Cellular responses of Aspergillus sydowii to growth at extreme chaotropic concentrations of MgCl₂

Studies on microbial adaptations to salt stress, particularly in fungi, have focused mainly on high concentrations of NaCl. The effects of other inorganic salts on biological systems are much less well known. This study investigates the effects of MgCl₂ (0-2.0 M), a chaotropic salt, on the halophilic fungus Aspergillus sydowii (EXF-12860) and compares these results with those obtained with the kosmotropic NaCl. While A. sydowii thrived at MgCl₂ concentrations previously considered lethal (up to 2.0 M), differences in morphological, physiological, metabolic, and molecular responses were observed. At 0.5-1.0 M, growth rates were similar under both salts, but higher MgCl₂ concentrations (1.5-2.0 M) significantly inhibited growth, reduced hyphal elongation, and decreased cell density. During growth in the presence of NaCl, a wide range of carbon sources was utilized, but high MgCl₂ interfered with substrate metabolism and uncoupled growth from metabolic activity. Both salts induced the production of compatible solutes (glycerol, erythritol), with higher accumulation under MgCl₂. Oxidative stress responses were also stronger under MgCl₂, including increased catalase and glutathione peroxidase activity. Transcriptomic analyses revealed significant changes in gene expression under MgCl₂ stress, with upregulation of ion transport, cell wall remodelling, glycerol biosynthesis, and oxidative defense pathways. In contrast, NaCl triggered responses focused on osmotic balance and maintenance of cell integrity. These findings emphasize the need for a deeper investigation of microbial tolerance mechanisms in chaotropic environments.