Seawater carbonate chemistry and photosynthesis of a coccolithophorid in a laboratory experiment

Mixing of seawater subjects phytoplankton to fluctuations in photosynthetically active radiation (400-700 nm) and ultraviolet radiation (UVR; 280-400 nm). These irradiance fluctuations are now superimposed upon ocean acidification and thinning of the upper mixing layer through stratification, which alters mixing regimes. Therefore, we examined the photosynthetic carbon fixation and photochemical performance of a coccolithophore, Gephyrocapsa oceanica, grown under high, future (1,000 µatm) and low, current (390 µatm) CO2 levels, under regimes of fluctuating irradiances with or without UVR. Under both CO2 levels, fluctuating irradiances, as compared with constant irradiance, led to lower nonphotochemical quenching and less UVR-induced inhibition of carbon fixation and photosystem II electron transport. The cells grown under high CO2 showed a lower photosynthetic carbon fixation rate but lower nonphotochemical quenching and less ultraviolet B (280-315 nm)-induced inhibition. Ultraviolet A (315-400 nm) led to less enhancement of the photosynthetic carbon fixation in the high-CO2-grown cells under fluctuating irradiance. Our data suggest that ocean acidification and fast mixing or fluctuation of solar radiation will act synergistically to lower carbon fixation by G. oceanica, although ocean acidification may decrease ultraviolet B-related photochemical inhibition.

海水混合过程会使浮游植物暴露于光合有效辐射（photosynthetically active radiation，400~700 nm）与紫外线辐射（ultraviolet radiation，UVR；280~400 nm）的动态波动之中。当前，此类辐照度波动正与海洋酸化、层化作用引发的上层混合层变薄相互叠加，进而改变海水混合模式。为此，我们针对颗石藻（coccolithophore）物种大洋桥石藻（Gephyrocapsa oceanica）开展了受控培养实验：分别设置当前CO₂水平（390 μatm）与未来高浓度CO₂水平（1000 μatm）两个实验组，并设置含/不含UVR的辐照度波动处理，探究其光合碳固定与光化学性能的变化。与恒定辐照度处理相比，两种CO₂水平下的辐照度波动均降低了细胞的非光化学淬灭（nonphotochemical quenching）水平，同时减轻了UVR对光合碳固定与光系统II（photosystem II）电子传递的抑制效应。在高CO₂水平下培养的细胞，其光合碳固定速率更低，但非光化学淬灭水平也同步降低，且受紫外线B（ultraviolet B，UV-B；280~315 nm）诱导的抑制作用更弱。在辐照度波动条件下，高CO₂组细胞的光合碳固定受紫外线A（ultraviolet A，UV-A；315~400 nm）诱导的增强效应显著减弱。本研究数据表明，尽管海洋酸化可缓解紫外线B介导的光化学抑制作用，但海洋酸化与快速混合或太阳辐射波动的协同效应，将降低大洋桥石藻的光合碳固定能力。