Abstract
Calcareous octocorals are ecologically important calcifiers, but little is known about their biomineralization physiology, relative to scleractinian corals. Many marine calcifiers promote calcification by up-regulating pH at calcification sites against the surrounding seawater. Here, we investigated pH in the red octocoral Corallium rubrum which forms sclerites and an axial skeleton. To achieve this, we cultured microcolonies on coverslips facilitating microscopy of calcification sites of sclerites and axial skeleton. Initially we conducted extensive characterisation of the structural arrangement of biominerals and calcifying cells in context with other tissues, and then measured pH by live tissue imaging. Our results reveal that developing sclerites are enveloped by two scleroblasts and an extracellular calcifying medium of pH 7.97 ± 0.15. Similarly, axial skeleton crystals are surrounded by cells and a calcifying medium of pH 7.89 ± 0.09. In both cases, calcifying media are more alkaline compared to calcifying cells and fluids in gastrovascular canals, but importantly they are not pH up-regulated with respect to the surrounding seawater, contrary to what is observed in scleractinians. This points to a potential vulnerability of this species to decrease in seawater pH and is consistent with reports that red coral calcification is sensitive to ocean acidification.
Highlights
Molecular phylogenetic research suggests that cnidarians originated 741 million years ago (Ma) and that the divergence between Hexacorallia and Octocorallia occurred prior to the Cambrian (543 Ma)[1]
In order to obtain structural information regarding the site of calcification, we investigated the interface between the tissues and the biominerals, together with the different steps involved in axial skeleton and sclerite formation
In the present study we developed the culture of microcolonies of Corallium rubrum which allowed us to obtain, for the first time in an Octocorallian species, direct in vivo pH measurements in different cell types and at sites of calcification
Summary
Molecular phylogenetic research suggests that cnidarians originated 741 million years ago (Ma) and that the divergence between Hexacorallia and Octocorallia occurred prior to the Cambrian (543 Ma)[1]. There is relatively little histological information about the site of calcification in octocorals i.e. about the relationship between the calcifying tissues and the two biominerals (axial skeleton and sclerites) that members of this group produce. 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 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]. Our results are discussed in the context of physiological regulation of biomineralization in scleractinian corals and octocorals
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