Paper Reviewed
Sevilgen, D.S., Venn, A.A., Hu, M.Y., Tambutté, E., de Beer, D., Planas-Bielsa, V. and Tambutté, S. 2019. Full in vivo characterization of carbonate chemistry at the site of calcification in corals. Science Advances 5: eaau7447.

The impact of predicted ocean acidification on coral calcification remains a matter of debate given the wide range of responses that have been observed in laboratory and field studies to date. However, early pessimism on this topic has given way to optimism in recent years as a growing body of research has demonstrated a biological control on calcification. The latest study to shed some light in this regard comes from Sevilgen et al. (2019).

Writing in the journal Science Advances, the team of seven researchers notes that coral skeleton formation (i.e., calcification) takes place within an extra-cellular calcifying medium (ECM), a "semi-enclosed compartment of a few nano- to micrometers thickness that is 'sandwiched' between the skeleton and the calcifying calicoblastic epithelium" and "separates the ECM from a direct contact with the surrounding [seawater] environment." It is widely accepted by the scientific community that the chemical composition of the ECM is the key factor controlling calcification and that corals can biochemically modify the carbonate chemistry within the ECM by raising the pH, carbonate (CO3^2-) and Ca²⁺ values within the ECM (relative to surrounding seawater) in order to maintain favorable rates of calcification (i.e., a higher aragonite saturation state, Omega;arag). However, evidence of such alterations has primarily come from indirect approaches using geochemical proxies as opposed to direct measurements in the ECM itself -- until now.

In a first-of-a-kind study, Sevilgen et al. (2019) inserted microsensors capable of measuring pH, CO3^2- and Ca²⁺ directly into the ECM on the growing edge of Stylophora pistillata corals to "quantify the full carbonate system of the ECM based solely on direct in vivo measurements" (dissolved inorganic carbon [DIC] and Omega;arag were calculated using the in vivo data using standard equations of carbonate chemistry). Results indicated that there was an immediate increase in the three measured parameters upon insertion of the microsensors into the ECM, as well as an immediate decrease when the sensors were retracted out of the ECM and back into the surrounding seawater. More specifically, the authors report that pH was elevated by approximately 0.44 units, CO3^2- by 468 µmol kg-1and Ca²⁺ by 2 mM in the ECM relative to surrounding seawater. Consequently, carbonate chemistry calculations using these measurements revealed that [DIC] was elevated by 43% and Omega;arag by four-fold in the ECM.

The results of this study solidify the thesis that corals can modify the carbonate chemistry parameters within their ECM to create a more favorable medium for calcification. Such biologic control will likely play a key role in their ability to adapt to future projections of ocean acidification, helping them to maintain calcification as oceanic pH declines.