Seawater carbonate chemistry and lipid content, particulate organic carbon/nitrogen, growth rate, and morphology of Emiliania huxleyi@en

Coccolithophores are a calcifying unicellular phytoplankton group that are at the base of the marine food web, and their lipid content provides a source of energy to consumers. Coccolithophores are vulnerable to ocean acidification and warming, therefore it is critical to establish the effects of climate change on these significant marine primary producers, and determine potential consequences that these changes can have on their consumers. Here, we quantified the impact of changes in pH and temperature on the nutritional condition (lipid content, particulate organic carbon/nitrogen), growth rate, and morphology of the most abundant living coccolithophore species, Emiliania huxleyi. We used a regression type approach with nine pH levels (ranging from 7.66 to 8.44) and two temperatures (15°C and 20°C). Lipid production was greater under reduced pH, and growth rates were distinctly lower at 15°C than at 20°C. The production potential of lipids, which estimates the availability of lipids to consumers, increased under 20°C, but decreased under low pH. The results indicate that, while consumers will benefit energetically under ocean warming, this benefit will be mitigated by ocean acidification. The carbon to nitrogen ratio was higher at 20°C and low pH, indicating that the nutritional quality of coccolithophores for consumers will decline under climate change. The impact of low pH on the structural integrity of the coccosphere may also mean that coccolithophores are easier to digest for consumers. Many responses suggest cellular stress, indicating that increases in temperature and reductions in pH may have a negative impact on the ecophysiology of coccolithophores.颗石藻（Coccolithophores）是一类钙化单细胞浮游植物类群，居于海洋食物网的基底位置，其脂质含量可为海洋消费者提供能量来源。颗石藻易受海洋酸化与气候变暖的影响，因此明确气候变化对这类重要海洋初级生产者的影响，以及此类变化可能给其消费者带来的潜在后果，具有关键意义。 本研究量化了pH与温度变化对当前最优势的颗石藻物种——赫氏颗石藻（Emiliania huxleyi）的营养状况（脂质含量、颗粒有机碳/氮（particulate organic carbon/nitrogen）比值）、生长速率及形态的影响。实验采用回归分析方法，设置9个pH梯度（范围7.66至8.44）与两个温度梯度（15°C与20°C）。 实验结果显示：低pH条件下脂质产量更高，15°C下的生长速率显著低于20°C；脂质生产潜力（用于评估消费者可获取的脂质供应量）在20°C条件下升高，但在低pH条件下降低。研究表明，尽管海洋变暖可使消费者在能量层面获益，但该益处会被海洋酸化抵消。20°C与低pH条件下的碳氮比值更高，表明气候变化下颗石藻对消费者的营养质量将出现下降。低pH对颗石球（coccosphere）结构完整性的影响，还可能使消费者更易消化颗石藻。诸多响应特征均提示存在细胞应激反应，表明升温与pH降低可能对颗石藻的生态生理产生负面影响。