title: Osmoadaptation strategy of the most halophilic fungus, Wallemia ichthyophaga, growing optimally at salinities above 15% NaCl.
abstract: Wallemia ichthyophaga is a fungus from the ancient basidiomycetous genus Wallemia (Wallemiales, Wallemiomycetes) that grows only at salinities between 10% (wt/vol) NaCl and saturated NaCl solution. This obligate halophily is unique among fungi. The main goal of this study was to determine the optimal salinity range for growth of the halophilic W. ichthyophaga and to unravel its osmoadaptation strategy. Our results showed that growth on solid growth media was extremely slow and resulted in small colonies. On the other hand, in the liquid batch cultures, the specific growth rates of W. ichthyophaga were higher, and the biomass production increased with increasing salinities. The optimum salinity range for growth of W. ichthyophaga was between 15 and 20% (wt/vol) NaCl. At 10% NaCl, the biomass production and the growth rate were by far the lowest among all tested salinities. Furthermore, the cell wall content in the dry biomass was extremely high at salinities above 10%. Our results also showed that glycerol was the major osmotically regulated solute, since its accumulation increased with salinity and was diminished by hypo-osmotic shock. Besides glycerol, smaller amounts of arabitol and trace amounts of mannitol were also detected. In addition, W. ichthyophaga maintained relatively small intracellular amounts of potassium and sodium at constant salinities, but during hyperosmotic shock, the amounts of both cations increased significantly. Given our results and the recent availability of the genome sequence, W. ichthyophaga should become well established as a novel model organism for studies of halophily in eukaryotes.
authors.affiliation: Department of Biology, Biotechnical Faculty, University of Ljubljana, Ljubljana, Slovenia.
authors.forename: Janja
authors.initials: J
authors.lastname: Zajc
authors.forename: Tina
authors.initials: T
authors.lastname: Kogej
authors.forename: Erwin A
authors.initials: EA
authors.lastname: Galinski
authors.forename: José
authors.initials: J
authors.lastname: Ramos
authors.forename: Nina
authors.initials: N
authors.lastname: Gunde-Cimerman
mesh_terms: D000222:Adaptation, Physiological; D001487:Basidiomycota / Q000187:drug effects* / Q000254:growth & development / Q000378:metabolism / Q000502:physiology*; D018533:Biomass; D005990:Glycerol / Q000378:metabolism; D009997:Osmotic Pressure; D012462:Saline Solution, Hypertonic; D054712:Salinity; D013312:Stress, Physiological
chemical_list: D012462:Saline Solution, Hypertonic; D005990:Glycerol