## 3 Results and Discussion
### Stress Induces Changes in the Redox State of _A. yydowii_  
Fungi and plants are prone to generate Reactive Oxygen Species (ROS) in the presence of a high salt concentration [125, 126, 127, 128, 129]. There is a causal link between salinity and potential oxidative stress, which is the result of the unbalanced generation vs. scavenging of ROS [128, 129, 130]. Whereas ROS are generated as normal by-products of aerobic metabolism, their accumulation alters the balance between oxidized and reduced glutathione. High ROS levels can damage macromolecules such as lipids, proteins, and nucleic acids, which can lead to cell death [131], although there is also evidence that ROS are signaling molecules that can regulate growth and cell differentiation [132, 133]. However, to our knowledge, there is no evidence linking ROS levels and halophile fungi adaptation to extreme salinities. Therefore, we investigated oxidative and antioxidant responses of _A. sydowii_ (lipoperoxidation, protein oxidation, GSH and SOD activity levels), under different salinity conditions. Hydroperoxides were not detected under our experimental conditions, perhaps because cells were already adapted at the tested times. As shown in Figure 8, SOD activity (percentage) and GSH concentration were higher when the fungus was growing in 2.0 M of NaCl reaching 11.2% on day 11 and 290.6 \(\upmu\)g/mL on day 5, respectively (Figure 8). On day 11, SOD activity increased significantly (7.4%) when the fungus was growing without salt as compared with the optimal growing condition (0.5 M NaCl). Meanwhile, GSH was accumulated significantly at 2.0 M NaCl, with the lowest accumulation in the optimal growth condition (115-145 \(\upmu\)g/mL) (Figure 8). The protein advanced oxidation products (PAOPs) were accumulated only in the hypersaline condition (2.0 M NaCl) reaching up to 101.2 \(\upmu\)M on day 5, suggesting that this is the most stressful condition for _A. sydowii_. Malondialdehydedehyde (MDA) accumulated at similar levels in all conditions by day 5 and 11. However, on day 7 there was an inverse correlation between MDA levels and the presence of salt. MDA is a byproduct of polyunsaturated fatty acid peroxidation mediated by free radicals or by lipoxygenases [134; 135]. The heat map in Figure 8 shows that antioxidant mechanisms were reduced under optimal salinity as compared to hypoosmotic and hyperosmotic conditions. These results indicate that non-optimal salinity growth conditions (0 and 2.0 M NaCl) favor a higher production of ROS, in contrast to what is observed under optimal growth conditions (0.5 M NaCl). This is consistent with the detection of high levels of both antioxidants and oxidative markers in salt-tolerant bacterial isolates from a saline lake cultured in a media with 20% (3.4 M) NaCl and also with the resistance of _H. werneckii_ to oxidative stress, which show a correlation between salinity and resistance to oxidative stress [136]. Figure 7: Regulation of intracellular K\({}^{+}\)/Na\({}^{+}\) ratio in response to salinity in _A. sydowii_ cells. Dh: _Debaryomyces hansenii_ was used as a positive control. Data are means \(\pm\) SD. Statistical significance was determined using the Holm-Sidak method, with alpha = 0.05. Each row was analyzed individually, without assuming a consistent SD. (\({}^{*}\)_p_ < 0.05, \({}^{**}\)_p_ < 0.01, \({}^{***}\)_p_ < 0.0001). Figure 9 shows _A. sydowii_ dynamic response of oxidative markers and antioxidants when the fungus was challenged by osmotic stress. The fungus was grown for 7 days in MMG without salt, 0.5 M or 2.0 M of NaCl, then shifted to the other conditions and  
Figure 8: Oxidative stress and antioxidant markers evaluated in _A. sydowii_ cultures under stable salinity conditions. Superoxide dismutase (SOD) and glutathione (GSH) (antioxidants markers; green in the heat maps), malondialdehydehyde (MDA) and protein advance oxidation products (PAOP) (oxidative damage markers; brown in the heat maps) were analyzed. Data are means \(\pm\) SD.