<i>Ceratostigma willmottianum</i> mineralizes atmospheric carbon dioxide into calcium carbonate in a high-calcium environment

Calcium carbonate biomineralization is an ancient evolutionary feature of life that plays a key role in environmental adaptation. In plants, calcium carbonate 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 calcium carbonate biomineralization. We found that secretion crystals are mainly composed of calcium carbonate (>90%), and the chalk gland consists of 16 cells with 4 secretory pores on the surface. Calcium carbonate accumulation was highly dependent on atmospheric carbon dioxide and independent of soil dissolved inorganic carbon. Calcium carbonate accumulation occurred mainly during the day, with diurnal variations in the carbon source, mainly atmospheric carbon dioxide during the day and metabolic carbon dioxide at night. Hydration of carbon dioxide to bicarbonate (HCO3-) occurred within the leaves, and the reaction rate was controlled by the activity of extracellular carbonic anhydrases. C. willmottianum showed a high tolerance to calcium stress, potentially related to enhanced calcium compartmentalization and calcium carbonate excretion in the chalk gland under high-calcium environments. The conversion of atmospheric carbon dioxide into calcium carbonate by C. willmottianum may represent an ecological adaptation of plants to high-calcium environments. These results provide cases and theoretical references for studying calcium carbonate biomineralization mechanisms and plant calcium adaptation.