Compositional information on the calcification medium in reef-building corals derived from identification of mineral phases

Reef-building corals are animals inhabiting the shallow sea in the tropical or sub-tropical regions and have the external skeletons formed by calcium carbonate. It is worthy noted that coral skeletons are formed by mineral “aragonite”, although calcite is the thermodynamically stable mineral phase of calcium carbonates in the coral living environment. It has been suggested that aragonite crystallizes in the so-called “calcification medium” filling the small calcification space less than a few micron meters between skeletons and living organisms, however, there is a little knowledge about mechanisms and process of coral biomineralization. Compositional information on the calcification medium could be an important factor controlling crystallization of aragonite. However, the information about the calcification medium is little, because the calcification space is too small to be measured directly. Thus, in chapter 2, I observed skeletal textures and crystal morphology of the massive reef-building corals from the microto nano scale in order to obtain some information about the growth mechanism of aragonite. Furthermore, identification of mineral phases could be important to estimate chemical composition of the calcification medium, because the mineral phases forming the skeletons must be crystallized in it. Thus, I performed two kinds of experimental approaches based on mineral identification in the skeletons to obtain some compositional information on the calcification medium in chapter 3 and 4. The skeletal texture and crystal morphology of the massive reef-building coral Porites lobata were observed from the microto nanometer scale using an analytical transmission electron microscope (ATEM). The skeletal texture consists of centers of calcification (COCs) and fiber areas. Fiber areas contain bundles of needle-like aragonite crystals that are elongated along the crystallographic c-axis and are several hundred nanometers to one micrometer in width and several micrometers in length. The size distribution of aragonite crystals is relatively homogeneous in the fiber area. Growth lines are observed sub-perpendicular to the direction of aragonite growth. These growth lines occur in 1–2 μm intervals and reflect a periodic contrast in the thickness of an ion-spattered sample and pass through the interior of some aragonite crystals. These observations suggest that aragonite crystals precipitate inorganically in the calcification medium and the medium maintains a CaCO3-supersaturated state during fiber growth. It is also suggested that growth lines are possibly formed due to changes in the chemical and physical conditions of the medium during fiber growth. Identification of mineral phases of massive reef-building coral skeleton were performed from the microto nanometer scale using an ATEM and a Fourier transform infrared spectrometer (FT–IR). The most notable result was the discovery of halite (NaCl) in the coral skeleton. From the TEM observation, average size of halite crystals is about 60-80 nm and their crystal shape exhibits the square cross-sectional form that is typical euhedral shape of halite. In addition, some special relations of crystallographic orientation can be found between halite and host aragonite. These microstructural and crystallographic features of this halite suggest that it precipitated almost simultaneously with aragonite during coral calcification. The existence of such halite not only represents new evidence for the validity of extracellular calcification, but also provides chemical information about the calcification medium. Moreover, the absence of gypsum (CaSO4・2H2O) and anhydrite (CaSO4) in association with the halite indicates that a selective ion transport mechanism operates from seawater to the calcification space in coral. The primary deposition of a mineral phase other than aragonite in coral skeleton has been confirmed for the first time. Acropora digitifera was cultured in artificial seawater formulated at 5.0, 2.5 and 1.2 of Mg/Ca ratios in order to investigate the effect of seawater Mg/Ca ratios on mineral phases occurred in the coral skeleton. In this study, larval coral polyp was grown from egg to remove influence of already formed aragonite and symbiotic algae. X-ray diffraction analysis, Raman scattering analysis and TEM observation showed that only aragonite could be identified in all skeletons. Because this result is consistent with the evidence that only aragonite precipitates inorganically in model seawaters with more than 1.0 of Mg/Ca ratio at 25°C (Morse et al., 1997), it is suggested that the calcification medium has a similar value to the Mg/Ca ratio in the surrounding seawater. Ries et al. (2006) showed that calcite can precipitate in the skeletons of some coral specimens with symbiotic algae, cultured in artificial seawater with less than 3.5 of the Mg/Ca ratio. These evidences indicate that symbiotic algae play an important role for decreasing the Mg/Ca ratio in the calcification medium against seawater.