Thermal Stress and Antioxidant Activity in Sea Anemones, Exaiptasia pallida

Abstract

When exposed to stress, coral reefs undergo bleaching, a process in which the host‐symbiont relationship is disrupted. Stressors can be environmental including ocean acidification, exposure to ultraviolet radiation, or rising ocean temperatures due to global warming. The sea anemone, Exaiptasia pallida, serves as a model organism of coral reef biology since both species share symbiotic relationships with their algal symbionts and can undergo bleaching resulting in the expulsion of algal symbionts. This is the ultimate phenotypic result of high levels of stress experienced by the coral reef. Stress factors can lead to an increase in free radicals, like reactive oxygen species (ROS), that will adversely alter lipids, proteins, and DNA and trigger bleaching events. Although oxygen free radicals are natural by‐products of metabolic processes in most organisms, they are closely regulated through endogenous systems, such as antioxidants like superoxide dismutase (SOD). We hypothesized that as temperatures increase, the levels of SOD activity in the sea anemone will increase over time. We used commercially available E. pallida to establish a ‘stock’ aquaria to rear anemones under optimal conditions for at least two weeks before use. The stock aquaria were maintained at 28‐30% salinity at 22℃ during a 12‐hour light/dark period, at a pH of approximately 8.4, with low levels nitrate, nitrite, and ammonia (checked weekly). E. pallida were moved from the stock tank and placed into one of three identical tanks, maintained at increasing temperatures of 22℃ (control), 24℃, and 29℃. We sampled 5 anemones from each tank at two‐week intervals for 8 weeks. Seawater was removed by aspiration, anemones were weighed, flash‐frozen in liquid nitrogen, and stored at ‐80℃. We extracted proteins from each sample by homogenization on ice. For each sample, total protein concentrations were established with a Bradford assay and SOD concentration in units of activity was established with a commercially available SOD assay. By observation, the anemones reared at in the higher range of thermal stress appeared smaller and individuals tended to die faster during the trial. E. pallida reared under increased temperatures of 24℃ and 29℃ showed an increase in SOD activity from week 6 to week 8 compared to the control tank at 22℃. These data are based on two replicates and further trials are currently in progress. Our findings describe the outcome of elevated temperature on levels of metabolism and increased levels of superoxide dismutase activity as a byproduct of stress. In addition, disruption of a host‐symbiont relationship was suggested by the decrease in size and death of anemones at higher temperatures. Additional evidence is necessary to support our hypothesis and future experiments will focus on increasing the rearing time, increased temperatures, and other antioxidant defenses like xanthine oxidase or cyclooxygenases