Controlling crystallisation and dissolution of biogenic CaCO3via dissolved magnesium cations†

20 January 2024

Environmental Science.: Advances

Toby Morton-Collings, Minjun Yang and Richard G. Compton, Environ. Sci.: Adv., 2024,3, 402-410,

View Journal Article / Working Paper

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.