Distribution and Functions of Calcium Mineral Deposits in Photosynthetic Organisms

CaCO3 precipitates occur inside a few cyanobacteria and green algae. More common is precipitation on the surface of cyanobacteria, a range of algae and aquatic plants, and in invaginations of the cell wall in terrestrial plants (cystoliths). In coccolithophores and calcified dinoflagellates, CaCO3 is precipitated with organic matter in intracellular vesicles and the resulting structures are externalised. The precipitation of CaCO3 on the surface of photosynthesising structures is related to the consumption of CO2 in photosynthesis. CO2 production by root respiration can solubilise soil CaCO3. A few cyanobacteria and eukaryotic algae can bore through solid CaCO3 by removing Ca2+ and adding H+ at the site of boring, generating soluble inorganic C that can be used in photosynthesis. Ca(COO)2 is precipitated in the vacuoles of many algae and plants, and the cell walls of some plants. An outcome of precipitation of CaCO3 using CO3= produced from CO2, and Ca2+, is the production of H+; the same is the case for precipitation of Ca(COO)2 from (COOH)2 and Ca2+. The H+ produced by Ca(COO2) can be used to neutralise OH− produced in NO3− assimilation in the shoot without increasing cell osmolarity. There is no evidence of CaCO3 fulfilling this role. Another outcome of CaCO3 and Ca(COO)2 precipitation is Ca2+ immobilisation, though with little evidence of remobilisation of Ca2+ under Ca2+ deficiency. Other consequences of CaCO3 and Ca(COO)2 precipitation are light scattering and increased density, and ‘alarm photosynthesis’. Defence against herbivores and pathogens is better established for Ca(COO)2 than for CaCO3, and pollen release from anthers is a function of Ca(COO)2 but not CaCO3.