A comparison of calcification mechanisms in haploid and diploid cells of the coccolithophore Calcidiscus leptoporus (Murray & Blackman 1898).

Abstract

Coccolithophores are prominent marine pelagic calcifiers due to their production of calcite coccoliths. Diploid coccolithophores produce heterococcoliths intracellularly, with an organic cellulose baseplate scale acting as a nucleating substrate. However, coccolith production in the haploid life phase has not been extensively studied. Most haploid coccolithophores produce distinct holococcoliths that were previously thought to be produced in an extracellular space. However, we recently observed intracellular holococcolith production in haploid Coccolithus braarudii. We therefore compared the calcification mechanisms of the 2 N and N phases of Calcidiscus leptoporus, an understudied yet ecologically important species, to better understand ultrastructure features related to calcification and explore aspects of the calcification pathway in each life-history phase. We show that both life phases of C. leptoporus produce coccoliths intracellularly and that holococcoliths are likely nucleating on body scales in a dilated vacuole space rather than within a tightly associated coccolith vesicle (CV) as in diploid heterococcolith production. Both life phases were sensitive to the SERCA Ca²⁺ATPase inhibitor thapsigargin, suggesting conservation in the transcellular Ca²⁺ transport pathways utilized for intracellular calcification, namely through calcium accumulation in the endoplasmic reticulum. However, the Si analogue germanium led to severe disruption of the CV and coccolith morphogenesis only in the diploid life phase. This work provides strong evidence to support a model of intracellular holococcolith production and clearly demonstrates that calcification in both life phases shares a common cellular pathway. Nevertheless, differences in the spatial arrangement of the CV membrane exert significant control over crystal growth that has profound consequences for coccolith morphology.