| Summary: | The variability in the extracellular release of organic ligands by <em>Emiliania huxleyi</em> under four different pCO<sub>2</sub> scenarios (225, 350, 600 and 900 μatm), was determined. Growth in the batch cultures was promoted by enriching them only with major nutrients and low iron concentrations. No chelating agents were added to control metal speciation. During the initial (IP), exponential (EP) and steady (SP) phases, extracellular release rates, normalized per cell and day, of dissolved organic carbon (DOC<em><sub>ER</sub></em>), phenolic compounds (PhC<em><sub>ER</sub></em>), dissolved combined carbohydrates (DCCHO<em><sub>ER</sub></em>) and dissolved uronic acids (DUA<em><sub>ER</sub></em>) in the exudates were determined.<br /> The growth rate decreased in the highest CO<sub>2</sub> treatment during the IP (＜48 h), but later increased when the exposure was longer (more than 6 days). DOC<em><sub>ER</sub></em> did not increase significantly with high pCO<sub>2</sub>. Although no relationship was observed between DCCHO<em><sub>ER</sub></em> and the CO<sub>2</sub> conditions, DCCHO was a substantial fraction of the freshly released organic material, accounting for 18% to 37%, in EP, and 14% to 23%, in SP, of the DOC produced. Growth of <em>E. huxleyi</em> induced a strong response in the PhC<em><sub>ER</sub></em> and DUA<em><sub>ER</sub></em>. While in EP, PhC<em><sub>ER</sub></em> were no detected, the DUA<em><sub>ER</sub></em> remained almost constant for all CO<sub>2</sub> treatments. Increases in the extracellular release of these organic ligands during SP were most pronounced under high pCO<sub>2</sub> conditions. Our results imply that, during the final growth stage of <em>E. huxleyi</em>, elevated CO<sub>2</sub> conditions will increase its excretion of acid polysaccharides and phenolic compounds, which may affect the biogeochemical behavior of metals in seawater.
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