Calcification, photophysiology and treatment parameters for laboratory experiment

Ocean acidification and coastal nutrient enrichment threaten the persistence of nearshore ecosystems, yet little is known about their combined effects on marine organisms. Here, we show that elevated nitrogen concentrations, in the forms of nitrite + nitrate and ammonium, offset the negative effects of near-future OA on calcification of the reef-building crustose coralline alga, Porolithon onkodes. Projected near-future pCO2 levels (~850 µatm) decreased P. onkodes calcification by 30% relative to ambient conditions. Conversely, nitrogen enrichment increased P. onkodes calcification by 90% and 130% in ambient and high pCO2 treatments, respectively, relative to ambient controls. pCO2 and nitrogen enrichment interactively affected instantaneous photophysiology. Relative electron transport rates (rETR) were highest in high pCO2 and high nitrogen conditions. Nitrogen enrichment alone increased concentrations of the photosynthetic pigments chlorophyll a, phycocyanin and phycoerythrin by ~80-450%, regardless of pCO2. These results demonstrate that nutrient enrichment can mediate organismal responses to OA, which has far-reaching implications for nearshore coral reefs that experience persistent or episodic nutrient enrichment via eutrophication or consumer excretions. Multi-stressor OA experiments increasingly are becoming important for improving our ability to predict the response of marine organisms and coral reefs to simultaneously occurring local and global change stressors.

海洋酸化（OA, Ocean Acidification）与沿海营养富集正威胁近岸生态系统的存续，但目前对二者联合作用对海洋生物的影响尚所知有限。本研究发现，以亚硝酸盐与硝酸盐及铵盐形式存在的升高氮浓度，可抵消近未来海洋酸化对造礁壳状珊瑚藻（Porolithon onkodes）钙化过程的负面影响。相对于环境条件，预测的近未来二氧化碳分压（pCO2）水平（约850 µatm）会使石孔藻（Porolithon onkodes）的钙化作用降低30%。与之相反，相对于环境对照组，氮富集分别使环境pCO2与高pCO2处理组的石孔藻钙化作用提升90%与130%。二氧化碳分压与氮富集对瞬时光生理特性存在交互影响，相对电子传递速率（relative electron transport rates，rETR）在高pCO2与高氮条件下达到最高。无论pCO2水平如何，单独的氮富集均可使光合色素叶绿素a、藻蓝蛋白与藻红蛋白的浓度提升约80%~450%。本研究结果表明，营养富集可调节生物对海洋酸化的响应，这对于那些通过富营养化或消费者排泄物持续或间歇性承受营养富集的近岸珊瑚礁具有深远意义。多胁迫因子海洋酸化实验正逐渐成为提升我们预测海洋生物与珊瑚礁应对同时发生的局地与全球变化胁迫因子响应能力的重要研究手段。