Abstract
Coral calcification is dependent on the mutualistic partnership between endosymbiotic zooxanthellae and the coral host. Here, using newly developed geochemical proxies (δ11B and B/Ca), we show that Porites corals from natural reef environments exhibit a close (r2 ∼0.9) antithetic relationship between dissolved inorganic carbon (DIC) and pH of the corals’ calcifying fluid (cf). The highest DICcf (∼ × 3.2 seawater) is found during summer, consistent with thermal/light enhancement of metabolically (zooxanthellae) derived carbon, while the highest pHcf (∼8.5) occurs in winter during periods of low DICcf (∼ × 2 seawater). These opposing changes in DICcf and pHcf are shown to maintain oversaturated but stable levels of carbonate saturation (Ωcf ∼ × 5 seawater), the key parameter controlling coral calcification. These findings are in marked contrast to artificial experiments and show that pHcf upregulation occurs largely independent of changes in seawater carbonate chemistry, and hence ocean acidification, but is highly vulnerable to thermally induced stress from global warming.
Highlights
Coral calcification is dependent on the mutualistic partnership between endosymbiotic zooxanthellae and the coral host
The exact mechanism(s) by which coral calcification is linked to endosymbiont photosynthesis has, remained largely enigmatic at the polyp scale (Fig. 1) the zooxanthellae are physically separated from the site of calcification[13,14,15] and, apart from pH, few direct measurements exist[16] of the chemical conditions necessary to constrain the biocalcification process
We find that over annual timescales there is an inverse correlation between pH of the calcifying fluid (pHcf) and DIC in the calcifying fluid (DICcf)
Summary
Coral calcification is dependent on the mutualistic partnership between endosymbiotic zooxanthellae and the coral host. The highest DICcf (B Â 3.2 seawater) is found during summer, consistent with thermal/light enhancement of metabolically (zooxanthellae) derived carbon, while the highest pHcf (B8.5) occurs in winter during periods of low DICcf (B Â 2 seawater) These opposing changes in DICcf and pHcf are shown to maintain oversaturated but stable levels of carbonate saturation (Ocf B Â 5 seawater), the key parameter controlling coral calcification. Of equal but largely overlooked importance, are the mechanisms via which the various pH-dependent species of DIC (that is, CO2, HCO3À or CO32 À ) are produced, transported, and inter-converted at the site of calcification It has long been recognized[12,13] that light plays a key role in driving rates of calcification, and that light-enhanced calcification occurs as a result of the photosynthetic activity of endosymbiont dinoflagellates (zooxanthellae), providing both energy and additional carbon needed to drive calcification. This acts to maintain relatively stable levels of aragonite saturation in the calcifying fluid, and near-optimal rates of coral calcification, despite large seasonally driven variations in metabolically supplied DIC
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
