Differential hypo-osmotic stress responses and regulatory mechanisms of <i>Aspergillus sydowii</i> in amphipod guts and hadal sediments

Hadal amphipods, which play a vital role in deep-sea ecosystems, harbor gut microbiota that significantly influence host physiology and environmental adaptation. However, understanding of the culturable gut fungi remains limited, particularly their response to the deep-sea osmotic environment. Here, this study reports the successful isolation of <i>Aspergillus sydowii</i> XTO612 from the gut of <i>Hysterocrates gigas</i>. Comparative physiological profiling with <i>A. sydowii</i> DM1, originating from hadal sediment, revealed significant interspecific divergence in hypo-osmotic stress responses (0.1 M NaCl). Low osmolarity conditions were applied as an environmental stressor, and alterations in secondary metabolites, metabolic activity, micromorphology, and reactive oxygen species were observed in the two <i>A. sydowii</i> strains under varying osmotic pressures. The results showed enhanced stress responsiveness in <i>A. sydowii</i> XTO612. Transcriptomic analyses under hypo-osmotic conditions revealed comprehensive regulatory strategies in <i>A. sydowii</i> XTO612, including modulation of membrane permeability, cell wall restructuring, energy metabolism, and osmolyte biosynthesis pathways to optimize osmotic homeostasis. The divergent osmoregulatory strategies in two conspecific marine fungal strains under identical conditions reveal habitat-driven evolution of distinct osmotic regulation mechanisms, highlighting hypo-osmotic stress as a key factor in shaping natural product biosynthesis.IMPORTANCEHadal amphipods play crucial roles in deep-sea ecosystems, yet their gut fungi remain unexplored, representing a major gap in understanding microbial response strategy in extreme environments. While most studies focus on high-osmotic stress, we reveal the unique hypo-osmotic regulatory mechanisms of <i>Aspergillus sydowii</i> XTO612, a novel gut-derived strain from hadal amphipods. Comparative analyses demonstrate distinct stress responses, including cell wall remodeling, metabolic reprogramming, and osmolyte biosynthesis, highlighting habitat-driven evolutionary divergence. This study significantly broadens our understanding of fungal osmoregulation, providing groundbreaking insights into fungal regulation under low-osmotic conditions, with potential implications for biotechnology and microbial response strategies in the deep sea.