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

自1980年代以来，珊瑚白化的频率、严重程度和地理范围都急剧增加，预计这一趋势将继续下去，对热带地区造成重大的环境和经济影响。这种漂白-珊瑚失去了它们的光合作用内共生甲藻（zooxanthemum; Symbiodinium spp.）- 涉及由高温和高光强度的组合效应引起的增加的氧化应激。虽然珊瑚和浮游植物在白天产生活性氧（ROS）是常规的，但在环境温度相对较小的变化下，虫黄藻抗氧化防御的失败变得灾难性，因为珊瑚礁的生活接近其上限温度。这种失败的机制尚不清楚，但分为两类：（1）温度/辐照条件超出了珊瑚及其内共生体的热适应能力，或（2）营养缺乏阻碍了虫黄藻抗氧化防御的必要增强。 在这个项目中，工作的假设是，低环境浓度的溶解铁，锌，铜，也许锰（铁，锌，铜，锰）在贫营养的热带表面沃茨，结合调节金属供应zooxanthropium的珊瑚宿主，限制补偿性升高的金属依赖的抗氧化酶与ROS的生产上升，这种资源限制有助于珊瑚漂白。这一假设将分三个阶段进行研究：与纯克隆培养的虫黄藻分离株;在珊瑚菌落培养实验;和在澳大利亚大堡礁地区的样本，观察到敏感或耐珊瑚漂白。纯培养实验的主要目标是：1）确定哪些已知金属参与抗氧化酶铁、铜、锌、锰等金属元素在虫黄藻中起重要作用; 2）确定了虫黄藻的金属营养阈值（在供应和细胞内金属配额两者中）低于该值时，在动物黄嘌呤中发生不受控制的氧化应激，以及3）确定这些关系在漂白敏感和不敏感的共生藻物种之间是否显著不同。除了验证珊瑚/藻类共生的研究结果外，珊瑚实验还将用于确定漂白指标的时间和幅度是否随金属营养而变化，以及对漂白敏感的珊瑚是否可以通过增加其金属配额而变得更有抵抗力。微量金属和抗氧化酶之间的联系是建立在其他生物系统，但尚未在珊瑚/虫黄藻协会检查。拟议的工作汇集了痕量金属/浮游植物相互作用，浮游植物光生理学和氧化应激，珊瑚礁中的光氧化防御以及海洋共生体的分子生物学方面的专家，以提供珊瑚漂白的机制理解，增加对珊瑚漂白全球趋势的预测见解。该项目将支持两名博士的教育和研究培训。这些学生将测试作为论文一部分的工作中不可或缺的假设。两名博士后科学家将参与该项目的规划、管理和研究，为他们提供完善职业发展和职业成功所需的指导技能的机会。计划举办关于“珊瑚和全球气候变化”的公开讲座。研究结果将提供影响漂白事件严重程度的因素的见解，并可能提出现实的缓解策略，以尽量减少漂白在本地化的环境或经济敏感地区。

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.