Emiliania huxleyi sensitivity to ecological changes: a new methodological approach to ascertain if and which environmental parameter influences coccolith sizes and shapes

Coccolithophores are a group of unicellular marine phytoplankton primary producers of biogenic calcite in the open ocean. During their diploid life-cycle stage, coccolithophores produce calcite plates called heterococcoliths. These circular to elliptical coccoliths have very ornate structures and form an exoskeleton, called coccosphere, generally composed of a single layer of plates. Coccolithophore algae produce coccoliths one at a time, with an intracellular growth process. This begins with nucleation of a proto-coccolith ring of simple crystals and continues by upward and outward growth of these crystals into a complex unit to form complete coccoliths. Coccoliths are then extruded to the cell surface and continue to be generated until a complete coccosphere covering is created. The cell recurrently produces incomplete and malformed coccoliths: incomplete coccoliths occur if the growth process is arrested due to premature extrusion of the coccolith from the cell, while malformation is due to “irregular coccolith formation as a result of departure from the normal growth process” (Young and Westbroek 1991), implying the malfunction of the coccolith-shaping machinery per se. Malformations are difficult to evaluate and most studies use a qualitative and subjective approach with the identification of arbitrary categories (e.g. normal, very malformed, malformed and incomplete coccoliths). The use of morphometrics has the potential to significantly improve the knowledge of shape and size variations in coccolithophore taxa and the ability to describe their evolutionary history and response to ecological changes. Here, we present a new morphometrical method to quantitatively characterize coccolith sizes and shapes and discriminate normal versus malformed specimens and the degree of malformations. This methodology was applied to coccoliths of Emiliania huxleyi grown under different ecological conditions. The obtained results evidenced that E. huxleyi is very sensitive to chemical alterations of seawater and specifically, that introduction of toxic trace metals and increased CO2 concentrations might have the potential to disturb the calcification process causing an increase in the number of aberrant coccoliths, an alteration of the calcite content per coccolith and a general decrease in the cellular calcification rate. Young, J.R. & Westbroek, P. (1991): Genotypic variation in the coccolithophorid species Emiliania huxleyi. Marine Micropaleontology, 18(1-2), 5-23.