Differential gene expression in skeletal organic matrix proteins of scleractinian corals associated with mixed aragonite/calcite skeletons under low mMg/Ca conditions.

Ikuko Yuyama,Tomihiko Higuchi

doi:10.7717/peerj.7241

Ikuko Yuyama, Tomihiko Higuchi

Open Access

https://doi.org/10.7717/peerj.7241

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Journal: PeerJ	Publication Date: Jul 15, 2019
Citations: 20	License type: CC BY 4.0

Affiliation: The University of Tokyo, University of Tsukuba

Abstract

Although coral skeletons generally comprise aragonite crystals, changes in the molar Mg/Ca ratio (mMg/Ca) in seawater result in the incorporation of calcite crystals. The formation mechanism of aragonite and calcite crystals in the scleractinian coral Acropora tenuis was therefore investigated by RNA-seq analysis, using early growth stage calcite (mMg/Ca = 0.5) and aragonite (mMg/Ca = 5.2)-based corals. As a result, 1,287 genes were up-regulated and 748 down-regulated in calcite-based corals. In particular, sixty-eight skeletogenesis-related genes, such as ectin, galaxin, and skeletal aspartic acid-rich protein, were detected as up-regulated, and six genes, such as uncharacterized skeletal organic matrix protein 5, down-regulated, in low-Mg/Ca conditions. Since the number of down-regulated genes associated with the skeletal organic matrix of aragonite skeletons was much lower than that of up-regulated genes, it is thought that corals actively initiate construction of an aragonite skeleton by the skeletal organic matrix in low-Mg/Ca conditions. In addition, different types of skeletal organic matrix proteins, extracellular matrix proteins and calcium ion binding proteins appeared to change their expression in both calcite-formed and normal corals, suggesting that the composition of these proteins could be a key factor in the selective formation of aragonite or calcite CaCO3.

Highlights

Calcium carbonate deposition by scleractinian corals is directly linked to the development of coral reefs, providing the structural basis of high species diversity coral reef ecosystems
Of the two major crystal types of calcium carbonate produced by marine calcifying organisms, aragonite is common in modern scleractinian corals, calcite occurs in precious corals as well as foraminifera (Porter, 2007; Porter, 2010)
X-ray diffraction (XRD) revealed that the coral skeleton produced in m Mg/Ca = 5.2 had an aragonite specific pattern, whereas that produced in molar Mg/Ca ratio (mMg/Ca) = 0.5 had a calcite specific pattern (Fig. 1)

Summary

Introduction

Calcium carbonate deposition by scleractinian corals is directly linked to the development of coral reefs, providing the structural basis of high species diversity coral reef ecosystems. Of the two major crystal types of calcium carbonate produced by marine calcifying organisms, aragonite is common in modern scleractinian corals, calcite occurs in precious corals (genus Corallium) as well as foraminifera (Porter, 2007; Porter, 2010). Differential gene expression in skeletal organic matrix proteins of scleractinian corals associated with mixed aragonite/calcite skeletons under low mMg/Ca conditions. Fossil records indicate that Mg concentrations, resulting from fluctuations in past seawater mMg/Ca ratios, have impacted greatly on the selective nucleation of aragonite/calcite crystals (Ries, 2010). During the Cretaceous period, characterized by low mMg/Ca, scleractinian corals (with aragonite skeletons) have been poorly reported form the fossil record whereas rudist bivalves, which produced calcite, were the main reef builder (Stanley & Hardie, 1998; Stanley Jr, 2003; Janiszewska et al, 2017)

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