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。这种白化——珊瑚失去了光合作用的内共生鞭毛藻（虫黄藻；共生藻属）——包括在高光强下温度升高的综合影响下引起的氧化应激增加。虽然珊瑚和浮游植物在白天产生活性氧（ROS）是常规的，但在相对较小的环境温度变化下，虫黄藻抗氧化防御的失败会成为灾难性的，因为礁珊瑚生活在接近其上限的温度下。这种失败的机制尚不清楚，但可以分为两类：(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)虫黄藻中必要的抗氧化防御能力的增强受到营养缺乏的阻碍。在本项目中，工作假设是，低环境浓度的溶解铁、锌、铜和锰（Fe、Zn、Cu和Mn）在低营养热带地表水中，再加上珊瑚宿主对虫黄藻金属供应的调节，限制了金属依赖性抗氧化酶的补偿性升高，同时增加了活性氧的产生，这种资源限制导致了珊瑚白化。这一假设将分三个阶段进行研究：畜黄藻分离物的纯克隆培养；在珊瑚培养实验中；在澳大利亚大堡礁地区的样本中，观察到它们易受或抵抗珊瑚白化。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（Fe, Cu, Zn, Mn）在虫黄藻中是重要的；2)确定金属营养的阈值（包括供应和细胞内金属配额），低于这些阈值，虫黄藻就会发生不受控制的氧化应激；3)确定这些关系在漂白敏感和不敏感的共生菌物种之间是否有显著差异。除了验证珊瑚/藻类共生的研究结果外，珊瑚实验将用于确定白化指标的时间和程度是否随金属营养而改变，以及对白化敏感的珊瑚是否可以通过增加金属配额来增强抵抗力。微量金属和抗氧化酶之间的联系在其他生物系统中已经得到了很好的证实，但在珊瑚/虫黄体的关联中尚未得到检验。建议的工作汇集了微量金属/浮游植物相互作用、浮游植物光生理和氧化应激、珊瑚礁珊瑚的光氧化防御以及海洋共生分子生物学方面的专家，以提供对珊瑚白化的机制理解，增加对全球珊瑚白化趋势的预测性见解。该项目将支持两名博士生的教育和研究培训，他们将检验与工作相关的假设，作为其论文的一部分。两名博士后科学家将参与该项目的规划、管理和研究，为他们提供完善专业发展和职业成功所需的指导技能的机会。计划举办“珊瑚与全球气候变化”的公开讲座。这些发现将为影响白化事件严重程度的因素提供见解，并可能提出现实的缓解策略，以尽量减少局部环境或经济敏感地区的白化。

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.