Mechanisms of Calcification in Coccolithophores

Abstract

Coccolithophores are unicellular marine algae characterised by the production of calcite coccoliths. As a result of their calcification they contribute significantly to global biogeochemical cycles. Comprehensive understanding of the mechanisms behind calcification remains elusive, due in part to the research focus on one species, Emiliania huxleyi; the most globally abundant of all coccolithophores. It is imperative to investigate calcification in other species to better understand this biogeochemically important process, especially as the ecological success of E. huxleyi may be due to certain physiological differences with other species. This study set out to explore differences between species in the mechanisms of calcification in three primary areas. Firstly, the physiological requirement for calcification remains poorly understood, particularly as non-calcifying strains of E. huxleyi grow normally in laboratory culture. This study identified a contrast in the requirement for calcification between E. huxleyi and the ecologically important Coccolithus braarudii. Calcification disruption had no negative impacts on E. huxleyi but resulted in major growth defects in C. braarudii demonstrating an obligate requirement for calcification in this species. Secondly, the previous identification of Si transporters in some coccolithophores was further investigated using a combination of physiological and expression studies to identify that Si plays a role in heterococcolith calcification during their diploid life stage. C. braarudii Si transporters were also found to be regulated in response to available Si and shown to be expressed in natural populations. Finally, coccolith associated polysaccharides (CAPs) are an integral component of the calcification mechanism known to modulate the precipitation of calcite. The data presented here show that extracellular CAPs differ in structure and composition between species and that they also play an important role in the organisation of the coccosphere, expanding upon their role and importance in calcification. These findings mark crucial physiological differences between coccolithophore species. The identification of a requirement for calcification in coccolithophores highlights the importance of maintaining a coccosphere. The requirement for Si in some species suggests major physiological differences between species which may influence their ecology. Consequently, these contrasting physiological characteristics may contribute to significant differences in the response of coccolithophores to future ocean conditions.

More information

Identifiers

ORCID for Luke Tyrrell: orcid.org/0000-0002-1002-1716

Catalogue record

Export record