Amorphous calcium carbonate particles form coral skeletons

Area of Science:

• Biomineralization
• Coral Reef Ecology
• Marine Geochemistry

Background:

• The classical model of coral skeleton formation assumes precipitation from solution.
• Observed "vital effects" in coral skeletons deviate from thermodynamic equilibrium, suggesting alternative formation mechanisms.
• Understanding coral biomineralization is crucial for predicting reef responses to climate change.

Purpose of the Study:

• To investigate the mechanism of skeleton formation in the coral Stylophora pistillata.
• To determine if corals utilize amorphous precursor particles for skeleton accretion.
• To compare the growth rates of particle-mediated versus solution-mediated calcification.

Main Methods:

• Direct spectromicroscopy was employed to analyze coral skeleton formation.
• Characterization of amorphous calcium carbonate (ACC) precursors within coral tissues.
• Comparison of crystal growth rates between ACC particle attachment and ion-by-ion precipitation.

Main Results:

• Two forms of amorphous calcium carbonate (ACC) precursors (hydrated and anhydrous) were identified within coral tissues.
• These ACC particles, approximately 400 nm in size, are formed in the tissue, attach to the skeleton, and crystallize into aragonite.
• ACC particle attachment facilitates crystal growth over 100 times faster than ion-by-ion precipitation.

Conclusions:

• Coral skeletons are formed via the attachment and crystallization of amorphous calcium carbonate (ACC) particles generated within coral tissues.
• This rapid, particle-mediated growth offers a physiological advantage in competitive reef environments.
• The internal formation of ACC may confer greater resilience to ocean acidification than previously assumed, potentially explaining past survival during high CO2 events.