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Contribution of Marine Phytoplankton to Seawater Alkalinity

Contribution of Marine Phytoplankton to Seawater Alkalinity
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Rising atmospheric carbon dioxide (CO2) concentration over the past two centuries have led to greater CO2 uptake by the ocean, resulting in the increase of hydrogen ion concentration and a decrease in carbonate ion concentrations. The changes of carbonate chemistry in seawater such as ocean acidification can be described by measurements of CO2 parameters in which AT (total alkalinity) is the most measured parameter. Understanding of the changes in seawater chemistry requires the performance of a high quality measurement of the four measureable properties of the carbon dioxide system, namely AT, CT (total inorganic carbon), fCO2 (CO2 fugacity), and pH. This thesis work concentrates on improving our understanding of the carbonate chemistry by the estimation of unprecedented source of AT during phytoplankton growth, and the geological significance of such in over-determined oceanic AT. To attain these goals, first, the contribution of phytoplankton cells to AT was studied. Second, the contribution of freshly produced dissolved organic matter (DOM) to AT was estimated with six phytoplankton culture experiments. Finally, the geological significances of the unidentified sources of AT was investigated in the open ocean. Phytoplankton and bacterial cells make a significant contribution to the measured alkalinity of unfiltered seawater
their contribution is probably 3~5 
mol kg-1, 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. The accuracy of the stoichiometry between changes in AT and NO3- is important in estimating the oceanic uptake of fossil fuel CO2 and the calcium carbonate dissolution rate, where corrections for changes in AT induced by the formation and destruction of organic matter are critical. AT was directly measured by seawater titration and calculated from the pair of pH and CT. We estimated the relationship between AT-MEAS (or AT-CAL) and NO3- to confirm the established relationships based on culture experiments and the analysis of water column AT and NO3-. During photosynthesis, phytoplankton release dissolved organic compounds containing basic functional groups that readily react with protons during seawater titration, thereby contributing to alkalinity (AT). We have cultured six species which are distributed in worldwide and play an important role in biological carbon cycles. The magnitude of the contribution of dissolved organic compounds (DOM) to seawater alkalinity (AT
DOM) is estimated by differences between AT-MEAS and AT-CAL assuming that AT-CAL does not include the effects of DOM to the seawater alkalinity. The results show that AT-DOM is species dependent
suggesting that individual phytoplankton species exude dissolved organic compounds with unique proton accepting capacities. Contribution of dissolved organic matter to seawater alkalinity can affect the computations of the CO2 fugacity and carbonate ion concentration. Various scales of ocean acidification experiments are usually carried out under high initial nutrient concentrations, yielding high DOM production. Unless the AT-DOM is accounted for, this might lead to affecting the interpretation of ocean acidification experiments which are a high priority field of research. The stability of AT-DOM produced in these cultures depends on the lability of DOM. AT-DOM has been monitored from cultures with different bacterial activity. In cultures with high bacterial abundances, both AT-DOM and DOC concentration rapidly decreased with time compared to the cultures with relatively low biological activity. These results indicate that it is not easy to detect AT-DOM in the open ocean because a labile fraction of DOM is subject to rapid consumption by bacteria cells and rapid dilution by surrounding water that contains little labile DOM. Our results indicate that the contribution of dissolved organic matter to seawater alkalinity can be significant in varying scales of culture experiments, where it is currently unrecognized or considered insignificant. Without accurately accounting for the effect of dissolved organic matter on seawater AT, concentrations of inorganic carbon components calculated from pairs of carbon parameters including AT will suffer in considerable error.
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