Impact of Acute and Long-Term Hypoxia and Hyperoxia on Antioxidant Metabolism and Gene Expression in Juvenile Rainbow Trout (Oncorhynchus mykiss)

Abstract

Aquatic organisms experience oxidative stress due to environmental stressors that promote oxygen exchange, such as eutrophication, algal photosynthetic activity, or changes in water temperature. This study aimed to clarify the effects of oxidative stress on the liver of juvenile rainbow trout (Oncorhynchus mykiss) exposed to hypoxia and hyperoxia by examining the physiological roles of hydrogen peroxide, glutathione (GSH) redox status, malondialdehyde (MDA) concentration, antioxidant enzyme activity, and antioxidant gene expressions. Dissolved oxygen levels were maintained at 4.0 ± 0.5 mg/L for hypoxia, 7.5 ± 0.5 mg/L for normoxia, and 12 ± 1.2 mg/L for hyperoxia. Liver samples were collected from each experimental group following exposure (6, 12, 24, 48, and 72 h) and chronic exposure (28 days). Under both hypoxia and hyperoxia conditions, reduced GSH levels and the oxidative stress index (OSI) decreased compared to normoxia (control group), whereas oxidized glutathione (GSSG) levels, the GSH/GSSG ratio, and MDA concentrations increased. Hydrogen peroxide levels were unstable. Superoxide dismutase (SOD) activity remained consistently lower than that of the control group. Catalase (CAT) activity decreased with chronic exposure to hyperoxia. Glutathione peroxidase (GPx) activity fluctuated with varying oxygen treatments. Glutathione-S-transferase (GST) activity increased after 12 h of hypoxia exposure but decreased with acute hyperoxia; however, chronic hyperoxia exposure eventually increased GST activity. Glutathione reductase (GR) activity is generally reduced under hypoxia. The expressions of SOD and CAT increased at 24 h but decreased at other times. GPx expression increased under chronic hypoxia but decreased under hyperoxia, while GST expression decreased with chronic treatments. Hypoxia and hyperoxia influence the antioxidant defense system in the fish liver through different pathways. While a coordinated relationship between gene expression and enzyme activities is observed under acute exposures, this coordination diminishes during chronic exposures, leading to the depletion of defense mechanisms. This suggests that the capacity of aquatic organisms to adapt to oxidative stress is limited and that post-transcriptional mechanisms play a significant role in regulating antioxidant responses.