Physiological responses to cold stress in the gills of discus fish (Symphysodon aequifasciatus) revealed by conventional biochemical assays and GC-TOF-MS metabolomics.

Discus fish (Symphysodon aequifasciatus) is a cichlid that is among the most popular fish for warm-water aquaria and also frequently used as the model animal for environmental science. However, little is known about the responses of S. aequifasciatus to low temperatures caused by environmental variation. Here, by using conventional biochemical assays and gas chromatography time-of-flight mass spectrometry metabolomics, we investigated the physiological responses of S. aequifasciatus gills exposed for 30 days to two temperature regimes: 28 °C and 20 °C. Low temperature resulted in elevated production of reactive oxygen species but not increased malondialdehyde. This might be partially related to protective responses in the antioxidant system, revealed by increased activities of superoxide dismutase and glutathione peroxidase, and level of reduced glutathione (GSH), compensating for the depletion of catalase activity. A total of 35 metabolites were identified as potential biomarkers of cold stress, showing the most influenced pathways including starch and sucrose metabolism, pentose phosphate pathway, glycerolipid metabolism, sphingolipid metabolism, glutathione metabolism, and arginine and proline metabolism. Moreover, the activation of glutathione metabolism agreed with the increased GSH level detected by biochemical assays. Overall, the results of this study suggest that low temperature can activate a protective antioxidant defence response and modify the metabolic pathways in gills of S. aequifasciatus, providing insights into the physiological regulation in response to cold stress in this tropical fish.