Molecular and mineral responses of the scleractinian Montipora digitata grown in artificial Cretaceous seawater
Biomineralization is a taxonomically ubiquitous process by which organisms form minerals that they use for support, protection, or nutrient storage, such as shells, skeletons, and bones. Among corals (Class Anthozoa, Phylum Cnidaria) calcification is widespread and both aragonite and calcite structures can be observed.  Coral biomineralization has been extensively studied, primarily due to the ecological role these organisms play in the formation and accretion of coral reefs. Nevertheless, how corals acquired the ability to form skeletons characterized by different calcium carbonate (CaCO3) polymorphs, i.e. calcite and aragonite, remains elusive. One standing question revolves around whether corals can biologically control the deposition of either polymorph. This has important evolutionary implications as changes in  seawater chemistry through the Earth's history, especially the Mg/Ca ratio, appear to favor  the deposition of either one polymorph or the other.To address this question, we have set-up  calcite-inducing, Cretaceous-like marine aquaria experiments to grow coral species from both clades of biomineralizing anthozoans, namely the subclass Octocorallia (Heliopora coerulea) and the order Scleractinia (Montipora digitata). We employed a diverse array of mineralogical analyses, including electron backscatter diffraction (EBSD) and energy dispersive spectroscopy (EDS), to determine and examine the presence of environmentally-induced modifications within the coral skeleton. We then used RNA sequencing to investigate the transcriptional response of the corals to different polymorph favoring environments. In conjunction, these data provide insights on how the diversity of skeletal structures in corals evolved and may inform our predictions about the response of these ecologically important organisms to future changes in ocean chemistry .