MEPS 158:61-74 (1997)  -  doi:10.3354/meps158061

In physiological studies of marine phytoplankton in culture, the CO₂ equilibrium is typically calculated from measurement of pH_NBS (calibrated in dilute National Bureau of Standards buffers) and a single point titration of alkalinity (A_T). This approach has widespread appeal because it is simple, inexpensive, and requires very low sample volumes. However, its continued application ignores advances in analytical and theoretical oceanic CO₂ chemistry that suggest fundamental flaws in the assumption pH_NBS = -log[H⁺] as a consequence of the high ionic strength of seawater and liquid residual junction errors in the electrode. Here we compare directly measured pCO₂ in sterile artificial seawater with calculated values, adopting the usual assumptions and commonly used equilibrium constants. Calculated values either overestimated (ca +15 to +23% error) or underestimated (-9 to -11% error) directly measured pCO₂ depending upon whether constants were derived, respectively, on the pH_NBS or 'seawater' (pH_SWS) scales. With the currently accepted equilibrium constants on the 'total hydrogen ion' pH scale (pH_TOT), and converting pH_NBS to pH_SWS using an apparent activity coefficient f_H (optimum value 0.85) and then to pH_TOT, excellent agreement was achieved between calculated and measured pCO₂, both in sterile seawater and in cultures of the diatom Thalassiosira pseudonana. However, f_H is generally unknown and is specific to electrode and electrode condition, making calculated pCO₂ a rather nebulous concept. Without knowledge of f_H, calculated pCO₂ had a total uncertainty of an order (~120 ppmv at atmospheric equilibrium 360 ppmv) similar to the variation in atmospheric pCO₂ between glacial periods and the present (~160 ppmv). This method therefore clearly lacks the resolution required to address the biogeochemical significance of key physiological questions. Future studies should measure pCO₂ directly, and then if required calculate CO_2(aq) from pCO₂ and the solubility coefficient. Alternatively, since analytical precision of calculated pCO₂ was excellent, and accuracy is potentially good when f_H is known, we advocate improved interdisciplinary collaboration in order to improve this pH & A_T approach as a simple but effective tool for the study of marine phytoplankton physiology under controlled conditions.

Dissolved carbon dioxide · Marine phytoplankton · Cultures · pCO₂ · pH · Alkalinity · Total CO₂