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., CO32−, B(OH)4−) 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.

Characterization of a novel autonomous analyzer for seawater total alkalinity: Results from laboratory and field tests

“…An adsorption of dissolved organic matter (DOM) onto the filter material interfering with A T measurement is not given to our knowledge. Furthermore, within the scope of this study, DOM contributions to A T are not significant in the open ocean (e.g., McElligott et al 1998;Lee et al 2000;Millero et al 2002, Ko et al 2016).…”

Alkalinity, inorganic carbon and CO2 flux variability during extreme rainfall years (2010-2011) in two polluted tropical estuaries NE Brazil

“…Such underestimation in the pCO 2 may vary around 30% to 50% (Kuliński et al, 2014). From the low oxygen saturation found in the Barra de Jangadas estuary, <30% throughout the year, and in the Capibaribe estuary during the dry season, as well as the high DBO 5,20 values and total phosphorus it is possible to assume that the alkalinity from dissolved organic matter probably influence the alkalinity regulation in these estuaries as has recently been discussed by Kullinsky et al (2014), Abril et al (2014) and Ko et al, (2016). So further studies are necessary to understand which processes control the alkalinity and the inorganic and organic carbon variability in the Capibaribe and BJ estuaries, including possible submarine groundwater discharge and organic alkalinity contributions.…”

“…The prevalence of anaerobic reactions in anoxic estuarine waters and sediments is a drawback to carbonate system calculations using pH and total alkalinity since the real amount of carbonate alkalinity is unknown. The contribution of the organic alkalinity to the TA may be relevant, thus leading to an underestimation of the calculated pCO 2 values using pH and TA (Ko et al, 2016). Such underestimation in the pCO 2 may vary around 30% to 50% (Kuliński et al, 2014).…”

Deviation of Boron Concentration From Predictions Using Salinity in Coastal Environments

“…The resulting conjugate bases in seawater probably react with protons during seawater titration. Therefore, if they are abundant in coastal systems, they may contribute as much as 5–10 μmol/kg to A T (Ko et al, ). The high concentration of particulate organic matter and bacterial cells in these environments can also contribute to A T (Kim et al, ).…”