Effects of Hyposalinity on Osmoregulation, Oxidative Stress, and Microbial Disruption in Chromis notata (Temminck & Schlegel, 1843).

Climate change causes substantial alterations in marine environments, including salinity reduction due to glacial melting, increased rainfall, and freshwater influx, which impose stress on marine organisms. Hypoosmotic stress leads to increased production of reactive oxygen species, thereby disrupting physiological processes, such as osmoregulation, oxidative responses, and gut microbial stability, in marine fish. Here, we investigated the responses of Chromis notata, a stenohaline damselfish, exposed to hyposaline conditions (27 and 20 psu), to better understand the effects of hyposalinity on osmoregulation, oxidative stress, and gut microbiota. Plasma osmolality was measured alongside Na/K-ATPase (NKA) activity in gill tissue to assess osmoregulatory changes. The plasma levels of hydrogen peroxide (H₂O₂) and lipid peroxidation (LPO) levels were measured as oxidative stress markers. Furthermore, 16S rRNA sequencing and RNA sequencing were conducted to analyze gut microbial diversity and transcriptomic responses, respectively. Plasma osmolality and NKA activity markedly decreased, whereas H₂O₂ and LPO levels remarkably increased under low-salinity conditions. The gut microbiome in the low-salinity groups exhibited decreased α-diversity and increased abundance of Proteobacteria, including pathogenic genera, whereas Lactobacillus abundance was reduced. Upregulated genes were associated with immune and inflammatory responses, including complement activation, and salt transmembrane transporter activity, whereas downregulated genes were linked to the lateral plasma membrane and mitochondrial membrane. These findings suggest that hyposaline induces oxidative stress and disrupts gut microbiome stability in C. notata, thereby triggering complex physiological and molecular responses. These findings provide insights into the challenges encountered by marine fish in coastal and oceanic ecosystems due to climate change.