Temperature modulates coccolithophorid sensitivity of growth, photosynthesis and calcification to increasing seawater pCO2

Increasing atmospheric CO2 concentrations are expected to impact pelagic ecosystem functioning in the near future by driving ocean warming and acidification. While numerous studies have investigated impacts of rising temperature and seawater acidification on planktonic organisms separately, little is presently known on their combined effects. To test for possible synergistic effects we exposed two coccolithophore species, Emiliania huxleyi and Gephyrocapsa oceanica, to a CO2 gradient ranging from ~0.5-250 µmol/kg (i.e. ~20-6000 µatm pCO2) at three different temperatures (i.e. 10, 15, 20°C for E. huxleyi and 15, 20, 25°C for G. oceanica). Both species showed CO2-dependent optimum-curve responses for growth, photosynthesis and calcification rates at all temperatures. Increased temperature generally enhanced growth and production rates and modified sensitivities of metabolic processes to increasing CO2. CO2 optimum concentrations for growth, calcification, and organic carbon fixation rates were only marginally influenced from low to intermediate temperatures. However, there was a clear optimum shift towards higher CO2 concentrations from intermediate to high temperatures in both species. Our results demonstrate that the CO2 concentration where optimum growth, calcification and carbon fixation rates occur is modulated by temperature. Thus, the response of a coccolithophore strain to ocean acidification at a given temperature can be negative, neutral or positive depending on that strain's temperature optimum. This emphasizes that the cellular responses of coccolithophores to ocean acidification can only be judged accurately when interpreted in the proper eco-physiological context of a given strain or species. Addressing the synergistic effects of changing carbonate chemistry and temperature is an essential step when assessing the success of coccolithophores in the future ocean.

大气二氧化碳浓度持续升高将通过驱动海洋增温和酸化，在不久的将来对远洋生态系统功能产生影响。尽管已有大量研究分别探讨了海水升温与酸化对浮游生物的影响，但目前学界对二者的联合效应仍知之甚少。为探究潜在的协同效应，本研究将两种颗石藻（coccolithophore）——艾氏艾美球虫（Emiliania huxleyi）与大洋桥石藻（Gephyrocapsa oceanica）——置于三组不同温度条件下，施加浓度范围约0.5~250 µmol/kg（对应二氧化碳分压（pCO2）约20~6000 µatm）的梯度处理：其中艾氏艾美球虫的培养温度为10、15、20°C，大洋桥石藻的培养温度为15、20、25°C。两种颗石藻在所有温度条件下，其生长、光合作用及钙化速率均呈现出依赖于二氧化碳浓度的最优曲线响应模式。升温总体上提升了生长与生产速率，并改变了代谢过程对二氧化碳浓度升高的敏感性。在低温至中温区间内，生长、钙化及有机碳固持速率对应的二氧化碳最优浓度仅发生微弱变化。但在中温至高温区间内，两种颗石藻的二氧化碳最优浓度均显著向高浓度方向偏移。本研究结果表明，对应最优生长、钙化及碳固持速率的二氧化碳浓度会受到温度的调控。因此，在特定温度条件下，某颗石藻株系对海洋酸化的响应可为负向、中性或正向，具体取决于该株系的温度最优值。这一发现强调，唯有结合特定株系或物种的生态生理背景进行解读，才能准确评估颗石藻细胞对海洋酸化的响应。在评估未来海洋中颗石藻的生存状况时，探讨碳酸盐化学与温度变化的协同效应是必不可少的关键环节。