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Coral reef ecosystems supported by environmentally sensitive reef-building corals face serious threats from human activities. Our understanding of these reef threats is hampered by the lack of sufficiently sensitive coral environmental impact assessment systems. In this study, we established a platform for metabolomic analysis at the single-coral-polyp level using state-of-the-art mass spectrometry (probe electrospray ionization/tandem mass spectrometry; PESI/MS/MS) capable of fine-scale analysis. We analyzed the impact of the organic UV filter, benzophenone (BP), which has a negative impact on corals. We also analyzed ammonium and nitrate samples, which affect the environmental sensitivity of coral-zooxanthella (Symbiodiniaceae) holobionts, to provide new insights into coral biology with a focus on metabolites. The method established in this study breaks new ground by combining PESI/MS/MS with a technique for coral polyps that can control the presence or absence of zooxanthellae in corals, enabling functions of zooxanthellae to be assessed on a polyp-by-polyp basis for the first time. This system will clarify biological mechanisms of corals and will become an important model system for environmental impact assessment using marine organisms. [ABSTRACT FROM AUTHOR]

Since the beginning of the last century, bacteria, including cyanobacteria, have been known to be involved in the extracellular formation and precipitation of CaCO3. It is also known that some marine bacteria form calcite granules in Ca-containing artificial media. However, a detailed analysis of these granules has not yet been performed. The objective of the present study was to isolate marine bacteria that form CaCO3 granules in a culture medium to analyze the structure of the granules in detail. Pseudovibrio sp. 01OK 105-5-5, belonging to the class Alphaproteobacteria, was isolated from an ascidian in a coral reef at On-na, Okinawa, Japan. It produced extracellular granules of CaCO3 in a Ca-containing artificial medium. X-ray diffraction analysis, infrared spectroscopy, and inductively coupled plasma atomic emission spectrometry demonstrated that the extracellular granules contained Mg-rich calcite-like crystal polymorphs. This crystal form of CaCO3 was similar to that of Mg-rich calcite found in the skeletons of many marine invertebrates. This bacterium provides a promising tool for studying the mechanisms involved in the formation of Mg-rich biogenic calcite. [ABSTRACT FROM AUTHOR]

Calcareous skeletons of stony corals are composed of a low-crystalline core (center of calcification: COC) and surrounding fibrous crystals. We characterized the overall framework of juvenile Acropora digitifera corals to clarify the entire calcic architecture and the contribution of abiotic processes. Skeletal and tubular frameworks contain string and sheet COCs, respectively, consisting of strained aragonite nanograins. Parallel and curved structures of fibrous aragonite develop around the COC cores through geometric selection and a gradual change in the growth direction of the c-direction-elongated nanorods. Based on a study of the nanoscale morphology of the coral frameworks by comparing them to fibrous crystals grown in artificial systems, it appears that subsequent precipitation of the fibrous aragonite on the COC is primarily regulated by environmental abiotic parameters. The architecture of the COC network is deduced to be designed biologically and then thickened by the fibrous crystals that are produced with morphology formation through physicochemical processes although the vital activity controls the total mineralization. [ABSTRACT FROM AUTHOR]

Over the past several decades, coral reef ecosystems have experienced recurring bleaching events. These events were predominantly caused by thermal anomalies, which vary widely in terms of severity and spatio-temporal distribution. Acropora corals, highly prominent contributors to the structural complexity of Pacific coral reefs, are sensitive to thermal stress. Response of Acropora corals to extremely high temperature has been well documented. However, studies on the effects of moderately high temperature on Acropora corals are limited. In the summer of 2016, a moderate coral bleaching event due to moderately high temperature was observed around Sesoko Island, Okinawa, Japan. The objective of this study was to examine thermal tolerance patterns of Acropora corals, across reefs with low to moderate thermal exposure (degree heating weeks ~2–5°C week). Field surveys on permanent plots were conducted from October 2015 to April 2017 to compare the population dynamics of adult Acropora corals 6 months before and after the bleaching events around Sesoko Island. Variability in thermal stress response was driven primarily by the degree of thermal stress. Wave action and turbidity may have mediated the thermal stress. Tabular and digitate coral morphologies were the most tolerant and susceptible to thermal stress, respectively. Growth inhibition after bleaching was more pronounced in the larger digitate and corymbose coral morphologies. This study indicates that Acropora populations around Sesoko Island can tolerate short-term, moderate thermal challenges. [ABSTRACT FROM AUTHOR]