Seawater carbonate chemistry and specific growth rate, respiration rate, net photosynthetic rate and photochemical parameters of Phaeodactylum tricornutum

Experimentally elevated pCO2 and the associated pH drop are known to differentially affect many aspects of the physiology of diatoms under different environmental conditions or in different regions. However, contrasting responses to elevated pCO2 in the dark and light periods of a diel cycle have not been documented. By growing the model diatom Phaeodactylum tricornutum under 3 light levels and 2 different CO2 concentrations, we found that the elevated pCO2/pH drop projected for future ocean acidification reduced the diatom's growth rate by 8–25% during the night period but increased it by up to 9–21% in the light period, resulting in insignificant changes in growth over the diel cycle under the three different light levels. The elevated pCO2 increased the respiration rates irrespective of growth light levels and light or dark periods and enhanced its photosynthetic performance during daytime. With prolonged exposure to complete darkness, simulating the sinking process in the dark zones of the ocean, the growth rates decreased faster under elevated pCO2, along with a faster decline in quantum yield and cell size. Our results suggest that elevated pCO2 enhances the diatom's respiratory energy supplies to cope with acidic stress during the night period but enhances its death rate when the cells sink to dark regions of the oceans below the photic zone, with implications for a possible acidification-induced reduction in vertical transport of organic carbon.

实验性升高的二氧化碳分压（pCO₂）及其伴随的pH下降，已知会在不同环境条件或不同区域下对硅藻生理的多个方面产生差异化影响。然而，硅藻在昼夜周期（diel cycle）的黑暗与光照阶段对升高的pCO₂的对比响应尚未被记录。通过在3种光照强度和2种不同CO₂浓度下培养模式硅藻三角褐指藻（Phaeodactylum tricornutum），我们发现，未来海洋酸化预测的升高pCO₂/pH下降会使该硅藻夜间生长速率降低8–25%，但在光照阶段使其生长速率提高9–21%，最终导致三种不同光照强度下整个昼夜周期的生长变化不显著。升高的pCO₂会提高呼吸速率，无论生长光照强度如何以及处于光照还是黑暗阶段；同时，它会增强硅藻白天的光合性能。在模拟海洋暗区下沉过程的持续完全黑暗条件下，升高的pCO₂会导致生长速率下降更快，同时量子产率（quantum yield）和细胞大小的下降也更迅速。我们的结果表明，升高的pCO₂在夜间会增强硅藻的呼吸能量供应以应对酸性胁迫，但当细胞下沉到透光层以下的海洋暗区时，会提高其死亡率——这暗示着酸化可能导致有机碳垂直运输的减少。