Organic alkalinity produced by phytoplankton and its effect on the computation of ocean carbon parameters

Abstract

In seawater dissolved organic acids produced by phytoplankton dissociate into conjugate bases and an amount of hydrogen ions that maintain electroneutrality, leaving the seawater alkalinity (AT) unchanged. However, the resulting conjugate bases react with protons during seawater titration and thereby contribute to the titration alkalinity (AT−ORG) whereas the contributions of other species (e.g., (Formula presented.), (Formula presented.)) to AT are proportionally lowered. Production of such dissolved organic acids was confirmed in each of six phytoplankton cultures and in a coastal environment. In phytoplankton monocultures with initial concentrations of ∼70 μM nitrate and ∼5 μM phosphate, the contribution of organic acids to the seawater alkalinity (AT−ORG) was found to be 15−40 μmol kg−1 on the complete consumption of added nutrients, with the magnitude of AT−ORG depending on the phytoplankton species involved. In the coastal environment the contribution of AT−ORG was as high as 15 μmol kg−1. Analysis of back titration data of culture and coastal samples further enabled identification of the functional groups responsible, which included two distinct charge groups (pKa1 = 4.4−4.9; pKa2 = 6.1−6.9). Thus, if the effect of organic acids on seawater AT is not accounted for, the concentrations of inorganic carbon components calculated from pairs of carbon parameters, including AT, will be inaccurate in culture studies and in studies of productive coastal environments.