Radioisotope tracer studies of inorganic carbon and Ca in microbially derived CaCO₃

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 Ca²⁺ and inorganic carbon in CaCO₃ precipitation by the unicellular green alga Nannochloris atomus. Implications of the effects of these physiological aspects on CaCO₃ precipitation and the effects of microbial calcification on CaCO₃ δ¹³C ratios are discussed. Results from pulse/chase experiments indicate that intracellular Ca²⁺ is incorporated into extracellular CaCO₃. Intracellular inorganic carbon leaks from cells within 10 to 12 s after injection of unlabelled NaHCO₃, providing a source of inorganic carbon for extracellular CaCO₃. Cellular expulsion of calcium plays a key role in increasing the CaCO₃ saturation state at the site of calcification. The δ¹³C ratios of microbial carbonates may vary depending on the amount of photorespiratory CO₂ incorporated.