Effects of the pH/ pCO ₂ control method on medium chemistry and phytoplankton growth

The control of key chemical parameters in phytoplankton cultures, such as pCO ₂, 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 CO ₂ fixation, nutrient uptake and, for coccolithophores, calcite precipitation. Even in cultures where pCO ₂ or pH is maintained constant, other chemical parameters change substantially at high cell densities. Experimentally we observed that various methods of adjustment of pCO ₂/pH – acid or base addition, use of buffers or pH-stats, or bubbling of CO ₂-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 pCO ₂/pH was controlled by these different methods. The pH/ pCO ₂ control method also did not affect the rates of growth or calcification of the coccolithophore Emiliania huxleyi at seawater pH. At lower pH/higher pCO ₂, 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 pCO ₂/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.