Temperature-dependent shifts in gut microbiota and metabolome of olive flounder (Paralichthys olivaceus): implications for cold-water aquaculture expansion and probiotic applications.

Background: In recent years, rising temperatures due to climate change have become significant stressors in aquatic environments, impacting disease incidence, growth, and gut microbiota in fish. Cold-water species, such as the olive flounder (Paralichthys olivaceus), are particularly vulnerable to increasing water temperatures. Despite its economic importance as a species farmed in East Asia, research on temperature-dependent shifts in the gut microbiota and metabolome of olive flounder remains limited. This study investigates the effects of water temperature on the gut microbiota and metabolome of olive flounder using full-length 16 S rRNA sequencing with Oxford Nanopore Technologies and metabolomics analysis with high-resolution liquid chromatography-mass spectrometry (LC-MS). The analysis compares individuals exposed to three water temperatures (18 °C, 22 °C, and 26 °C).

Results: Temperature significantly influenced the composition of gut microbiota, with an increase in Gammaproteobacteria abundance at higher temperatures. Potential pathogens such as Vibrio and Photobacterium increased from 22 °C to 26 °C, while Pseudomonas declined, suggesting an elevated risk of pathogen infection at 26 °C. Functional predictions revealed that gut bacteria regulated host metabolism, particularly carbohydrate, amino acid, and lipid pathways. Metabolomic analysis showed reduced levels of polyunsaturated fatty acids (PUFAs) and phosphatidylcholine (PC)-related metabolites at higher temperatures. Notably, the umami flavor-related compound aspartic acid decreased, while the bitter flavor-related compound phenylalanine increased. Correlation analysis identified significant associations between bacterial genera, such as Comamonas,Pseudomonas,Sphingomonas, and Stentotrophomonas (positive correlation), and Legionella and Phaeobacter (negative correlation), with shifts in PUFAs and PC metabolites.

Conclusions: This study demonstrates that environmental temperature significantly affects the gut microbiota and muscle metabolites of olive flounder. Higher temperatures diversified gut bacterial communities and altered metabolite profiles, with reductions in PUFAs and PC-related compounds linked to specific bacterial genera. These findings highlight the potential of these bacterial genera as biomarkers or probiotics for improving aquaculture practices and environmental adaptation strategies. By establishing a strong correlation between gut microbiota and muscle metabolites, this research provides insights that could contribute to sustainable flounder farming and enhance resilience to climate change.