Ceratostigma willmottianum mineralizes atmospheric CO2 into CaCO3 in a high-calcium environment

Abstract Calcium carbonate (CaCO3) biomineralization is an ancient evolutionary feature of life that plays a key role in environmental adaptation. In plants, CaCO3 deposition is found in several taxa; however, current knowledge of its formation and ecological adaptive implication is limited. Here, we used the chalk gland plant Ceratostigma willmottianum to gain insight into CaCO3 biomineralization. We found that secretion crystals are mainly composed of CaCO3 (> 90%), and the chalk gland consists of sixteen cells with four secretory pores on the surface. CaCO3 accumulation was highly dependent on atmospheric carbon dioxide (CO2) and independent of soil dissolved inorganic carbon (DIC). CaCO3 accumulation occurred mainly during the day, with diurnal variations in the carbon source, mainly atmospheric CO2 during the day and metabolic CO2 at night. Hydration of CO2 to bicarbonate (HCO3­) occurred within the leaves, and the reaction rate was controlled by the activity of extracellular carbonic anhydrases (CAs). C. willmottianum showed a high tolerance to calcium stress, potentially related to enhanced calcium compartmentalization and CaCO3 excretion in the chalk gland under high-calcium environments. The conversion of atmospheric CO2 into CaCO3 by C. willmottianum may represent an ecological adaptation of plants to high-calcium environments. These results provide cases and theoretical references for studying CaCO3 biomineralization mechanisms and plant calcium adaptation.