Effects of the pH/pCO₂ control method on medium chemistry and phytoplankton growth

Abstract. 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. In our hands, the bubbling of cultures seemed to induce more variable results than other methods of pCO₂/pH control. While highly convenient, the addition of pH buffers to the medium apparently induces changes in trace metal availability and cannot be used under trace metal-limiting conditions.