An aposymbiotic primary coral polyp counteracts acidification by active pH regulation

Yoshikazu Ohno,Atsushi Suzuki,Kazuhiko Sakai,Akira Iguchi,Chuya Shinzato,Takashi Nakamura,Mayuri Inoue

doi:10.1038/srep40324

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.

Highlights

Corals build their skeletons using extracellular calcifying fluid located in the tissue–skeleton interface
Ocean acidification, which is driven by seawater uptake of carbon dioxide (CO2) and is recognized as a severe threat to calcifying organisms[5], causes a decrease in the pH of the exterior seawater followed by that in the Ωarag, and decreases in the pH in the SCM (pHSCM) and calcification[6], suggesting that exterior seawater and calcifying fluid are strongly linked by a paracellular pathway[7,8,9,10]
We have shown that corals can regulate pHSCM more dynamically than was previously believed, which would be regulated by several types of pH-regulating ion transporters that coordinate with multicellular signaling in coral tissue

Summary

Introduction

Corals build their skeletons using extracellular calcifying fluid located in the tissue–skeleton interface. 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. It is worth noting that an in vivo pH imaging method using live coral tissues permits direct pH measurements at calcification sites, providing detailed information of pH dynamics in coral tissue[1,6] Applying this advanced physiological technique will facilitate deeper understanding both of the physiological characteristics of the SCM and the manner in which the pHSCM during coral calcification responds to ambient seawater conditions, such as changes seawater pH6

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Results