Contribution of phytoplankton and bacterial cells to the measured alkalinity of seawater

We report the first direct measurements of the contribution of phytoplankton and bacterial cells to the measured alkalinity of unfiltered seawater. Phytoplankton and bacterial cells suspended in seawater make a significant contribution to the measured alkalinity of unfiltered seawater; their contribution is probably next to that of borate ion in most seawater samples. This nonnegligible contribution of particulate organic matter to the measured alkalinity is due largely to the presence of negatively charged surface groups on the phytoplankton and bacterial cells that react with protons during titration with hydrochloric acid. The contribution of organic particles to the measured alkalinity of unfiltered seawater could potentially be an important factor when evaluating the accuracy of presently available carbonate thermodynamic models using at-sea carbon system parameters that include measured alkalinity.