Morphological Response of the Halophilic Fungal Genus Wallemia to High Salinity

Abstract

The basidiomycetous genus Wallemia is an active inhabitant of hypersaline environments, and it has recently been described as comprising three halophilic and xerophilic species: Wallemia ichthyophaga, Wallemia muriae, and Wallemia sebi. Considering the important protective role the fungal cell wall has under fluctuating physicochemical environments, this study was focused on cell morphology changes, with particular emphasis on the structure of the cell wall, when these fungi were grown in media with low and high salinities. We compared the influence of salinity on the morphological characteristics of Wallemia spp. by light, transmission, and focused-ion-beam/scanning electron microscopy. W. ichthyophaga was the only species of this genus that was metabolically active at saturated NaCl concentrations. W. ichthyophaga grew in multicellular clumps and adapted to the high salinity with a significant increase in cell wall thickness. The other two species, W. muriae and W. sebi, also demonstrated adaptive responses to the high NaCl concentration, showing in particular an increased size of mycelial pellets at the high salinities, with an increase in cell wall thickness that was less pronounced. The comparison of all three of the Wallemia spp. supports previous findings relating to the extremely halophilic character of the phylogenetically distant W. ichthyophaga and demonstrates that, through morphological adaptations, the eukaryotic Wallemia spp. are representative of eukaryotic organisms that have successfully adapted to life in extremely saline environments.

Osmoadaptation Strategy of the Most Halophilic Fungus, Wallemia ichthyophaga, Growing Optimally at Salinities above 15% NaCl

“…A previous study showed that the relatively thick cell wall of W. ichthyophaga was almost 3-fold thicker at high salinity (7). Determination of the cell wall content in the dry biomass of W. ichthyophaga at various salinities revealed that the dry biomass contained 26% cell wall at the lowest salinity, which is in accordance with the measurements from literature (e.g., in S. cerevisiae, the cell wall makes up from 15 to 30% of the dry weight of the cell, at most [41]).…”

“…Recently, it was shown that no other basidiomycetous yeast was able to grow below an a w of 0.9, adjusted either with NaCl or with sorbitol (6). Furthermore, W. ichthyophaga shows a preference for certain solutes that lower the a w : it grows better on media with high concentrations of NaCl than on media with high concentrations of glucose (7,8). This and the fact that it can thrive only in media with NaCl contents above 10% (wt/vol) and up to saturation (32% NaCl) make W. ichthyophaga the most halophilic fungus known to date (5,7,9).…”

“…A recent study of W. ichthyophaga has revealed specific morphological adaptations to high NaCl concentrations, such as a 3-fold thickening of the cell wall and an almost 4-fold increase in the size of multicellular clumps compared to growth at low salinity. These morphological phenomena are believed to have an important role in successful growth under extremely saline conditions (7). Furthermore, analysis of the W. ichthyophaga genome sequence revealed a significant enrichment of salt-responsive genes coding for hydrophobins (4), small cell wall proteins involved in diverse cellular functions, such as modification of the movement of solutes across the cell wall, and giving strength and rigidity to the cell wall (27,28).…”

“…Recently, a few morphological, as well as molecular-biological, studies of W. ichthyophaga were published (4,7,8,22,23). In liquid saline media, W. ichthyophaga grows meristematically.…”

“…This could be of great importance in hypersaline environments (4). In addition, W. ichthyophaga cells are abundantly covered with extracellular polysaccharides (7), which prevent rock-inhabiting fungi from desiccation (29) and might also have a protective function in W. ichthyophaga at high salinity (7). In addition to this, some molecular mechanisms of halotolerance in W. ichthyophaga, such as the adaptation of a gene crucial for glycerol production (GPD1 [23]), an insight into the HOG pathway by the study of Hog1 mitogenactivated protein (MAP) kinase (22), and also genome and transcriptome sequence analysis (4), have been uncovered recently.…”