Photochemical responses of the diatom Skeletonema costatum grown under elevated CO2 concentrations to short-term changes in pH

Variability in pH is a common occurrence in many aquatic environments, due to physical, chemical and biological processes. In coastal waters, lagoons, estuaries and inland waters, pH can change very rapidly (within seconds or hours) in addition to daily and seasonal changes. At the same time, progressive ocean acidification caused by anthropogenic CO2 emissions is superimposed on these spatial and temporal pH changes. Photosynthetic organisms are therefore unavoidably subject to significant pH variations at the cell surface. Whether this will affect their response to long-term ocean acidification is still unknown, nor is it known whether the short-term sensitivity to pH change is affected by the pCO2 to which the cells are acclimated. We posed the latter open question as our experimental hypothesis: Does acclimation to seawater acidification affect the response of phytoplankton to acute pH variations? The diatom Skeletonema costatum, commonly found in coastal and estuarine waters where short-term acute changes in pH frequently occur, was selected to test the hypothesis. Diatoms were grown at both 390 (pH 8.2, low CO2; LC) and 1000 (pH 7.9, high CO2; HC) µatm CO2 for at least 20 generations, and photosynthetic responses to short-term and acute changes in pH (between 8.2 and 7.6) were investigated. The effective quantum yield of LC-grown cells decreased by ca. 70% only when exposed to pH 7.6; this was not observed when exposed to pH 7.9 or 8.2. HC-grown cells did not show significant responses in any pH treatment. Non-photochemical quenching showed opposite trends. In general, our results indicate that while LC-grown cells are rather sensitive to acidification, HC-grown cells are relatively unresponsive in terms of photochemical performance.

诸多水生环境中，pH波动是物理、化学与生物过程共同作用下的常见现象。在近岸海域、潟湖、河口与内陆水体中，除昼夜与季节尺度的变化外，pH还可在数秒至数小时内发生剧烈波动。与此同时，人为二氧化碳排放引发的渐进式海洋酸化，叠加于上述时空尺度的pH波动之上。因此，光合生物不可避免地会在细胞表面遭遇显著的pH变化。目前尚不明确这是否会影响光合生物对长期海洋酸化的响应，也不清楚细胞所适应的二氧化碳分压（pCO2）是否会改变其对pH短期变化的敏感性。我们将后者这一悬而未决的问题确立为本次实验的研究假说：海水酸化驯化是否会影响浮游植物对急性pH波动的响应？本研究选取在近岸与河口水体中广泛分布、且经常遭遇短期急性pH波动的硅藻——中肋骨条藻（Skeletonema costatum），以验证上述假说。研究将硅藻分别置于390 µatm 二氧化碳分压（对应pH 8.2，低二氧化碳组，LC）与1000 µatm 二氧化碳分压（对应pH 7.9，高二氧化碳组，HC）的环境中培养至少20代，随后探究其对pH（8.2至7.6区间）短期急性变化的光合响应。仅当暴露于pH 7.6环境时，低二氧化碳组培养的细胞的有效量子产量下降约70%；而在pH 7.9或8.2的处理组中未观察到此现象。高二氧化碳组培养的细胞在所有pH处理条件下均未表现出显著响应。非光化学淬灭则呈现出相反的变化趋势。总体而言，本研究结果表明：低二氧化碳组培养的细胞对酸化较为敏感，而高二氧化碳组培养的细胞在光化学性能方面相对无明显响应。