The modulating effect of light intensity on the response of the coccolithophore Gephyrocapsa oceanica to ocean acidification

Global change leads to a multitude of simultaneous modifications in the marine realm among which shoaling of the upper mixed layer, leading to enhanced surface layer light intensities, as well as increased carbon dioxide (CO2) concentration are some of the most critical environmental alterations for phytoplankton. In this study, we investigated the responses of growth, photosynthetic carbon fixation and calcification of the coccolithophore Gephyrocapsa oceanica to elevated inline image (51 Pa, 105 Pa, and 152 Pa) (1 Pa ~ 10 µatm) at a variety of light intensities (50-800 µmol photons/m**2/s). By fitting the light response curve, our results showed that rising inline image reduced the maximum rates for growth, photosynthetic carbon fixation and calcification. Increasing light intensity enhanced the sensitivity of these rate responses to inline image, and shifted the inline image optima toward lower levels. Combining the results of this and a previous study (Sett et al. 2014) on the same strain indicates that both limiting low inline image and inhibiting high inline image levels (this study) induce similar responses, reducing growth, carbon fixation and calcification rates of G. oceanica. At limiting low light intensities the inline image optima for maximum growth, carbon fixation and calcification are shifted toward higher levels. Interacting effects of simultaneously occurring environmental changes, such as increasing light intensity and ocean acidification, need to be considered when trying to assess metabolic rates of marine phytoplankton under future ocean scenarios.

全球变化引发海洋领域诸多同步发生的环境改变，其中上层混合层（upper mixed layer）变浅导致表层光照强度增强、二氧化碳（CO₂）浓度升高等，是对浮游植物（phytoplankton）而言最为关键的若干环境变化。本研究针对颗石藻（coccolithophore）桥石藻属大洋桥石藻（Gephyrocapsa oceanica），探究了其生长、光合固碳及钙化作用对升高的二氧化碳分压（pCO₂，51 Pa、105 Pa与152 Pa；1 Pa ≈ 10 微大气压（µatm））在50–800 μmol 光子/(平方米·秒)梯度光照强度下的响应。通过拟合光响应曲线（light response curve），本研究结果显示，升高的pCO₂会降低大洋桥石藻的生长、光合固碳与钙化的最大速率；光照强度的提升会增强这些生理速率对pCO₂的响应敏感性，并使pCO₂的最适值向更低水平偏移。结合本研究与此前针对同一藻株的研究（Sett等，2014）的结果可知，受限的低pCO₂与本研究中的高pCO₂抑制效应均会引发相似的响应，即降低大洋桥石藻的生长、固碳及钙化速率。在受限的低光照强度下，支撑最大生长、固碳与钙化速率的pCO₂最适值会向更高水平偏移。在评估未来海洋情境下海洋浮游植物的代谢速率时，需同时考虑光照强度升高与海洋酸化（ocean acidification）等同步发生的环境变化之间的交互效应。