A simple microscopy approach quantifies biomineralized CO2 in Coccolithus braarudii – a calcifying marine phytoplankton

Climate is partly controlled by atmospheric carbon dioxide levels. In turn atmospheric CO2 is substantially regulated by oceanic phytoplankton, specifically coccolithophores which biomineralize CO2 as CaCO3 in the form of tiny platelets which ultimately sink to the ocean floor. The scale of this biomineralization is massive; some 1015 g of CaCO3 are formed by phytoplankton per annum. This figure represents a tiny mass of CaCO3 per phytoplankton multiplied by a huge number of individual coccolithophores. For modelling the climate, it is essential to measure the biomineralized CaCO3 at single entity as well as bulk level. In this work we show that a simple light microscopy approach can, by monitoring the undersaturation driven dissolution of calcite in deionised (DI) water, quantify sub-nanograms of calcite mass of heterococcoliths secreted by marine phytoplankton. We show that the biogenic calcite found in coccoliths dissolves at a comparable mass-transport rate to that of inorganic particulate calcite in DI water. We further show that undersaturation driven dissolution is not suitable for quantifying coccolithophores due to competing osmotic pressure effects.