English: Illustration of a thought experiment on the energetic effort for armor construction in three major shell-forming phytoplankton taxa as a function of carbonate chemistry conditions.

The frustule (diatom shell) is considered the most inexpensive armor under all circumstances because diatoms typically outcompete all other groups when silicate is available. The coccosphere is relatively inexpensive under sufficient [CO2], high [HCO3−], and low [H+] because substrate is saturating and protons are easily released into seawater (Taylor et al., 2011). In contrast, the construction of thecal elements, which are organic (cellulose) plates that constitute the dinoflagellate shell, should rather be favored at high H+ concentrations because these usually coincide with high [CO2]. Under these conditions dinoflagellates could down-regulate the energy-consuming operation of carbon concentrating mechanisms to fuel the production of organic source material for their shell. Therefore, a shift in carbonate chemistry conditions toward high [CO2] may promote their competitiveness relative to coccolithophores. It must be kept in mind, however, that such a hypothetical gain in competitiveness due to altered carbonate chemistry conditions would not automatically lead to dinoflagellate dominance because a huge number of factors other than carbonate chemistry have an influence on species composition as well.

• A.R. Taylor, A. Chrachri, G. Wheeler, H. Goddard, C. Brownlee (2011) "A voltage-gated H+ channel underlying pH homeostasis in calcifying coccolithophores", PLoS Biology, 9: e1001085. doi:10.1371/journal.pbio.1001085.