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

Abstract

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.