Reconsidering the role of carbonate ion concentration in calcification by marine organisms

Marine organisms precipitate 0.5–2.0 Gt of carbon as calcium carbonate (CaCO₃) every year with a profound impact on global biogeochemical element cycles. Biotic calcification relies on calcium ions (Ca²⁺) and usually on bicarbonate ions (HCO₃⁻) as CaCO₃ substrates and can be inhibited by high proton (H⁺) concentrations. The seawater concentration of carbonate ions (CO₃²⁻) and the CO₃²⁻-dependent CaCO₃ saturation state (Ω_CaCO3) seem to be irrelevant in this production process. Nevertheless, calcification rates and the success of calcifying organisms in the oceans often correlate surprisingly well with these two carbonate system parameters. This study addresses this dilemma through the rearrangement of carbonate system equations which revealed an important proportionality between [CO₃²⁻] or Ω_CaCO3and the ratio of [HCO₃⁻] to [H⁺]. Due to this proportionality, calcification rates will always correlate as well with [HCO₃⁻] / [H⁺] as they do with [CO₃²⁻] or Ω_CaCO3 when temperature, salinity, and pressure are constant. Hence, [CO₃²⁻] and Ω_CaCO3 may simply be very good proxies for the control by [HCO₃⁻] / [H⁺], where [HCO₃⁻] serves as the inorganic carbon substrate and [H⁺] functions as a calcification inhibitor. If the "substrate–inhibitor ratio" (i.e., [HCO₃⁻] / [H⁺]) rather than [CO₃²⁻] or Ω_CaCO3 controls biotic CaCO₃ formation, then some of the most common paradigms in ocean acidification research need to be reviewed. For example, the absence of a latitudinal gradient in [HCO₃⁻] / [H⁺] in contrast to [CO₃²⁻] and Ω_CaCO3 could modify the common assumption that high latitudes are affected most severely by ocean acidification.