Labile dissolved organic carbon (DOC) is a major pollutant in coastal marine environments affected by anthropogenic impacts, and may significantly contribute to coral bleaching and subsequent mortality on coastal reefs. DOC can cause bleaching indirectly through the rapid proliferation of copiotrophic and pathogenic bacteria. Here we demonstrate that labile DOC compounds can also impair the coral-dinoflagellate symbiosis by directly affecting coral physiology on both the host and algal symbiont level. In a controlled aquarium experiment, we monitored over several weeks key physiological parameters of the tropical coral Stylophora pistillata exposed to ambient and elevated labile DOC levels (0.1 and 1.0 mM) in combination with low and high nitrogen (i.e. ammonium) conditions (0.2 and 4.0 μM). At the symbiont level, DOC exposure under low ammonium availability decreased the photosynthetic efficiency accompanied by ∼75 % Chl a and ∼50 % symbiont cell reduction. The photosynthetic functioning of the symbionts recovered once the DOC enrichment ceased indicating a reversible shift between autotrophic and heterotrophic metabolism. At the host level, the assimilation of exogenous DOC sustained the tissue carbon reserves, but induced a depletion of the nitrogen reserves, indicated by ∼35 % decreased protein levels. This suggests an imbalanced exogenous carbon to nitrogen supply with nitrogen potentially limiting host metabolism on the long-term. We also demonstrate that increased ammonium availability delayed DOC-induced bleaching likely by keeping symbionts in a photosynthetically competent state, which is crucial for symbiosis maintenance and coral survival. Overall, the present study provides further insights into how coastal pollution can de-stabilize the coral-algal symbiosis and cause coral bleaching. Therefore, reducing coastal pollution and sustaining ecological integrity are critical to strengthen the resilience of coral reefs facing climate change.

中文翻译：

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在不稳定的碳污染期间，升高的铵盐会延缓珊瑚-鞭毛虫共生的损害。

不稳定的溶解有机碳（DOC）是受人为因素影响的沿海海洋环境中的主要污染物，并且可能极大地导致珊瑚白化和随后在沿海礁石上的死亡。DOC可以通过嗜营养性和致病性细菌的快速繁殖间接引起漂白。在这里，我们证明了不稳定的DOC化合物还可以通过直接影响宿主和藻类共生体水平的珊瑚生理来损害珊瑚-鞭毛共生。在一个受控的水族馆实验中，我们在数周内监测了热带珊瑚Stylophora pistillata的关键生理参数，这些细菌暴露于环境中和不稳定的DOC水平（0.1和1.0 mM）以及低氮和高氮（铵）条件（0.2和4.0）的共同作用下（μM）。在共生水平上，低铵可利用性下的DOC暴露降低了光合效率，同时伴随〜75％的Chla和约50％的共生细胞减少。一旦DOC富集停止，共生体的光合功能恢复，表明自养和异养代谢之间可逆转变。在宿主水平上，外源DOC的吸收维持了组织的碳储量，但诱导了氮储量的消耗，这表明蛋白质水平降低了约35％。这表明长期以来，氮与碳之间的外源碳供应不平衡，这可能会限制宿主的新陈代谢。我们还证明，通过将共生体保持在光合作用的状态下，增加的铵可利用性可以延缓DOC诱导的漂白，这对共生维持和珊瑚生存至关重要。总体而言，本研究为沿海污染如何破坏珊瑚-藻类共生稳定并导致珊瑚白化提供了进一步的见解。因此，减少沿海污染和维持生态完整性对于增强面对气候变化的珊瑚礁的复原力至关重要。