Abstract
Calcification processes are largely unknown in scleractinian corals. In this study, live confocal imaging was used to elucidate the spatiotemporal dynamics of the calcification process in aposymbiotic primary polyps of the coral species Acropora digitifera. The fluorophore calcein was used as a calcium deposition marker and a visible indicator of extracellular fluid distribution at the tissue-skeleton interface (subcalicoblastic medium, SCM) in primary polyp tissues. Under continuous incubation in calcein-containing seawater, initial crystallization and skeletal growth were visualized among the calicoblastic cells in live primary polyp tissues. Additionally, the distribution of calcein-stained SCM and contraction movements of the pockets of SCM were captured at intervals of a few minutes. Our experimental system provided several new insights into coral calcification, particularly as a first step in monitoring the relationship between cellular dynamics and calcification in vivo. Our study suggests that coral calcification initiates at intercellular spaces, a finding that may contribute to the general understanding of coral calcification processes.
Highlights
Coral skeletons contribute to maintain a high level of biodiversity in ecosystems associated with coral reef formations
Calcein is a water-soluble molecule that cannot penetrate the cell membrane. These characteristics have led to the use of calcein for hard tissue staining in a wide variety of marine organisms
We mainly used a spinning-disk confocal imaging system equipped with an Eclipse Ti-U inverted epifluorescence microscope (Nikon, Tokyo, Japan), hand-made reflection light, CSU-X1 laser-scanning unit (Yokogawa, Tokyo, Japan), and ImagEM C9100-13 electron-multiplying charge-couple device (EM-CCD) camera (Hamamatsu Photonics, Hamamatsu, Japan)
Summary
Coral skeletons contribute to maintain a high level of biodiversity in ecosystems associated with coral reef formations. Numerous studies have investigated the physiological and molecular aspects of coral calcification mechanisms, the actual calcification mechanism, the initial nucleation and subsequent calcium carbonate (CaCO3) crystal deposition, has been debated for more than a century [1]. Mechanistic aspects of this process, those concerning calcium transport at the level of the calcifying cells (i.e., calicoblastic cells), remain unclear. The present study proposes a detailed observational method of coral calcification in live tissues of early life stages which enables us to deeply understand the physiological aspects of coral calcification
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