilic Bacteria have been reported in this group (Grant et al. 2001). As in such extreme environments the salt concentration can reach the saturation, halophiles succeed to survive by adopting several strategies: they accumulate in the cytoplasm huge quantity of organic compounds which function as osmo-protectants or store in the cytoplasm potassium chloride offering osmotic balance between cells and medium. Furthermore, they have also developed unique enzymes that are stable and active in the presence of salts (Lanyi 1974), and interesting biomolecules with protective functions as exopolysacchyarides (EPS), pigments, and polyhydroxyalkanoates (PHA) (Yin et al. 2015). Some halophilic microorganisms exhibit antimicrobial properties as they produce bacteriocin-like peptides, the halocins (Rodriguez-Valera et al. 1982). Consequently, the halophilic microorganisms can be regarded as amazing tools for producing compounds of significant importance for industries as they can found application in many fields, such as pharmaceutical, food, and cosmetic. This review reports an overview of the most useful "halophilic products" produced by Archaea, Bacteria and Eukarya, and their biotechnological potentials.