Temperature-dependent physiological responses of the marine medaka oryzias melastigma: Implication on pollutant responses at thermal extremes

Abstract

According to the latest report of Intergovernmental Panel on Climate Change (IPCC), there will be increasing incidence of extreme temperature events worldwide due to the anthropogenically driven climate change. Most aquatic animals are ectothermic and their metabolism is highly temperature-dependent. At both high and low temperature extremes, aquatic ectotherms are expected to face a mismatch in energy demand, and eventually become more vulnerable to other stressors such as chemical pollutants. This study aims to gain better understanding on the effects of temperature on the marine medaka fish Oryzias melastigma through measurement of oxygen consumption rate (OCR) and heat shock proteins (HSPs) expression. Thermal acclimation was conducted in both acute and chronic stepwise manners; the experimental temperature was either increased or deceased by 1°C/10 min and 2°C/day, respectively. The results showed that OCR was relative low from 26 to 30 °C and gradually increased and peaked around 24°C and 36-39°C. Their OCR reduced when the temperature was further increased or decreased. At the thermal extremes, the fish might have switched to anaerobic respiration for compensating the mismatched energy demand. Also, a significant up-regulation was recorded at 10, 15, 32 and 38°C for HSP 70 and at 10 and 38°C for HSP 90. The over expression of these molecular chaperones represents a stress response to protecting the cellular structure and function at the thermal extremes. It is deduced that overall energy reserve will be depleted at such stressful conditions because the animal needs energy to synthesize functional proteins such as HSPs. Given that detoxification is also energy demanding, further exposure to chemical toxicants at the thermal extremes probably worsens the situation and hence increases the chemical toxicity to the fish. In this presentation, we will share our latest findings about the combined effect of thermal extremes and selected pollutants on O. melastigma.