| The objective of my research was to examine the physiological impact of hypoxia on the estuarine fish Leiostomus xanthurus (spot) in order to develop a causal understanding of the relationship between hypoxia and cellular level responses of exposed organisms. The research was conducted in three parts: (1) an examination of the response of antioxidant and metabolic enzymes to hypoxia; (2) an examination of the response of antioxidant enzymes to hypoxic preconditioning, periods of sub-lethal hypoxic exposure followed by reoxygenation; and (3) an examination of the response of antioxidant enzymes to a subsequent hypoxic challenge after hypoxic preconditioning. Specifically, the antioxidant enzymes superoxide dismutase (SOD), catalase (CAT), and glutathione peroxidase (GPx) were assayed as well as citrate synthase (CS) and lactate dehydrogenase (LDH), as indicators of aerobic and anaerobic metabolism, respectively. All experiments were conducted within the laboratory. In response to varying degrees of oxygen saturation, SOD activity levels increased after 12 hours of exposure to 10% (∼0.8 mg/L) relative to 50% (∼4.0 mg/L) oxygen saturation. LDH activity also increased after 12 hours of exposure to 10% oxygen saturation, this time relative to 25% (2.0 mg/L) oxygen saturation. A single, variable duration exposure to hypoxic preconditioning was shown to increase activity levels of SOD and decrease activity levels of CAT immediately following exposure. Activity levels continued to steadily change over the normoxic recovery period, with the degree of change proportional to the duration of the hypoxic preconditioning. Hypoxic preconditioning, applied using a cyclic protocol, did not have an immediate impact on activity levels of antioxidant enzymes. When preconditioned fish were subsequently exposed to a hypoxic challenge, 12 of the 16 significant comparisons (75%) resulted in preconditioned fish showing reduced activity levels when compared to non-preconditioned fish. This suggests that preconditioning in spot could protect against subsequent hypoxic exposure by reducing the production of reactive oxygen species, rather than by increasing activity of the antioxidant enzymes. Finally, the high level of inter-individual variation observed in these fish does not appear to be linked to prior exposure history; instead it may reflect variable genetic expression between individuals. |
