Effects of Trace Metal Limitation on Oxidative Stress in Zooxanthellae and Its Role in Coral Bleaching

Coral bleaching has increased dramatically in frequency, severity, and geographic extent since the 1980s and this trend is anticipated to continue, causing major environmental and economic impacts in tropical regions. This bleaching - the loss by corals of their photosynthetic endosymbiotic dinoflagellates (zooxanthellae; Symbiodinium spp.) - involves increased oxidative stress arising from the combined effects of elevated temperature at high light intensities. Although the production of reactive oxygen species (ROS) in corals and phytoplankton is routine during daylight hours, the failure of antioxidant defenses in zooxanthellae becomes catastrophic under comparatively small changes in environmental temperature, because reef corals live close to their upper thermal limits. The mechanisms underlying this failure are not understood, but fall into two categories: (1) the temperature/irradiance conditions lie beyond the capacity for thermal acclimatization by corals and their endo-symbionts, or (2) the necessary enhancement of antioxidant defenses in zooxanthellae is hindered by nutrient deficiencies. In this project, the working hypothesis is that low ambient concentrations of dissolved iron, zinc, copper and perhaps manganese (Fe, Zn, Cu, and Mn) in oligotrophic tropical surface waters, combined with regulation of metal supply to zooxanthellae by the coral host, restrict the compensatory elevation of metal-dependent antioxidant enzymes with rising ROS production, and this resource limitation contributes to coral bleaching. This hypothesis will be investigated in three stages: with pure clonal cultures of zooxanthella isolates; in coral colony culture experiments; and in samples on areas of the Great Barrier Reef, Australia, observed to be susceptible or resistant to coral bleaching. The primary goals of the pure culture experiments are to 1) identify which of the known metals involved in antioxidant enzymes (Fe, Cu, Zn, Mn) are important in zooxanthellae, 2) determine the thresholds of metal nutrition (both in supply and intracellular metal quotas) below which onset of uncontrolled oxidative stress occurs in the zooxanthellae, and 3) ascertain whether these relationships differ significantly among bleaching sensitive and insensitive Symbiodinium species. In addition to verifying the findings in coral/algal symbioses, coral experiments will be used to determine whether the timing and magnitude of bleaching indicators change with metal nutrition, and whether bleaching-sensitive corals can become more resistant by increasing their metal quotas. The linkage between trace metals and antioxidant enzymes is well established in other biological systems but has not been examined in coral/zooxanthellar associations. The proposed work brings together experts in trace metal/ phytoplankton interactions, phytoplankton photo-physiology and oxidative stress, photo-oxidative defenses in reef corals, and molecular biology of marine symbioses to provide mechanistic understanding of coral bleaching, increasing predictive insights to the global trend of coral bleaching. This project will support the education and research training of two Ph.D. students who would test hypotheses integral to the work as parts of their dissertations. Two postdoctoral scientists will participate in the planning, management, and research of the project, providing opportunities to refine their professional development and their mentoring skills necessary for career success. Public lectures on corals and global climate change are planned. The findings will provide insights to the factors influencing the severity of bleaching events, and may suggest realistic mitigation strategies to minimize bleaching in localized environmentally or economically sensitive regions