Single-entity coccolithophore electrochemistry shows size is no guide to the degree of calcification

Abstract

We report single-entity measurements of the degree of calcification of individual phytoplankton cells. Electrogenerated acid is used to dissolve the calcium carbonate (CaCO₃) shell (coccosphere) of individual coccolithophores and the changes in size are monitored by simultaneous optical measurements, allowing the CaCO₃ content of the single phytoplankton cell to be inferred via analysis of the dissolution kinetics. Three species of coccolithophore, E. huxleyi, G. oceanica and C. braarudii, were studied with CaCO₃ masses measured over 3 orders of magnitude ranging from 2.6 pg to 8.3 ng of CaCO₃ per cell. Surprisingly, within a species, the CaCO₃ content is not correlated to the size of the coccosphere but the volume of its shell. Further, the variation of the CaCO₃ content with the cell size and the particulate inorganic carbon (PIC) to particulate organic carbon (POC) ratio at different stages of the growth phase is reported.