Yeasts in hypersaline habitats

Abstract

For a long time, halotolerant yeasts were known exclusively as contaminants of food preserved with high concentrations of salt or sugar. Their presence in natural thalassohaline hypersaline environments was unknown until 2000, when they were first reported to be active inhabitants of man-made solar salterns in Slovenia. Since then, they have been described on the surface of halophytic plants, in salt mines, in cold and temperate saline lakes, in brine and bittern of different solar salterns on three continents and in MgCl2-dominated waters of the Dead Sea. Yeasts in these environments can be described as halotolerant, extremely halotolerant and even extremely chaotolerant. The dominant representatives are different ascomycetous black yeast species, mainly of the genera Hortaea and Phaeotheca; non-melanised ascomycetous yeasts from the genera Candida, Debaryomyces, Meyerozyma, Metschnikowia, Pichia and Yarrowia; and basidiomycetous yeasts from the genera Bulleromyces, Cryptococcus, Cutaneotrichosporon, Papiliotrema, Rhodosporidium, Rhodotorula, Solicoccozyma, Sterigmatomyces and Vishniacozyma. Until the discovery and description of indigenous saltern mycobiota, the physiological and molecular mechanisms relating to salt tolerance in eukaryotic microorganisms were studied using salt-sensitive Saccharomyces cerevisiae as the model organism. Nowadays, most studies focus on halotolerant yeast species like Debaryomyces hansenii, Aureobasidium pullulans and Hortaea werneckii, which have been isolated globally from natural hypersaline environments and can tolerate up to 10%, 15% and 30% NaCl, respectively. Studies of halotolerant yeasts at the molecular level continue to unravel the complexity of the adaptations needed for yeasts to cope with the problems of ion toxicity and low water activity that are characteristic of hypersaline environments.