Coccolithophore biomineralization: New questions, new answers

Abstract

Coccolithophores are unicellular phytoplankton that are characterized by the presence intricately formed calcite scales (coccoliths) on their surfaces. In most cases coccolith formation is an entirely intracellular process - crystal growth is confined within a Golgi-derived vesicle. A wide range of coccolith morphologies can be found amongst the different coccolithophore groups. This review discusses the cellular factors that regulate coccolith production, from the roles of organic components, endomembrane organization and cytoskeleton to the mechanisms of delivery of substrates to the calcifying compartment. New findings are also providing important information on how the delivery of substrates to the calcification site is co-ordinated with the removal of H(+) that are a bi-product of the calcification reaction. While there appear to be a number of species-specific features of the structural and biochemical components underlying coccolith formation, the fluxes of Ca(2+) and a HCO3(-) required to support coccolith formation appear to involve spatially organized recruitment of conserved transport processes.

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“…Three acidic polysaccharides, termed PS-1, -2 and -3, have been identified in coccolith plates of P. carterae [13]. These biomacromolecules are hypothesised to be synthesised in the golgi (the coccolith vesicle), forming the template [10,24]. The golgi contains fluid isolated from the cytoplasm [30].…”

“…However, only uronic acids are considered responsible for calcite inhibition [59]. The role of organic coating on coccoliths is still relatively unclear [23], but it is hypothesised to both inhibit calcite crystal growth [24] and prevent dissolution [44,60]. It is also likely that the organic coating also gives coccoliths their superior mechanical properties; however, to the authors’ knowledge, no studies have yet measured their strength.…”

“…Coccolithophores offer a paradigm to ‘puzzle-solve’ the mechanisms of biomineralisation which remain poorly understood [24]. E. huxleyi and P. carterae have both been successfully grown as a model tool in climate change studies [61], indirectly answering several fundamental questions in coccolithophore biology that relate specifically to biomineralisation [24].…”

“…E. huxleyi and P. carterae have both been successfully grown as a model tool in climate change studies [61], indirectly answering several fundamental questions in coccolithophore biology that relate specifically to biomineralisation [24]. While these studies help explain certain observations in the biomineralisation process, many questions remain unanswered, particularly in relation to the biological control of the process [28]; questions such as, what regulates the ornate structures with such precision and uniformity?…”

“…The mineralisation process, which occurs inside an intracellular compartment, is still poorly understood [11], but is responsible for the precise control of the coccolith morphology [22]. Their complicated, ornate, hierarchical structure starts on a nanometer scale, and is formed from a collection of specialised proteins/genes that are regulated by Ca 2+ ion environmental flux and cell cycle progression [23,24]. Across different species, coccosphere architecture can vary markedly; however, within the same species, it is replicated precisely from generation-to-generation.…”