Abstract Microbial calcification significantly impacts the cycling and deposition of inorganic carbon. This research employs 45Ca and 14C techniques as radioisotopic tracers to examine the role of cellular cycling of Ca 2+ and inorganic carbon in CaCO 3 precipitation by the unicellular green alga Nannochloris atomus. Implications of the effects of these physiological aspects on CaCO 3 precipitation and the effects of microbial calcification on CaCO 3 δ 13C ratios are discussed. Results from pulse/chase experiments indicate that intracellular Ca 2+ is incorporated into extracellular CaCO 3. Intracellular inorganic carbon leaks from cells within 10 to 12 s after injection of unlabelled NaHCO 3, providing a source of inorganic carbon for extracellular CaCO 3. Cellular expulsion of calcium plays a key role in increasing the CaCO 3 saturation state at the site of calcification. The δ 13C ratios of microbial carbonates may vary depending on the amount of photorespiratory CO 2 incorporated.