Influence of changing carbonate chemistry on morphology and weight of coccoliths formed by Emiliania huxleyi

The coccolithophore Emiliania huxleyi is a marine phytoplankton species capable of forming small calcium carbonate scales (coccoliths) which cover the organic part of the cell. Calcification rates of E. huxleyi are known to be sensitive to changes in seawater carbonate chemistry. It has, however, not yet been clearly determined how these changes are reflected in size and weight of individual coccoliths and which specific parameter(s) of the carbonate system drive morphological modifications. Here, we compare data on coccolith size, weight, and malformation from a set of five experiments with a large diversity of carbonate chemistry conditions. This diversity allows distinguishing the influence of individual carbonate chemistry parameters such as carbon dioxide (CO₂), bicarbonate (HCO₃⁻), carbonate ion (CO₃²⁻), and protons (H⁺) on the measured parameters. Measurements of fine-scale morphological structures reveal an increase of coccolith malformation with decreasing pH suggesting that H⁺ is the major factor causing malformations. Coccolith distal shield area varies from about 5 to 11 μm². Changes in size seem to be mainly induced by varying [HCO₃⁻] and [H⁺] although influence of [CO₃²⁻] cannot be entirely ruled out. Changes in coccolith weight were proportional to changes in size. Increasing CaCO₃ production rates are reflected in an increase in coccolith weight and an increase of the number of coccoliths formed per unit time. The combined investigation of morphological features and coccolith production rates presented in this study may help to interpret data derived from sediment cores, where coccolith morphology is used to reconstruct calcification rates in the water column.