Effects of the pH/&amp;lt;i&amp;gt;p&amp;lt;/i&amp;gt;CO&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt; control method on medium chemistry and phytoplankton growth

D Shi,Y Xu,F M M Morel

doi:10.5194/bg-6-1199-2009

Abstract

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.

Highlights

There is a growing consensus that the ongoing increase in atmospheric carbon dioxide, CO2, as a result of anthropogenic activities will lead to a variety of physical, chemical and physiological effects on marine phytoplankton (Feely et al, 2004; Doney, 2006)
Experiments with T. weissflogii started with 20–100 cells ml−1 and those with E. huxleyi started with 1000 cells ml−1 for Center for Culture of Marine Phytoplankton (CCMP) 374 and 150– 500 cells ml−1 for PLY M219
Cell number and volume, which can be converted to biomass, were determined using a Z2 Coulter® Particle Count and Size Analyzer (Beckman), and the specific growth rates were computed during exponential growth with a linear regression of natural logarithm vs. time

Summary

Introduction

There is a growing consensus that the ongoing increase in atmospheric carbon dioxide, CO2, as a result of anthropogenic activities will lead to a variety of physical, chemical and physiological effects on marine phytoplankton (Feely et al, 2004; Doney, 2006). Upon dissolution in the surface ocean, the additional CO2 causes re-equilibration of the seawater carbonate system, increasing the concentrations of aqueous CO2 (usually quantified by its partial pressure pCO2) and bicarbonate ion, HCO−3 , while decreasing that of the carbonate ion, CO23− These changes in the distribution of the various species of the dissolved inorganic carbon, DIC, which is the main acid-base buffer of seawater, result in an increase in the hydrogen ion, H+, concentration – i.e., a decrease in pH- and these interrelated chemical changes are commonly referred to collectively as ocean acidification. Of all these effects, the elevated pCO2 and the lowered carbonate ion concentration have received the most attention. Changes in pH may affect a number of physiological processes, the activity of important extracellular enzymes (Xu et al, 2006)

Methods

Results

Conclusion