Data from: New insights into carbon acquisition and exchanges within the coral-dinoflagellate symbiosis under NH4+ and NO3- supply

Anthropogenic nutrient enrichment affects the biogeochemical cycles and nutrient stoichiometry of coastal ecosystems and is often associated with coral reef decline. However, the mechanisms by which dissolved inorganic nutrients, and especially nitrogen forms (ammonium versus nitrate) can disturb the association between corals and their symbiotic algae are subject to controversial debate. Here, we investigated the coral response to varying N : P ratios, with nitrate or ammonium as a nitrogen source. We showed significant differences in the carbon acquisition by the symbionts and its allocation within the symbiosis according to nutrient abundance, type and stoichiometry. In particular, under low phosphate concentration (0.05 µM), a 3 µM nitrate enrichment induced a significant decrease in carbon fixation rate and low values of carbon translocation, compared with control conditions (N : P = 0.5 : 0.05), while these processes were significantly enhanced when nitrate was replaced by ammonium. A combined enrichment in ammonium and phosphorus (N : P = 3 : 1) induced a shift in nutrient allocation to the symbionts, at the detriment of the host. Altogether, these results shed light into the effect of nutrient enrichment on reef corals. More broadly, they improve our understanding of the consequences of nutrient loading on reef ecosystems, which is urgently required to refine risk management strategies.

人为营养盐富集会改变沿岸生态系统的生物地球化学循环（biogeochemical cycles）与营养化学计量比（nutrient stoichiometry），且常与珊瑚礁退化相关联。然而，关于溶解态无机营养盐（dissolved inorganic nutrients）——尤其是不同形态氮（铵态氮ammonium与硝态氮nitrate）如何干扰珊瑚与其共生藻（symbiotic algae）之间的共生关系的机制，目前仍存在诸多争议。本研究针对以硝态氮或铵态氮为氮源、不同氮磷比（N:P）条件下的珊瑚响应展开了探究。研究结果显示，共生藻的碳获取过程以及碳在共生体系内的分配情况，会随营养盐丰度、形态及化学计量比的不同产生显著差异。具体而言，在低磷酸盐浓度（0.05 μM）条件下，与对照组（N:P=0.5:0.05）相比，3 μM硝态氮富集会导致珊瑚固碳速率显著下降，且碳转运量处于较低水平；而当将硝态氮替换为铵态氮时，上述过程则会得到显著增强。当同时对铵态氮与磷酸盐进行富集（N:P=3:1）时，营养盐分配会向共生藻偏移，以牺牲珊瑚宿主为代价。综上，本研究结果阐明了营养盐富集对造礁珊瑚的影响机制。从更广泛的层面来看，本研究加深了我们对营养盐负荷对珊瑚礁生态系统所产生影响的认知，而这一认知正是优化珊瑚礁风险管理策略所亟需的。