Increasing ocean temperatures associated with ongoing climate change have resulted in regional reductions in the cover of live coral and increasing concerns that coral reefs will be overgrown by macroalgae. The likelihood of macroalgal overgrowth will, however, depend on the thermal sensitivities of the macroalgae themselves. We exposed recently settled propagules of the common canopy-forming macroalga Sargassum swartzii and adult thalli of three species of Sargassum (S. swatzii, S. cristaefolium, S. polycystum) to three experimental temperatures: ambient, + 2 °C, and + 3.5 °C, reflective of summer minimum, mean, and maximum temperatures for the region. Growth and survival of Sargassum swartzii propagules were assessed over 48 days, and the growth, physical toughness, elemental composition, and susceptibility to herbivory of adult thalli were assessed after short-term exposure (2-weeks) to experimental temperatures. Growth and survival of S. swartzii propagules were reduced by 43% and 84%, respectively, when cultured at the elevated (+ 3.5 °C) temperature compared to ambient temperature. Similarly, elevated temperature resulted in a 17–49% decline in the growth of adult Sargassum thalli relative to controls. Susceptability of S. swartzii and S. cristaefolium to herbivory (i.e. mass removed by herbivores) was 50% less for thalli cultured at elevated (+ 3.5 °C) compared to ambient temperature, but this pattern was not related to changes in the physical or chemical properties of the thalli as a result of elevated temperature. The negative effects of elevated temperatures on the growth and survival of both Sargassum propagules and adult thalli will likely restrict the capacity of Sargassum, and potentially other macroalgae, to establish in new areas, and may also threaten the persistence of existing macroalgal meadows under future ocean temperatures. The thermal sensitivities of tropical Sargassum, together with those of corals, suggest ongoing ocean warming may lead to novel reef ecosystems that are low in both coral cover and macroalgal cover.

中文翻译：

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高温下热带大型藻类（马尾藻属）的生长和存活受损

与持续的气候变化相关的海洋温度升高导致活珊瑚覆盖的区域减少，并且人们越来越担心珊瑚礁将被大型藻类过度生长。然而，大型藻类过度生长的可能性将取决于大型藻类本身的热敏感性。我们将最近固定的形成树冠的大型海藻马尾藻和三种马尾藻（S. swatzii、S. cristaefolium、S. polycystum）的成年菌体的繁殖体暴露于三个实验温度：环境温度、+ 2 °C 和 + 3.5 °C，反映该地区夏季最低、平均和最高温度。在 48 天内评估了马尾藻繁殖体的生长和存活，以及生长、物理韧性、元素组成、在短期暴露于实验温度（2 周）后，评估了成年菌体对食草动物的敏感性和敏感性。与环境温度相比，在升高的 (+ 3.5 °C) 温度下培养时，斯瓦茨氏菌繁殖体的生长和存活率分别降低了 43% 和 84%。同样，相对于对照，升高的温度导致成年马尾藻菌体的生长下降 17-49%。与环境温度相比，S. swartzii 和 S. cristaefolium 对食草动物（即食草动物去除的质量）的敏感性比环境温度低 50%，但这种模式与物理或由于高温导致菌体的化学性质。温度升高对马尾藻繁殖体和成年菌体的生长和存活的负面影响可能会限制马尾藻和其他潜在大型藻类在新地区建立的能力，并且还可能威胁到未来海洋下现有大型藻类草甸的持久性温度。热带马尾藻的热敏感性以及珊瑚的热敏感性表明，持续的海洋变暖可能会导致珊瑚覆盖和大型藻类覆盖率低的新型珊瑚礁生态系统。