Abstract: Biological Control of Coral Calcification Under Environmental Stress Coral reefs rely on a biologically controlled biomineralization process that operates across multiple levels of organization, from molecular and cellular mechanisms to whole-organism responses. This process is increasingly threatened by global change, particularly ocean acidification (OA) and warming, which disrupts the intricate regulation of coral calcification.

Research conducted by Dr. Sylvie Tambutté’ s team and collaborators, has significantly advanced the understanding of these processes.

Recent discoveries highlight that coral biomineralization involves a two-step process:
intracellular formation of amorphous calcium carbonate (ACC) nanoparticles within vesicles of calcifying cells, followed by exocytosis into the extracellular calcifying medium (ECM). In addition to ACC, hydrated precursor phases such as monohydrocalcite (MHC) and calcium carbonate hemihydrate (CCHH) have been identified in freshly deposited coral skeletons before the formation of crystalline aragonite. These findings suggest more complex and diverse biomineralization pathways than previously understood, with significant implications for isotopic studies and climate reconstructions.

The calcification process is tightly regulated by a specialized epithelium, which controls organic matrix secretion, ion transport, and acid-base balance through septate junctions, proton and calcium transporters, and other molecular mechanisms that sustain an elevated pH in the ECM, favoring aragonite precipitation. However, environmental stressors disrupt these processes, leading to pH imbalances, altered ion homeostasis, and reduced carbonate availability, ultimately impairing skeletal growth and increasing porosity. Additionally, internal non-calcifying compartments such as the gastrovascular cavity and the mesoglea
indirectly influence calcification by modulating ion fluxes. The extent of these disturbances varies among coral species, with some exhibiting faster crystallization rates that enhance skeleton stability, while others retain prolonged amorphous phases, making them more susceptible to OA. This species-specific response provides insight into the differential resilience of coral taxa under climate change.

To investigate coral biomineralization across different levels of organization and assess the impacts of environmental stress, Dr. Tambutté’s team and collaborators have developed and applied advanced methodologies, including controlled coral cultures simulating climate change scenarios, in vivo cellular imaging coupled with ion-sensitive microelectrodes to measure pH and calcium dynamics, comparative molecular analyses to identify genes and transporters involved in stress resilience, and nanoscale mineral phase mapping to track precursor transformation. These approaches integrate molecular, cellular, and ecological perspectives, advancing our understanding of coral calcification under climate stress.

Understanding the fundamental mechanisms of coral biomineralization is not just a scientific challenge but a necessity for coral health. The increasing frequency and intensity of environmental changes are compromising the structural integrity and survival of coral reefs, ecosystems that provide essential services for marine biodiversity and coastal protection. Just as disturbances in biomineralization affect human health—such as in bone and dental pathologies—disruptions in coral calcification have direct consequences for reef resilience, ecosystem stability, and the health of marine organisms dependent on these habitats. A deeper understanding of these processes is therefore crucial to predict and mitigate the impacts of environmental stressors on coral health, ultimately guiding conservation efforts and supporting reef adaptation strategies in a rapidly changing ocean.