Abstract Competition between corals and macroalgae is frequently observed on reefs with the outcome of these interactions affecting the relative abundance of reef organisms and therefore reef health. Anthropogenic activities have resulted in increased atmospheric CO2 levels and a subsequent rise in ocean temperatures. In addition to increasing water temperature, elevated CO2 levels are leading to a decrease in oceanic pH (ocean acidification). These two changes have the potential to alter ecological processes within the oceans, including the outcome of competitive coral-macroalgal interactions. In our study, we explored the combined effect of temperature increase and ocean acidification on the competition between the coral Porites lobata and on the Great Barrier Reef abundant macroalga Chlorodesmis fastigiata. A temperature increase of +1 °C above present temperatures and CO2 increase of +85 ppm were used to simulate a low end emission scenario for the mid- to late 21st century, according to the Representative Concentration Pathway 2.6 (RCP2.6). Our results revealed that the net photosynthesis of P. lobata decreased when it was in contact with C. fastigiata under ambient conditions, and that dark respiration increased under RCP2.6 conditions. The Photosynthesis to Respiration (P:R) ratios of corals as they interacted with macroalgal competitors were not significantly different between scenarios. Dark calcification rates of corals under RCP2.6 conditions, however, were negative and significantly decreased compared to ambient conditions. Light calcification rates were negatively affected by the interaction of macroalgal contact in the RCP2.6 scenario, compared to algal mimics and to coral under ambient conditions. Chlorophyll a, and protein content increased in the RCP2.6 scenario, but were not influenced by contact with the macroalga. We conclude that the coral host was negatively affected by RCP2.6 conditions, whereas the productivity of its symbionts (zooxanthellae) was enhanced. While a negative effect of the macroalga (C. fastigiata) on the coral (P. lobata) was observed for the P:R ratio under control conditions, it was not enhanced under RCP2.6 conditions.

中文翻译：

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海洋变暖和酸化下的珊瑚-巨藻竞争

摘要 在珊瑚礁上经常观察到珊瑚和大型藻类之间的竞争，这些相互作用的结果会影响珊瑚礁生物的相对丰度，从而影响珊瑚礁的健康。人为活动导致大气 CO2 水平增加，随后海洋温度升高。除了水温升高外，二氧化碳浓度升高还会导致海洋 pH 值降低（海洋酸化）。这两个变化有可能改变海洋内的生态过程，包括珊瑚-大型藻类竞争性相互作用的结果。在我们的研究中，我们探讨了温度升高和海洋酸化对 Porites lobata 珊瑚和大堡礁丰富的大型藻类 Chlorodesmis fastigiata 之间竞争的综合影响。根据代表性浓度途径 2.6 (RCP2.6)，使用比当前温度增加 +1°C 的温度和 +85 ppm 的二氧化碳增加量来模拟 21 世纪中后期的低端排放情景。我们的结果表明，在环境条件下，与 C. fastigiata 接触时，P. lobata 的净光合作用降低，而在 RCP2.6 条件下，暗呼吸增加。珊瑚与大型藻类竞争者相互作用时的光合作用与呼吸 (P:R) 比率在不同场景之间没有显着差异。然而，与环境条件相比，RCP2.6 条件下珊瑚的暗钙化率为负值且显着降低。在 RCP2.6 情景中，大型藻类接触的相互作用对轻钙化率产生负面影响，与藻类模拟物和环境条件下的珊瑚相比。在 RCP2.6 情景中，叶绿素 a 和蛋白质含量增加，但不受与大型藻类接触的影响。我们得出结论，珊瑚宿主受到 RCP2.6 条件的负面影响，而其共生体（虫黄藻）的生产力得到提高。虽然在对照条件下观察到大藻 (C. fastigiata) 对珊瑚 (P. lobata) 的负面影响，但其 P:R 比在 RCP2.6 条件下并未增强。