Abstract Calcifying tropical macroalgae produce sediment, build three-dimensional habitats, and provide substrate for invertebrate larvae on reefs. Thus, lower calcification rates under declining pH and increasing ocean pCO2, or ocean acidification, is a concern. In the present study, calcification rates were examined experimentally under predicted end-of-the-century seawater pCO2 (1116 μatm) and pH (7.67) compared to ambient controls (pCO2 409 μatm; pH 8.04). Nine reef macroalgae with diverse calcification locations, calcium carbonate structure, photophysiology, and site-specific irradiance were examined under light and dark conditions. Species included five from a high light patch reef on the Florida Keys Reef Tract (FKRT) and four species from low light reef walls on Little Cayman Island (LCI). Experiments on FKRT and LCI species were conducted at 500 and 50 μmol photons m−2 s−1 in situ irradiance, respectively. Calcification rates independent of photosystem-II (PSII) were also investigated for FKRT species. The most consistent negative effect of elevated pCO2 on calcification rates in the tropical macroalgae examined occurred in the dark. Most species (89%) had net calcification rates of zero or net dissolution in the dark at low pH. Species from the FKRT that sustained positive net calcification rates in the light at low pH also maintained ~30% of their net calcification rates without PSII at ambient pH. However, calcification rates in the light independent of PSII were not sustained at low pH. Regardless of these low pH effects, most FKRT species daily net calcification rates, integrating light/dark rates over a 24h period, were not significantly different between low and ambient pH. This was due to a 10-fold lower dark, compared to light, calcification rate, and a strong correspondence between calcification and photosynthetic rates. Interestingly, low-light species sustained calcification rates on par with high-light species without high rates of photosynthesis. Low-light species' morphology and physiology that promote high calcification rates at ambient pH, may increase their vulnerability to low pH. Our data indicate that the negative effect of elevated pCO2 and low pH on tropical macroalgae at the organismal level is their impact on dark net calcification, probably enhanced dissolution. However, elevated pCO2 and low pH effects on macroalgae daily calcification rates are greatest in species with lower net calcification rates in the light. Thus, macroalgae able to maintain high calcification rates in the light (high and low irradiance) at low pH, and/or sustain strong biotic control with high [H+] in the bulk seawater, are expected to dominate under global change.

中文翻译：

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辐照度、光合作用和 pCO2 升高对热带珊瑚礁大型藻类净钙化的影响

摘要 钙化热带大型藻类产生沉积物，构建三维栖息地，并为珊瑚礁上的无脊椎动物幼虫提供基质。因此，在 pH 值下降和海洋 pCO2 增加或海洋酸化的情况下，钙化率降低是一个问题。在本研究中，与环境对照（pCO2 409 μatm；pH 8.04）相比，在本世纪末预测的海水 pCO2 (1116 μatm) 和 pH (7.67) 下通过实验检查钙化率。在明暗条件下检查了九种具有不同钙化位置、碳酸钙结构、光生理学和特定地点辐照度的珊瑚礁大型藻类。物种包括来自佛罗里达礁岛礁 (FKRT) 高光斑块礁的 5 种和来自小开曼岛 (LCI) 低光礁壁的 4 种。FKRT 和 LCI 物种的实验分别在 500 和 50 μmol 光子 m-2 s-1 原位辐照度下进行。还对 FKRT 物种研究了独立于光系统 II (PSII) 的钙化率。在被检查的热带大型藻类中，升高的 pCO2 对钙化率的最一致的负面影响发生在黑暗中。大多数物种 (89%) 在低 pH 值下在黑暗中的净钙化率为零或净溶解。来自 FKRT 的物种在低 pH 值的光照下保持正净钙化率，在环境 pH 值下，在没有 PSII 的情况下，其净钙化率也保持了约 30%。然而，与 PSII 无关的光照下的钙化率在低 pH 值下无法维持。不管这些低 pH 值影响如何，大多数 FKRT 物种的每日净钙化率，在 24 小时内整合光/暗率，在低pH值和环境pH值之间没有显着差异。这是由于与光、钙化速率以及钙化和光合速率之间的强对应性相比，黑暗低 10 倍。有趣的是，低光物种的钙化率与没有高光合作用速率的高光物种相当。在环境 pH 值下促进高钙化率的弱光物种的形态和生理学可能会增加它们对低 pH 值的脆弱性。我们的数据表明，升高的 pCO2 和低 pH 值对有机体水平的热带大型藻类的负面影响是它们对暗网钙化的影响，可能会增强溶解。然而，pCO2 升高和低 pH 值对大型藻类每日钙化率的影响在光照下净钙化率较低的物种中最大。因此，