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<p>A mesocosm experiment was conducted in Wuyuan Bay (Xiamen), China, to investigate the effects of elevated <i>p</i>CO₂ on the phytoplankton species <i>Phaeodactylum tricornutum</i> (<i>P. tricornutum</i>), <i>Thalassiosira weissflogii</i> (<i>T. weissflogii</i>) and <i>Emiliania huxleyi</i> (<i>E. huxleyi</i>) and their production ability of dimethylsulfide (DMS), dimethylsulfoniopropionate (DMSP), as well as four halocarbon compounds, bromodichloromethane (CHBrCl₂), methyl bromide (CH₃Br), dibromomethane (CH₂Br₂) and iodomethane (CH₃I). Over a period of 5 weeks, <i>P. tricornuntum</i> outcompeted <i>T. weissflogii</i> and <i>E. huxleyi</i>, comprising more than 99&thinsp;% of the final biomass. During the logarithmic growth phase (phase I), mean DMS concentration in high <i>p</i>CO₂ mesocosms (1000&thinsp;µatm) was 28&thinsp;% lower than that in low <i>p</i>CO₂ mesocosms (400&thinsp;µatm). Elevated <i>p</i>CO₂ led to a delay in DMSP-consuming bacteria concentrations attached to <i>T. weissflogii</i> and <i>P. tricornutum</i> and finally resulted in the delay of DMS concentration in the high <i>p</i>CO₂ treatment. Unlike DMS, the elevated <i>p</i>CO₂ did not affect DMSP production ability of <i>T. weissflogii</i> or <i>P. tricornuntum</i> throughout the 5-week culture. A positive relationship was detected between CH₃I and <i>T. weissflogii </i>and <i>P. tricornuntum</i> during the experiment, and there was a 40&thinsp;% reduction in mean CH₃I concentration in the high <i>p</i>CO₂ mesocosms. CHBrCl₂, CH₃Br, and CH₂Br₂ concentrations did not increase with elevated chlorophyll <i>a</i> (Chl <i>a</i>) concentrations compared with DMS(P) and CH₃I, and there were no major peaks both in the high <i>p</i>CO₂ or low <i>p</i>CO₂ mesocosms. In addition, no effect of elevated <i>p</i>CO₂ was identified for any of the three bromocarbons.</p>