Effects of the pH/ip/iCOsub2/sub control method on medium chemistry and phytoplankton growth

摘要

The control of key chemical parameters in phytoplankton cultures, such aspCO₂, pH and Ω (the saturation state of calcium carbonate), ismade difficult by the interdependence of these parameters and by the changesresulting from the growth of the organisms, such as CO₂ fixation,nutrient uptake and, for coccolithophores, calcite precipitation. Even incultures where pCO₂ or pH is maintained constant, other chemicalparameters change substantially at high cell densities. Experimentally weobserved that various methods of adjustment of pCO₂/pH – acid or baseaddition, 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 theexperiments. At seawater pH, we measured the same growth rates in culturesof the diatom Thalassiosira weissflogii where the pCO₂/pH was controlled by these differentmethods. The pH/pCO₂ control method also did not affect the rates ofgrowth or calcification of the coccolithophore Emiliania huxleyi at seawater pH. At lowerpH/higher pCO₂, in the E. huxleyi strain PLY M219, we observed increases in ratesof carbon fixation and calcification per cell, along with a slight increasein growth rate, except in bubbled cultures. In our hands, the bubbling ofcultures seemed to induce more variable results than other methods ofpCO₂/pH control. While highly convenient, the addition of pH buffers tothe medium apparently induces changes in trace metal availability and cannotbe used under trace metal-limiting conditions.