Abstract
It is well known that few halophilic bacteria and archaea as well as certain fungi can grow at the highest concentrations of NaCl. However, data about possible life at extremely high concentrations of various others kosmotropic (stabilizing; like NaCl, KCl, and MgSO4) and chaotropic (destabilizing) salts (NaBr, MgCl2, and CaCl2) are scarce for prokaryotes and almost absent for the eukaryotic domain including fungi. Fungi from diverse (extreme) environments were tested for their ability to grow at the highest concentrations of kosmotropic and chaotropic salts ever recorded to support life. The majority of fungi showed preference for relatively high concentrations of kosmotropes. However, our study revealed the outstanding tolerance of several fungi to high concentrations of MgCl2 (up to 2.1 M) or CaCl2 (up to 2.0 M) without compensating kosmotropic salts. Few species, for instance Hortaea werneckii, Eurotium amstelodami, Eurotium chevalieri and Wallemia ichthyophaga, are able to thrive in media with the highest salinities of all salts (except for CaCl2 in the case of W. ichthyophaga). The upper concentration of MgCl2 to support fungal life in the absence of kosmotropes (2.1 M) is much higher than previously determined to be the upper limit for microbial growth (1.26 M). No fungal representatives showed exclusive preference for only chaotropic salts (being obligate chaophiles). Nevertheless, our study expands the knowledge of possible active life by a diverse set of fungi in biologically detrimental chaotropic environments.
Highlights
Water is essential to life, and life can only exist within a narrow range of water availability in a particular environment, expressed as water activity
The most halophilic fungus known to date is Wallemia ichthyophaga as it requires at least 10% NaCl and grows in solutions saturated with NaCl (Zalar et al, 2005; Zajc et al, 2014)
We have tested growth of these strains on salts that act as kosmotropes (NaCl, KCl, and MgSO4) and chaotropes (CaCl2, MgCl2, and NaBr) that are present in these hypersaline environments
Summary
Water is essential to life, and life can only exist within a narrow range of water availability in a particular environment, expressed as water activity (aw). The lowest aw known to support life is 0.61, measured for the xerophilic fungus Xeromyces bisporus grown on sugar-based media (Pitt and Hocking, 2009), and for some halophilic Archaea and Bacteria (Stevenson et al, 2014). Many fungi are able to thrive at low aw, especially the numerous xerophilic filamentous fungi and osmophilic yeasts that grow on drying foods or on foods with high concentrations of sugars (Pitt and Hocking, 1977, 2009). The most halophilic fungus known to date is Wallemia ichthyophaga as it requires at least 10% NaCl and grows in solutions saturated with NaCl (Zalar et al, 2005; Zajc et al, 2014)
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