Anthropogenic carbon dioxide (CO2) dissolves into the ocean and leads to a suite of changes in the carbonate chemistry of surface seawater, collectively known as ocean acidification. Since marine phytoplankton are responsible for about half of the global primary production, their response to ocean acidification is likely to be significant for the productivity and/or structure of marine ecosystems.;In vast areas of the oceans, the vanishingly low concentration of iron (Fe), an essential trace nutrient, often limits the growth of marine phytoplankton. It is known that the availability of Fe to phytoplankton depends on its chemistry in seawater, which is highly sensitive to changes in pH. Because of the abundance of Fe in the photosynthetic apparatus, it is also possible that its cellular requirement in phytoplankton may be affected by the ambient CO2 concentration. In addition, the high Fe requirement of nitrogen (N2)-fixing organisms may make them particularly sensitive to changes in Fe bioavailability brought about by acidification.;In this thesis, I address the question of how seawater acidification will alter the availability and requirement of Fe in marine phytoplankton by conducting laboratory experiments with model organisms under well defined conditions. These laboratory experiments are complemented by some manipulation experiments with field samples of natural seawater. In all these experiments, I aim at uncovering the chemical and biological mechanisms responsible for the observed effects.;Experimentally, various methods of manipulating seawater pCO 2/pH -- bubbling of CO2-enriched air, acid/base addition and use of buffers -- give the same rates of growth in the diatom Thalassiosira weissflogii and the coccolithophore Emiliania huxleyi and of calcification in the calcifier. However, the bubbling of cultures tends to induce more variable results and the presence of organic buffers changes the availability of trace metals.;As seawater acidifies, the uptake of Fe chelated by a variety of organic ligands decreases in model diatoms and coccolithophores. Such an effect of ocean acidification results from changes in Fe chemistry caused by the decreasing pH and not a physiological response of the phytoplankton. In agreement with the laboratory data, a slower rate of Fe uptake by T. weissflogii with decreasing pH is observed in both coastal and oceanic Atlantic surface water samples where Fe bound to natural Fe-chelating ligands. The Fe requirement of model phytoplankton, however, remains unchanged with increasing pCO2.;The rates of both N2 fixation and growth of the Fe-limited cyanobacterium Trichodesmium decline at high pCO 2/low pH even though more Fe is added to maintain constant Fe availability. To compensate for the decreased rate of N2 fixation, which is due to decreasing pH rather than increasing pCO 2, the diazotroph synthesizes additional nitrogenase enzyme at the expense of Fe-containing photosynthetic proteins. Consequently the growth rate of the Fe-deficient N2-fixer decreases in acidified medium.;Overall, this thesis contributes to our understanding of the response of marine primary producers to ocean acidification caused by the ongoing increase in atmospheric CO2. It demonstrates that how the CO2-driven changes in the seawater chemistry can profoundly affect marine phytoplankton through both chemical and biological processes.
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