An aposymbiotic primary coral polyp counteracts acidification by active pH regulation

Abstract

Corals build their skeletons using extracellular calcifying fluid located in the tissue–skeleton interface. However, the mechanism by which corals control the transport of calcium and other ions from seawater and the mechanism of constant alkalization of calcifying fluid are largely unknown. To address these questions, we performed direct pH imaging at calcification sites (subcalicoblastic medium, SCM) to visualize active pH upregulation in live aposymbiotic primary coral polyps treated with HCl-acidified seawater. Active alkalization was observed in all individuals using vital staining method while the movement of HPTS and Alexa Fluor to SCM suggests that certain ions such as H+ could diffuse via a paracellular pathway to SCM. Among them, we discovered acid-induced oscillations in the pH of SCM (pHSCM), observed in 24% of polyps examined. In addition, we discovered acid-induced pH up-regulation waves in 21% of polyps examined, which propagated among SCMs after exposure to acidified seawater. Our results showed that corals can regulate pHSCM more dynamically than was previously believed. These observations will have important implications for determining how corals regulate pHSCM during calcification. We propose that corals can sense ambient seawater pH via their innate pH-sensitive systems and regulate pHSCM using several unknown pH-regulating ion transporters that coordinate with multicellular signaling occurring in coral tissue.

Primary Life Stage Boron Isotope and Trace Elements Incorporation in Aposymbiotic Acropora millepora Coral under Ocean Acidification and Warming

“…Furthermore, high-resolution pH imaging of primary polyp juveniles of aposymbiotic A. digitifera indicated active pH up-regulation at the site of calcification (Ohno et al, 2017). Our results confirm these observations in that our coral species also has a well-regulated internal pH at the calcification sites, which is considerably higher than the external seawater pH.…”

“…This response also likely increased the pH up-regulation resulting in a reconstructed pH (8.35, condition 4) that is even more elevated than the control condition (8.32). The enrichment of the δ 11 B ratio (conditions 2 and 4), is likely related to metabolic processes of the A. millepora juveniles and further support the conclusion that certain coral species have the ability to adapt to increasing OW and OA by up-regulating its internal pH (McCulloch et al, 2012;Ohno et al, 2017). We thus provide additional evidence of pH homeostasis or elevated pH control in scleractinian corals even at the primary polyp juvenile life-stage subjected to OA and OW.…”

Perspective on the response of marine calcifiers to global warming and ocean acidification—Behavior of corals and foraminifera in a high CO2 world “hot house”

“…Biomineralization of corals based on analyses of Ca isotopes It has been suggested that acidified seawater significantly decreases the growth rate of primary polyps, as reported for adult corals. However, recent studies suggest that corals regulate their internal pH and then counteract any acidification (Ohno et al 2017a;McCulloch et al 2012). In fact, the mechanism by which corals control the transport of Ca 2+ and other ions from seawater are largely unknown.…”

In vivo pH measurement at the site of calcification in an octocoral

“…Recent advances in studying the calcifying tissues and pH regulation at the site of calcification in scleractinian corals have been facilitated by the use of live tissue imaging on coral microcolonies cultivated on glass slides or coverslips. Indeed, this method of culture, initially developed for ultrastructural studies 15 has more recently been used in various studies (Table S1 ) including pH measurements by live tissue imaging with fluorescent dyes and confocal microscopy 5 , 7 , 12 , 13 , 16 . The use of live tissue imaging of microcolonies on glass slides has allowed direct observations and physiological measurements of the skeleton-tissue interface where calcification occurs.…”