Controlling crystallisation and dissolution of biogenic CaCO3via dissolved magnesium cations†

The surface of our oceans is teeming with single-cellular ‘plant’ organisms that biomineralise CaCO₃ (coccoliths). Globally, an estimate of over 10¹⁵ g of atmospheric CO₂ per annum is sequestered in the top layers of our ocean. Information of this process is crucial to modelling climate change and achieving net carbon neutrality not least because this rate of CO₂ sequestration is comparable to the rate of anthropogenic release of CO₂. While the dissolution kinetics of pure calcite (Icelandic Spar, Carrea marble and synthetically grown) have been well-studied in the past decades it remains unclear if biogenic CaCO₃ behaves differently, or not, to pure calcite in the marine environment. In this work, we utilise a light microscopy setup to study and compare the precipitation and dissolution of biogenic CaCO₃ in both the absence and presence of Mg²⁺, a known inhibitor, at concentrations similar to seawater. Notably, the time required for a micron-sized calcite particle to dissolve is doubled by approximately doubling the concentration of Mg²⁺ from 54.6 mM to 100 mM. The work produces two new, key insights. First, there is negligible difference between the rate of mass loss of biogenic and pure, laboratory grown CaCO₃ particles when placed in solutions supersaturated and undersaturated with respect to calcite. Second, the mass of the individual micron-sized biogenic coccoliths, ranging from 100–600 picograms, was inferred via image analysis of data from the complete dissolution of coccoliths in aqueous solutions containing seawater levels of Mg²⁺. This relatively simple light-based approach, allowing the mass of biogenic CaCO₃ platelets to be estimated at the single-entity level, shows promise for the development of a proof-of-concept sensor allowing CaCO₃ sequestration to be monitored real-time in our oceans.