Organic alkalinity produced by phytoplankton and its effect on computations 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 (A(gamma)) unchanged. However, the resulting conjugate bases react with protons during seawater titration and thereby contribute to the titration alkalinity (A(gamma-ORG)) whereas the contributions of other species (e.g., CO32-, B(OH)(4)(-)) to A gamma 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 similar to 70 mu M nitrate and similar to 5 mu M phosphate, the contribution of organic acids to the seawater alkalinity (A(gamma-ORG)) was found to be 15-40 mu mol kg(-1) on the complete consumption of added nutrients, with the magnitude of A(gamma-ORG) depending on the phytoplankton species involved. In the coastal environment the contribution of A(gamma-ORG) was as high as 15 mu mol 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 (pK(a1) = 4.4-4.9; pK(a2) = 6.1-6.9). Thus, if the effect of organic acids on seawater A(gamma) is not accounted for, the concentrations of inorganic carbon components calculated from pairs of carbon parameters, including A(gamma), will be inaccurate in culture studies and in studies of productive coastal environments.