Chronic low-level nutrient enrichment benefits coral thermal performance in a fore reef habitat

Global- and local-scale anthropogenic stressors have been the main drivers of coral reef decline, causing shifts in coral reef community composition and ecosystem functioning. Excess nutrient enrichment can make corals more vulnerable to ocean warming by suppressing calcification and reducing photosynthetic performance. However, in some environments, corals can exhibit higher growth rates and thermal performance in response to nutrient enrichment. In this study, we measured how chronic nutrient enrichment at low concentrations affected coral physiology, including endosymbiont and coral host response variables, and holobiont metabolic responses of Pocillopora spp. colonies in Mo'orea, French Polynesia. We experimentally enriched corals with dissolved inorganic nitrogen and phosphate for 15 months on an oligotrophic fore reef in Mo'orea. We first characterized symbiont and coral physiological traits due to enrichment and then used thermal performance curves to quantify the relationship between metabolic rates and temperature for experimentally enriched and control coral colonies. We found that endosymbiont densities and total tissue biomass were 54% and 22% higher in nutrient-enriched corals, respectively, relative to controls. Algal endosymbiont nitrogen content cell(-1) was 44% lower in enriched corals relative to the control colonies. In addition, thermal performance metrics indicated that the maximal rate of performance for gross photosynthesis was 29% higher and the rate of oxygen evolution at a reference temperature (26.8 degrees C) for gross photosynthesis was 33% higher in enriched colonies compared to the control colonies. These differences were not attributed to symbiont community composition between corals in different treatments, as C42, a symbiont type in the Cladocopium genus, was the dominant endosymbiont type found in all corals. Together, our results show that in an oligotrophic fore reef environment, nutrient enrichment can cause changes in coral endosymbiont physiology that increase the performance of the coral holobiont.

全球和局部范围的人为压力源一直是珊瑚礁衰退的主要驱动因素，导致珊瑚礁群落组成和生态系统功能发生变化。过量的营养富集可通过抑制钙化作用和降低光合性能，使珊瑚更容易受到海洋变暖的影响。然而，在某些环境中，珊瑚在营养富集的情况下可能会表现出更高的生长速率和耐热性能。在这项研究中，我们测量了低浓度的长期营养富集如何影响珊瑚的生理机能，包括内共生体和珊瑚宿主的反应变量，以及法属波利尼西亚莫雷阿岛的蔷薇珊瑚属群体的共生体代谢反应。我们在莫雷阿岛的一个贫营养的前礁上，用溶解的无机氮和磷酸盐对珊瑚进行了15个月的实验性富集。我们首先描述了因富集而产生的共生体和珊瑚的生理特征，然后利用热性能曲线来量化实验富集的珊瑚群体和对照珊瑚群体的代谢率与温度之间的关系。我们发现，与对照相比，营养富集的珊瑚内共生体密度和总组织生物量分别高出54%和22%。富集珊瑚的藻类内共生体单个细胞含氮量比对照群体低44%。此外，热性能指标表明，富集群体的总光合作用最大性能速率比对照群体高29%，在参考温度（26.8℃）下总光合作用的氧气释放速率比对照群体高33%。这些差异并非归因于不同处理的珊瑚之间的共生体群落组成，因为C42（一种属于角突藻属的共生体类型）是在所有珊瑚中发现的主要内共生体类型。总之，我们的研究结果表明，在贫营养的前礁环境中，营养富集可导致珊瑚内共生体生理机能发生变化，从而提高珊瑚共生体的性能。