Nutrient availability affects the response of the calcifying chlorophyte Halimeda opuntia (L.) J.V. Lamouroux to low pH@en

Atmospheric carbon dioxide emissions cause a decrease in the pH and aragonite saturation state of surface ocean water. As a result, calcifying organisms are expected to suffer under future ocean conditions, but their physiological responses may depend on their nutrient status. Because many coral reefs experience high inorganic nutrient loads or seasonal changes in nutrient availability, reef organisms in localized areas will have to cope with elevated carbon dioxide and changes in inorganic nutrients. Halimeda opuntia is a dominant calcifying primary producer on coral reefs that contributes to coral reef accretion. Therefore, we investigated the carbon and nutrient balance of H. opuntia exposed to elevated carbon dioxide and inorganic nutrients. We measured tissue nitrogen, phosphorus and carbon content as well as the activity of enzymes involved in inorganic carbon uptake and nitrogen assimilation (external carbonic anhydrase and nitrate reductase, respectively). Inorganic carbon content was lower in algae exposed to high CO2, but calcification rates were not significantly affected by CO2 or inorganic nutrients. Organic carbon was positively correlated to external carbonic anhydrase activity, while inorganic carbon showed the opposite correlation. Carbon dioxide had a significant effect on tissue nitrogen and organic carbon content, while inorganic nutrients affected tissue phosphorus and N:P ratios. Nitrate reductase activity was highest in algae grown under elevated CO2 and inorganic nutrient conditions and lowest when phosphate was limiting. In general, we found that enzymatic responses were strongly influenced by nutrient availability, indicating its important role in dictating the local responses of the calcifying primary producer H. opuntia to ocean acidification.大气二氧化碳排放会导致表层海水pH值下降以及文石饱和状态（aragonite saturation state）降低。在此背景下，钙化生物在未来海洋环境中大概率会受到负面影响，但其生理响应或取决于自身的营养盐状况。由于多数珊瑚礁生态系统均面临较高的无机营养盐负荷，或是营养盐可获得性的季节性变化，局域海域的珊瑚礁生物必须同时应对升高的二氧化碳水平与无机营养盐的改变。团扇藻（Halimeda opuntia）是珊瑚礁上的优势钙化初级生产者，可参与珊瑚礁体的加积作用。因此，本研究针对暴露于升高二氧化碳与无机营养盐环境下的H. opuntia开展了碳与营养盐平衡相关探究。我们测定了其组织中的氮、磷、碳含量，以及参与无机碳吸收与氮同化过程的酶活性，分别为胞外碳酸酐酶（external carbonic anhydrase）与硝酸还原酶（nitrate reductase）。高二氧化碳环境下的藻体无机碳含量更低，但二氧化碳与无机营养盐均未对钙化速率产生显著影响。有机碳含量与胞外碳酸酐酶活性呈正相关，而无机碳含量则呈现相反的相关性。二氧化碳对藻体组织的氮含量与有机碳含量具有显著影响，而无机营养盐则会影响组织磷含量与氮磷比（N:P ratio）。硝酸还原酶活性在同时处于升高二氧化碳与无机营养盐条件下的藻体中最高，而在磷酸盐限制条件下最低。总体而言，本研究发现酶促响应显著受营养盐可获得性调控，这表明营养盐状况在决定钙化初级生产者团扇藻（H. opuntia）针对海洋酸化（ocean acidification）的局域响应过程中发挥着关键作用。