Seawater carbonate chemistry and processes during experiments with Emiliania huxleyi (TW1), 2003

Precipitation of calcium carbonate by phytoplankton in the photic oceanic layer is an important process regulating the carbon cycling and the exchange of CO2 at the ocean-atmosphere interface. Previous experiments have demonstrated that, under nutrient-sufficient conditions, doubling the partial pressure of CO2 (pCO2) in seawater-a likely scenario for the end of the century-can significantly decrease both the rate of calcification by coccolithophorids and the ratio of inorganic to organic carbon production. The present work investigates the effects of high pCO2 on calcification by the coccolithophore Emiliania huxleyi (Strain TW1) grown under nitrogen-limiting conditions, a situation that can also prevail in the ocean. Nitrogen limitation was achieved in NO3-limited continuous cultures renewed at the rate of 0.5 d-1 and exposed to a saturating light level. pCO2 was increased from 400 to 700 ppm and controlled by bubbling CO2-rich or CO2-free air into the cultures. The pCO2 shift has a rapid effect on cell physiology that occurs within 2 cell divisions subsequent to the perturbation. Net calcification rate (C) decreased by 25% and, in contrast to previous studies with N-replete cultures, gross community production (GCP) and dark community respiration (DCR) also decreased. These results suggest that increasing pCO2 has no noticeable effect on the calcification/photosynthesis ratio (C/P) when cells of E. huxleyi are NO3-limited.

海洋透光层中浮游植物介导的碳酸钙沉积，是调控海洋碳循环及海-气界面二氧化碳（CO₂）交换的关键过程。既往实验已证实，在营养盐充足条件下，将海水中二氧化碳分压（pCO₂）提升至本世纪末的典型翻倍水平，可显著降低颗石藻的钙化速率，同时降低无机碳与有机碳的生产比值。本研究针对氮限制环境下培养的颗石藻物种赫氏颗石藻（Emiliania huxleyi，菌株TW1），探究了高pCO₂对其钙化作用的影响——该氮限制状态在海洋环境中同样普遍存在。实验采用硝酸根限制的连续培养体系，以0.5 d⁻¹的速率更新培养基，并设置饱和光照强度；通过向培养体系中通入富含CO₂或不含CO₂的空气，将pCO₂从400 ppm调控至700 ppm。pCO₂的改变会在扰动后的2次细胞分裂周期内，对细胞生理产生快速影响。净钙化速率（C）下降了25%；与此前营养盐充足培养的研究结果不同，总群落毛生产量（GCP）与暗群落呼吸速率（DCR）同样出现下降。上述结果表明，当赫氏颗石藻处于硝酸根限制状态时，pCO₂升高对钙化/光合作用比值（C/P）无显著影响。