Tracing Environmental and Biological Signatures in Diverse Coral Skeletal Systems Using Correlative Chemical Imaging

Biomineral systems exhibit complex hierarchical assemblies and chemical heterogeneities across lengths scales that are difficult to capture with a single analytical technique. Coral skeletons are excellent examples of skeletal systems where mineral polymorphism, trace-element incorporation and organic-inorganic interactions can complicate the interpretation of biological and environmental signals. Scleractinian (stony) corals primarily construct their hard exoskeletons from a crystalline polymorph of calcium carbonate (CaCO3) called aragonite [1]. Recent studies have reported acute toxicity of inorganic arsenic (As) in coral species native to Florida’s coral reef tract [2]. Elevated As levels pose an overlooked environmental stressor that may compromise coral reef health and resilience by disrupting key physiological and biochemical processes in the organism [3],[4]. Corals bioaccumulate and incorporate environmental pollutants from seawater into skeletal aragonite, serving as long-term environmental archives and biomarkers for reef health monitoring and restoration. The blue coral (Heliopora coerulea) is a unique reef-building octocoral that produces a hard aragonitic skeleton similar to stony corals [5] but with distinct blue pigmentation that has been speculated to be the result of iron salt deposition [6],[7]. Interestingly, Heliopora corals show higher resistance to environmental changes, such as thermal stress events like bleaching, compared to many stony corals, giving them a competitive advantage in warmer waters [8],[9]. Basic physiological studies of these reef-building octocorals have centered around skeletal density and calcification rate [10]. Discovering unknown aspects of biomineralization and trace element incorporation in Heliopora may bridge the gap in understanding their unique skeletal properties and evolutionary resilience. Correlative and multimodal characterization studies are essential for connecting elemental distribution patterns with molecular and crystallographic information in complex biomineral systems while preserving spatial relationships across length scales [11],[12]. In this work, coral skeletons were characterized by a multimodal framework that combined synchrotron X-ray fluorescence (XRF) mappings with Raman and FTIR microspectroscopy, scanning electron microscopy (SEM) (Fig. 1), and X-ray diffraction (XRD) to provide biological and environmental insights into the elemental, compositional and crystallographic heterogeneity of coral skeletons. Hard XRF mappings were collected at the Bionanoprobe (2-ID-D beamline) at the Advanced Photon Source, Argonne National Laboratory. This approach was applied to two distinct but unified case studies: (i) mapping As contamination in Montastraea cavernosa (M. cavernosa) corals from South Florida to resolve spatial association within the skeletal microstructure, and (ii) probing iron (Fe) and sulfur (S) distributions in H. coerulea corals from the Indo-Pacific Ocean to elucidate the origins of its blue skeletal pigmentation and associated mineral organization. XRF mappings revealed Fe enrichment in the outer surface layer of H. coerulea skeletons (Fig. 2). In both these systems, correlative analysis unravels intricate relationships between trace elements, mineralogical phases, and organic matrices that would otherwise be obscured by each technique in isolation [13]. Scanning electron micrograph of H. coerulea skeletal morphology. Photograph and light microscope image of polished longitudinal section of H. coerulea coral skeleton from Singapore. Synchrotron X-ray fluorescence mappings from a blue coral with distinct variations in coloration, making it ideal for comparative analysis, reveal Fe enrichment in the outer surface of the CaCO3 skeleton.