Huang Yulin, He Jiamin, Wang Yujie, Li Ling, Lin Senjie
State Key Laboratory of Marine Environmental Science, College of Ocean and Earth Sciences, Xiamen University, Xiamen, Fujian, China.
Department of Marine Sciences, University of Connecticut, Groton, Connecticut, USA.
Appl Environ Microbiol. 2025 Feb 19;91(2):e0059124. doi: 10.1128/aem.00591-24. Epub 2025 Jan 7.
Ocean warming due to climate change endangers coral reefs, and regional nitrogen overloading exacerbates the vulnerability of reef-building corals as the dual stress disrupts coral-Symbiodiniaceae mutualism. Different forms of nitrogen may create different interactive effects with thermal stress, but the underlying mechanisms remain elusive. To address the gap, we measured and compared the physiological and transcriptional responses of the Symbiodiniaceae to heat stress (31°C) when supplied with different types of nitrogen (nitrate, ammonium, or urea). Under heat stress (HS), cell proliferation and photosynthesis of declined, while cell size, lipid storage, and total antioxidant capacity increased, both to varied extents depending on the nitrogen type. Nitrate-cultured cells exhibited the most robust acclimation to HS, as evidenced by the fewest differentially expressed genes (DEGs) and less ROS accumulation, possibly due to activated nitrate reduction and enhanced ascorbate biogenesis. Ammonium-grown cultures exhibited higher algal proliferation and ROS scavenging capacity due to enhanced carotenoid and ascorbate quenching, but potentially reduced host recognizability due to the downregulation of N-glycan biosynthesis genes. Urea utilization led to the greatest ROS accumulation as genes involved in photorespiration, plant respiratory burst oxidase (RBOH), and protein refolding were markedly upregulated, but the greatest cutdown in photosynthate potentially available to corals as evidenced by photoinhibition and selfish lipid storage, indicating detrimental effects of urea overloading. The differential warming nitrogen-type interactive effects documented here has significant implication in coral-Symbiodiniaceae mutualism, which requires further research.IMPORTANCERegional nitrogen pollution exacerbates coral vulnerability to globally rising sea-surface temperature, with different nitrogen types exerting different interactive effects. How this occurs is poorly understood and understudied. This study explored the underlying mechanism by comparing physiological and transcriptional responses of a coral symbiont to heat stress under different nitrogen supplies (nitrate, ammonium, and urea). The results showed some common, significant responses to heat stress as well as some unique, N-source dependent responses. These findings underscore that nitrogen eutrophication is not all the same, the form of nitrogen pollution should be considered in coral conservation, and special attention should be given to urea pollution.
气候变化导致的海洋变暖危及珊瑚礁，区域氮超载加剧了造礁珊瑚的脆弱性，因为这种双重压力破坏了珊瑚与共生藻的共生关系。不同形态的氮可能与热应激产生不同的交互作用，但其潜在机制仍不清楚。为了填补这一空白，我们测量并比较了共生藻在供应不同类型的氮（硝酸盐、铵或尿素）时对热应激（31°C）的生理和转录反应。在热应激（HS）下，共生藻的细胞增殖和光合作用下降，而细胞大小、脂质储存和总抗氧化能力增加，且增加程度因氮类型而异。硝酸盐培养的细胞对HS表现出最强的适应性，差异表达基因（DEG）最少且活性氧积累较少就是证明，这可能是由于硝酸盐还原的激活和抗坏血酸生物合成的增强。铵培养的共生藻由于类胡萝卜素和抗坏血酸淬灭增强而表现出更高的藻类增殖和活性氧清除能力，但由于N-聚糖生物合成基因的下调，可能会降低宿主识别能力。尿素利用导致最大的活性氧积累，因为参与光呼吸、植物呼吸爆发氧化酶（RBOH）和蛋白质重折叠的基因明显上调，但从光抑制和自私的脂质储存可以看出，可供珊瑚利用的光合产物减少最多，表明尿素超载具有有害影响。这里记录的不同升温与氮类型的交互作用对珊瑚-共生藻共生关系具有重要意义，需要进一步研究。
重要性
区域氮污染加剧了珊瑚对全球海平面上升温度的脆弱性，不同类型的氮产生不同的交互作用。其发生方式了解甚少且研究不足。本研究通过比较珊瑚共生体在不同氮供应（硝酸盐、铵和尿素）下对热应激的生理和转录反应，探索了潜在机制。结果显示了对热应激的一些共同、显著反应以及一些独特的、依赖氮源的反应。这些发现强调，氮富营养化并非完全相同，在珊瑚保护中应考虑氮污染的形式，尤其应特别关注尿素污染。
