Niche-Specific Osmotic Stress Responses and Regulatory Mechanisms Under Hypo-Osmotic Conditions in Aspergillus sydowii Across Hadal Zone Habitats

Hadal amphipods are ubiquitous inhabitants of hadal ecosystems, where gut microbiota plays key roles in host physiology and environmental adaptation, establishing them as critical targets for investigating host-ecosystem interactions. Current research on hadal amphipod gut microbes is limited, especially for culturable fungi, with no reported fungal characterization from their digestive systems. This study reports the successful isolation of Aspergillus sydowii XTO612 from the gut of hadal amphipods. Comparative physiological profiling with A. sydowii DM1, originating from hadal sediment, revealed significant interspecific divergence in hypo-osmotic stress responses (0.1 M NaCl). Employing low-osmolarity conditions as an environmental stressor, we systematically compared osmotic adaptation mechanisms in two A. sydowii strains through a multi-parametric approach. The alterations of secondary metabolites, metabolic activity, micromorphology, and reactive oxygen species in hadal filamentous fungi under different osmotic pressures, demonstrating enhanced stress responsiveness in the A. sydowii XTO612. Transcriptomic analyses under hypo-osmotic conditions revealed comprehensive regulatory strategies in A. sydowii XTO612, including modulation of membrane permeability, cell wall restructuring, energy metabolism reprogramming, and osmolyte biosynthesis pathways to optimize osmotic homeostasis. Differences in physiological responses between two strains of the same genus shows that selective pressures in different habitats have driven marine filamentous fungi to develop unique environmental adaptation mechanisms during evolution.