## 1 Introduction  
Halophilic and halotolerant microorganisms, which thrive in saline environments, adapt strategies to cope with high concentrations of sodium chloride. The high ionic force in these environments provokes the inactivation of proteins and the osmotic pressure causeswater loss from non-adapted cells, while the activation of stress-response pathways increases the levels of reactive oxygen species (ROS) [1; 2]. Salt-adapted cells actively extrude metal cations to avoid toxicity and prevent water efflux by the accumulation of osmolytes, some of which also possesses ROS scavenging activity, among other mechanisms [1; 2; 3]. Osmolytes or compatible solutes are low molecular weight molecules that cells accumulate in high concentration to maintain osmotic balance [4; 5; 6; 7]. Halophilic bacteria use glycine, betaine, and glutamate as the main osmolytes [8] whereas fungi preferably accumulate trehalose and poly-alcohols such as mannitol, glycerol, arabitol, and sorbitol [9; 10; 11]. The main pathway that regulates the production of osmolytes and other salinity responses in yeasts and filamentous fungi is the High Osmolarity Glycerol (HOG) signaling pathway [12; 13; 14]. This is a MAPK signal transduction cascade that leads to differential gene expression driven by Hog due to the phosphorylation of target transcription factors [12; 13; 15]. For example, the synthesis of osmolytes such as glycerol, arabitol, and mannitol is controlled by transcriptional regulation of the genes _gpd_, _ardth_, _mpdh_, and post-translational regulation of the activity of the proteins through phosphorylation [16; 14; 17; 18]. Cellular responses in hyperosmotic media are well known in the halotolerant assprocetous yeasts _S. cerevisiae_[19; 20; 21; 22] and _Debaryomyces hansenii_[23; 24; 25; 26] and in the halotolerant ascomycetous filamentous fungus _Apergillus nidulans_[27]. In all these cases, the presence of salt entails stress responses that are superimposed with adaptation mechanisms to tolerate high salinity. _Hortaea verneckii_, an extreme halotolerant black yeast, and _Wallemia ichthyophaga_, an obligate halophile, have recently emerged as models to study hyper-salinity adaptations in Basidiomycetes [10; 23]. Osmolytes signatures, signaling pathways activation, cell wall adaptations, ionic balance, and membrane transporters have been studied in these models. However, previous studies in these and other halophilic fungi have been generally conducted in the minimum and maximum tolerated salinities [9; 10; 28]. Under these conditions, as in the studies of halotolerant fungi, the independent contribution of stress and salinity to physiological responses are difficult to discriminate. We have recently shown that in the case of the halophilic ascomycetous model _A. sydowii_, the physiological responses to salinity vary if the fungus is under additional salinity stress [29]. In this strain, hyperosmotic conditions (2.0 M NaCl) induce the transcriptional regulation of cell wall reorganization, membrane cation transporters, hydrophbin production, and glycerol synthesis. However, such regulatory mechanisms were not observed at a salt concentration (0.5 M NaCl) that is optimal for the growth of this fungus [29]. Thus, we hypothesize that, at the optimal salinity, the fungus accumulates osmolytes other than glycerol, regulates ionic balance, and maintains low oxidative stress levels by mechanisms that do not require a steady transcriptional change. The aim of the present work was to analyze the osmolyte signatures in salt-adapted _A. sydowii_ cells and their dynamic changes after hypoosmotic and hyperosmotic shocks. In this report we examined osmolyte accumulation, transcriptional regulation of key enzymes involved in their synthesis, the activation of HOG signaling pathway, and the sodium and potassium ion balance in _A. sydowii_ under hyperosmotic, hypoosmotic and optimal salinity conditions. Oxidative stress markers and antioxidant responses were also investigated to assert the level of stress in the evaluated conditions.