Currently, almost any article searched with the keywords “calcification” and “ocean acidification” (OA) will inevitably bring up a link between calcification and bulk seawater carbonate saturation state (Ω) as a justification for the study. Therefore, it seems timely to highlight that from a physiological point of view, there are mechanisms that invoke control of calcification by seawater bicarbonate (HCO3−) and proton (H+) concentrations rather than Ω in many marine organisms. While we agree with Waldbusser et al. (2015c) that there is a vast diversity of marine calcifiers able to regulate carbonate chemistry internally at the site of calcification, it seems adequate to assume a similar underlying physiological response to OA unless shown otherwise. We do not necessarily interpret the seeming Ω sensitivity of calcification as substrate limitation, as suggested by Waldbusser et al. (2015c), we simply question the mechanisms that would invoke bulk seawater Ω as a driving force of calcification in the first place. Waldbusser et al. (2015c) rightfully note the co-variability of seawater Ω with the ratio of [HCO3−] to [H+]. Recently, Bach (2015) demonstrated that a rearrangement of carbonate system equations results in an inevitable proportionality between Ω and the ratio of [HCO3−] to [H+]. Owing to this proportionality, calcification rates will always correlate as well with [HCO3−]/[H+] as they do with [CO32−] or Ω when temperature, salinity, and pressure are constant (Figure 1).
Cyronak T., Schulz K. G & Jokiel J. L. in press. Response to Waldbusser et al. (2015c): “Calcium carbonate saturation state: on myths and this or that stories”. ICES Journal of Marine Science. Article (subscription required).
