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The alkalinity of the organic constituents of marine phytoplankton and their participation in the total alkalinity (TA) change of seawater during photosynthesis are carefully assessed. Quantification of the contribution of phytoplankton chlorophyll, proteins and phosphorus compounds to the hydrogen ion balance of seawater in terms of total inorganic nitrogen (∆[NT] = ∆NH4 +] + ∆[N2] + ∆[NO2–] + ∆[NO3–]) and total inorganic phosphorus (∆[PT]) changes during photosynthesis yielded that the organic components of marine phytoplankton are alkaline by –0.06 ×  ∆[NT] – 0.49 × ∆[PT], and that the potential total alkalinity (TAP) during photosynthesis is TAP = TA – [NH4– + 0.93 × [NO2–] + [NO3–] + 0.08 × [NT] + 0.23 × [PT] for unfiltered seawater samples and TAP = TA – [NH4–] + 0.93 × [NO2–] + [NO3–] + 0.02 × [NT] + 0.26 × [PT] for filtered seawater samples. These equations correct the traditionally used expression TAP = TA + [NO3–]. The TAP anomalies are produced, in order of increasing importance, by N2 fixation, DMSP production and CaCO3 fixation.