Extremely Halotolerant and Halophilic Fungi Inhabit Brine in Solar Salterns Around the Globe

For a long time halotolerant and halophilic fungi have been known exclusively as contaminants of food preserved with high concentrations of either salt or sugar. They were first reported in 2000 to be active inhabitants of hypersaline environments, when they were found in man-made solar salterns in Slovenia. Since then, they have been described in different salterns and salt lakes on three continents. The mycobiota that inhabit these natural hypersaline environments are composed of phylogenetically unrelated halotolerant, extremely halotolerant, and halophilic fungi, which are represented not only by species previously known only as food contaminants, but also by new and rare species. The dominant representatives are different species of black yeast-like and related melanized fungi of the genus Cladosporium, different species within the anamorphic Aspergillus and Penicillium, and the teleomorphic Emericella and Eurotium, certain species of non-melanized yeasts, and Wallemia spp. Until the discovery and description of indigenous saltern mycobiota, the physiological and molecular mechanisms related to salt tolerance in eukaryotic microorganisms were studied using salt-sensitive model organisms. The most studied eukaryotic microorganism was Saccharomyces cerevisiae, which cannot adapt to hypersaline conditions. Species like Debaryomyces hansenii, Aureobasidum pullulans, Hortaea werneckii and Wallemia ichthyophaga, which have now been isolated globally from natural hypersaline environments, represent more suitable model organisms for the study of halotolerance in eukaryotes. Such studies in these species, and particularly with the extremely halotolerant H. werneckii and obligately halophilic W. ichthyophaga have continued to unravel the different strategies that these microorganisms can use to cope with the problems of ion toxicity and low water activity. The focus of this review is to present the main species of fungi inhabiting solar salterns around the world and the most suitable model fungi to study adaptations to life at high salinity.