Seawater carbonate chemistry, cell numbers and growth rate during experiments with dinoflagellates, 2007

The effects of dissolved inorganic carbon (DIC) on the growth of 3 red-tide dinoflagellates (Ceratium lineatum, Heterocapsa triquetra and Prorocentrum minimum) were studied at pH 8.0 and at higher pH levels, depending upon the pH tolerance of the individual species. The higher pH levels chosen for experiments were 8.55 for C. lineatum and 9.2 for the other 2 species. At pH 8.0, which approximates the pH found in the open sea, the maximum growth in all species was maintained until the total DIC concentration was reduced below ~0.4 and 0.2 mM for C. lineatum and the other 2 species, respectively. Growth compensation points (concentration of inorganic carbon needed for maintenance of cells) were reached at ~0.18 and 0.05 mM DIC for C. lineatum and the other 2 species, respectively. At higher pH levels, maximum growth rates were lower compared to growth at pH 8, even at very high DIC concentrations, indicating a direct pH effect on growth. Moreover, the concentration of bio-available inorganic carbon (CO2 + HCO3-) required for maintenance as well as the half-saturation constants were increased considerably at high pH compared to pH 8.0. Experiments with pH-drift were carried out at initial concentrations of 2.4 and 1.2 mM DIC to test whether pH or DIC was the main limiting factor at a natural range of DIC. Independent of the initial DIC concentrations, growth rates were similar in both incubations until pH had increased considerably. The results of this study demonstrated that growth of the 3 species was mainly limited by pH, while inorganic carbon limitation played a minor role only at very high pH levels and low initial DIC concentrations.本研究在pH 8.0及各物种耐受的更高酸碱度条件下，探究了溶解态无机碳（dissolved inorganic carbon, DIC）对3种赤潮甲藻——线形角藻（Ceratium lineatum）、三角异甲藻（Heterocapsa triquetra）及微小原甲藻（Prorocentrum minimum）——生长的影响。本实验选定的更高酸碱度条件为：线形角藻对应pH 8.55，其余两个物种对应pH 9.2。在pH 8.0（接近开阔海域的天然pH）条件下，所有供试物种的最大生长速率均可维持，直至总溶解态无机碳浓度分别降至约0.4 mM（线形角藻）与0.2 mM（其余两个物种）以下。供试物种的生长补偿点（维持细胞存活所需的无机碳浓度）分别约为0.18 mM（线形角藻）与0.05 mM（其余两个物种）。在更高酸碱度条件下，即便在极高溶解态无机碳浓度下，供试物种的最大生长速率仍低于pH 8.0条件下的水平，表明酸碱度对生长存在直接调控作用。此外，与pH 8.0条件相比，高酸碱度条件下维持生长所需的生物可利用无机碳（CO₂ + HCO₃⁻）浓度以及半饱和常数均显著升高。本研究设置了初始溶解态无机碳浓度分别为2.4 mM与1.2 mM的pH漂移实验，以探究在天然溶解态无机碳浓度范围内，酸碱度与溶解态无机碳何者为主要生长限制因子。在酸碱度显著升高之前，两组培养体系的生长速率无显著差异，且与初始溶解态无机碳浓度无关。本研究结果表明，3种供试赤潮甲藻的生长主要受酸碱度限制；仅在极高酸碱度且初始溶解态无机碳浓度较低的条件下，无机碳限制才会发挥次要作用。