## 4 Conclusions  
We have shown that _A. sydowii_ displays unique physiological responses when grown under optimal and non-optimal salinity conditions. We found that 0.5 M NaCl, a salt concentration that is stressful for many other fungal species, results an optimal growing condition for _A. sydowii, a_ salinity which did not trigger a stress response, and relied on the synthesis of compatible solutes to maintain the osmotic balance. Accordingly, we observed that glycerol-3-P dehydrogenase (_gpd_) gene induction and glycerol accumulation occur only as a response to saline stress, while other osmolytes are accumulated under optimal growth NaCl concentrations. The role of the constitutive phosphorylation of Hog kinase homologs in the accumulation of polyols observed under hypoosmotic conditions remains to be elucidated. However, it indicates a different regulation of the HOG pathway in moderate halophiles. Our results contribute to understand the response of the halophilic fungus _A. sydowii_ under diverse saline environments. The establishment of 0.5 M NaCl as the optimal growth condition for this fungus is in accordance with its natural environment as a coral reef pathogen, as in the sea water NaCl concentration is around 0.6 M. This knowledge would also allow implementation of _A. sydowii_ culture strategies for potential biotechnological applications, as this fungus is able to degrade hydrocarbons and lignocellulosic materials and can be a source of halotolerant enzymes. The following are available online at [[https://www.mdpi.com/article/10.3390/jof7060414/s1](https://www.mdpi.com/article/10.3390/jof7060414/s1)]([https://www.mdpi.com/article/10.3390/jof7060414/s1](https://www.mdpi.com/article/10.3390/jof7060414/s1)), Figure S1: Chromatograms of all used standards. Concentration: 5 mg/mL each; Figure S2: Chromatograms of selected samples from all salinity conditions, Table S1: Primer information for qPCR analysis of enzyme genes involved in the synthesis of compatible solutes. Table S2: Growth kinetic parameters of _Aspergillus sydowii_ grown at different NaCl concentrations. Table S3: \(p\) values from statistics analysis performed on experiments shown in Figure 1B. Figure S3: Reconstruction of MAPK phylogeny in selected Aspergillus, including Hog1, Hog2, and MpkC genes of _A. sydowii_. Table S4: Genes encoding compatible solute synthesis enzymes in _A. nidulans_ and their homologue genes of in _A. sydowii_. Table S5: \(p\) values obtained from statistics analysis performed on experiments shown in Figure 4. Figure S4: Expression levels of _hog1_ and _hog2_ genes. Table S6: \(p\) values obtained in the statistics analysis performed on experiments shown in Figure 8. Table S7: \(p\) values obtained in the statistics analysis performed on experiments shown in Figure 9.  
Conceptualization, Y.P.-L., E.C.R.-P., J.L.F.-M. and M.d.R.S.-C.; methodology, Y.P.-L., E.C.R.-P., J.R.T.-V., M.d.R.R.-H., N.S.S., F.-P.-G., M.C., O.S., N.d.C.S., A.P., J.A. and M.d.R.S.-C.; software, Y.P.-L., E.C.R.-P., A.S.-R.; validation, Y.P.-L., E.C.R.-P. and J.R.T.-V.; formal analysis, Y.P.-L., E.C.R.-P., R.A.B.-G. and M.d.R.S.-C.; investigation, Y.P.-L., E.C.R.-P., J.R.T.-V., N.S.S., F.-P.-G., M.C., O.S. ; data curation, Y.P.-L., E.C.R.-P., J.R.T.-V., N.d.C.S. and M.d.R.S.-C.; writing-original draft preparation, Y.P.-L., E.C.R.-P., J.L.F.-M. and M.d.R.S.-C.; writing-review and editing, Y.P.-L., E.C.R.-P., R.A.B.-G., J.L.F.-M., N.S.S., F.-P.-G., M.C., N.d.C.S., O.S.J., A.P. and M.d.R.S.-C.; visualization, Y.P.-L. and M.d.R.S.-C.; supervision, M.d.R.S.-C.; project administration, R.A.B.-G., J.L.F.-M. and M.d.R.S.-C.; funding acquisition, R.A.B.-G., J.L.F.-M. and M.d.R.S.-C. All authors have read and agreed to the published version of the manuscript.