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

Direct measurement of pCO2 in cultures of marine phytoplankton: how good is the estimate from pHNBS and single point titration of alkalinity?

David W. Crawford*, Paul J. Harrison

Department of Earth & Ocean Sciences (Oceanography), University of British Columbia, Vancouver, British Columbia, Canada V6T 1Z4
*Present address: Institute of Ocean Sciences, PO Box 6000, 9860 West Saanich Road, Sidney, British Columbia, Canada V8L 4B2. E-mail:

In physiological studies of marine phytoplankton in culture, the CO2 equilibrium is typically calculated from measurement of pHNBS (calibrated in dilute National Bureau of Standards buffers) and a single point titration of alkalinity (AT). 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 pHNBS = -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 pHNBS or 'seawater' (pHSWS) scales. With the currently accepted equilibrium constants on the 'total hydrogen ion' pH scale (pHTOT), and converting pHNBS to pHSWS using an apparent activity coefficient fH (optimum value 0.85) and then to pHTOT, excellent agreement was achieved between calculated and measured pCO2, both in sterile seawater and in cultures of the diatom Thalassiosira pseudonana. However, fH is generally unknown and is specific to electrode and electrode condition, making calculated pCO2 a rather nebulous concept. Without knowledge of fH, 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 CO2(aq) from pCO2 and the solubility coefficient. Alternatively, since analytical precision of calculated pCO2 was excellent, and accuracy is potentially good when fH is known, we advocate improved interdisciplinary collaboration in order to improve this pH & AT 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

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