Biogeosciences www.biogeosciences.net 1726-4170 1726-4189 6 7 2009 10.5194/bg-6-1199-2009 http://www.biogeosciences.net/6/1199/2009/ http://www.biogeosciences.net/6/1199/2009/bg-6-1199-2009.html http://www.biogeosciences.net/6/1199/2009/bg-6-1199-2009.pdf 1199 1207 2009-07-17 Effects of the pH/pCO2 control method on medium chemistry and phytoplankton growth D. Shi dshi@princeton.edu Y. Xu F. M. M. Morel Department of Geosciences, Princeton University, Princeton, New Jersey 08544, USA These two authors contributed equally to the work. The control of key chemical parameters in phytoplankton cultures, such as pCO2, pH and Ω (the saturation state of calcium carbonate), is made difficult by the interdependence of these parameters and by the changes resulting from the growth of the organisms, such as CO2 fixation, nutrient uptake and, for coccolithophores, calcite precipitation. Even in cultures where pCO2 or pH is maintained constant, other chemical parameters change substantially at high cell densities. Experimentally we observed that various methods of adjustment of pCO2/pH – acid or base addition, use of buffers or pH-stats, or bubbling of CO2-enriched air – can be used, the choice of one or the other depending on the goals of the experiments. At seawater pH, we measured the same growth rates in cultures of the diatom Thalassiosira weissflogii where the pCO2/pH was controlled by these different methods. The pH/pCO2 control method also did not affect the rates of growth or calcification of the coccolithophore Emiliania huxleyi at seawater pH. At lower pH/higher pCO2, in the E. huxleyi strain PLY M219, we observed increases in rates of carbon fixation and calcification per cell, along with a slight increase in growth rate, except in bubbled cultures. 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