Eutrophication with dissolved organic carbon (DOC) as a far under-investigated stressor, and ocean warming, can strongly affect coral reefs and hard corals as major reefs ecosystem engineers. However, no previous studies have investigated the metabolic responses of soft corals to DOC eutrophication, or its interaction with ocean warming. Thus, we investigated respiration and photosynthesis response of Xenia umbellata, a common mixotrophic soft coral from the Indo-pacific, to (1) three levels of DOC eutrophication simulated by glucose addition over the first 21 days of experiment and (2) ocean warming scenarios where the temperature was gradually increased from 26 °C (control condition) to 32 °C over another 24 days in an aquarium experiment. We found no significant difference in response to DOC treatments and all corals survived regardless of the DOC concentrations, whilst subsequent exposure to simulated ocean warming significantly decreased gross photosynthesis by approximately 50% at 30 °C, and 65% at 32 °C, net photosynthesis by 75% at 30 °C and 79% at 32 °C, and respiration by a maximum of 75% at 30 °C; with a slight increase at 32 °C of 25%. The ratio between gross photosynthesis and respiration decreased by the end of the warming period but remained similar between controls and colonies previously exposed to DOC. Our findings suggest that soft corals may be more resistant than hard corals to DOC eutrophication and in consequence, may potentially experiment in less magnitude the negative effects of increased temperature or subsequently both stressors. The results of this study may contribute to explain the successful role of soft corals in phase shifts as reported from many coral reefs. Where predicted declines in reef ecosystems health due to increased eutrophication levels can be exacerbated by future warming.

中文翻译：

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软珊瑚 Xenia umbellata 的光合作用和呼吸对变暖有反应，但对有机碳富营养化没有反应

溶解有机碳（DOC）作为一种远未得到充分研究的压力源造成的富营养化以及海洋变暖，可以强烈影响作为主要珊瑚礁生态系统工程师的珊瑚礁和硬珊瑚。然而，之前没有研究调查软珊瑚对 DOC 富营养化的代谢反应，或其与海洋变暖的相互作用。因此，我们研究了 Xenia umbellata（一种来自印度太平洋的常见混合营养软珊瑚）对 (1) 在实验的前 21 天通过添加葡萄糖模拟的三个 DOC 富营养化水平和 (2) 海洋变暖情景的呼吸和光合作用反应在水族箱实验中，在接下来的 24 天内，温度逐渐从 26 °C（对照条件）升至 32 °C。我们发现对 DOC 处理的反应没有显着差异，并且无论 DOC 浓度如何，所有珊瑚都能存活下来，而随后暴露于模拟海洋变暖中，总光合作用在 30 °C 时显着降低约 50%，在 32 °C 时净光合作用降低 65% 30 °C 时减少 75%，32 °C 时减少 79%，30 °C 时呼吸最多减少 75%；32°C 时略有增加 25%。在变暖期结束时，总光合作用和呼吸之间的比率有所下降，但对照组和之前暴露于 DOC 的菌落之间仍然相似。我们的研究结果表明，软珊瑚可能比硬珊瑚更能抵抗DOC富营养化，因此，可能会在较小程度上试验温度升高或随后的两种压力源的负面影响。这项研究的结果可能有助于解释软珊瑚在许多珊瑚礁报告的相移中的成功作用。未来的变暖可能会加剧因富营养化水平增加而导致的珊瑚礁生态系统健康状况的下降。