ROLE OF THE LIGHT-DARK CYCLE AND MEDIUM COMPOSITION ON THE PRODUCTION OF COCCOLITHS BY EMILIANIA HUXLEYI (HAPTOPHYCEAE)1

Production of coccoliths by cells of Emiliania huxleyi (Lohmann) Hay and Mohler was measured during exposure of the cells to two diel light-dark cycles (16:8 h). During the light period about eight coccoliths per cell were formed at a constant rate of one coccolith per 2 h. Cells divided during the first half of the dark period. No coccolith production took place during the dark period. With electron microscopy we found early-stage, coccolith-production compartments in cells after mitosis while still in the dark. No calcification was observed in these compartments. Cells grown on enriched seawater (Eppley's medium) tended to produce enough coccoliths to cover the cell in a single layer. When these cells reached the stationary phase coccolith production stopped. Coccolith production was induced by removal of extracellular coccoliths. Cells grown on medium containing 2% of the nitrate and phosphate of Eppley's medium tended to produce coccoliths in the stationary phase. This resulted in the formation of multiple layers of coccoliths. The multiple covering was restored after decalcification of stationary cells. Formation of multiple layers of coccoliths may help the cells reach deeper, nutrient-rich water by increasing the sinking rate of the cells.     

INCOMPLETENESS OF THE COCCOSPHERE AS A POSSIBLE STIMULUS FOR COCCOLITH FORMATION IN PLEUROCHRYSIS CARTERAE(PRYMNESIOPHYCEAE)1

Pleurochrysis carterae is a marine biflagellate that produces calcified structures called coccoliths. The coccoliths are formed inside the cells and released from the latter after formation. The light dependence of calcium incorporation in this species was studied using⁴⁵Ca as a tracer. Cells exposed to a repeating cycle of 16 h of light and 8 h of darkness incorporated calcium in extracellular coccoliths at a more or less constant rate throughout a cycle. The cells divided during the dark periods with a concomitant decrease in size. Their size increased during the light periods Coccolith formation in cells incubated in continuous darkness was greatly reduced and finally ceased. These cells did not divide and did not increase in size. Removal of extracellular coccoliths prior to the calcium incorporation experiments stimulated coccolith formation both in dark-incubated cells and in cells exposed to a repeating light-dark cycle. Cells in the stationary phase of growth ceased producing coccoliths. Calcification could be induced in these cells by removal of the extracellular coccoliths. Based on these findings we suggest that cells of Pleurochrysis carterae tend to produce a complete cover of coccoliths and that the available cell surface is a factor controlling coccolith formation.     

INCOMPLETENESS OF THE COCCOSPHERE AS A POSSIBLE STIMULUS FOR COCCOLITH FORMATION IN PLEUROCHRYSIS CARTERAE (PRYMNESIOPHYCEAE)1

Pleurochrysis carterae is a marine biflagellate that produces calcified structures called coccoliths. The coccoliths are formed inside the cells and released from the latter after formation. The light dependence of calcium incorporation in this species was studied using ⁴⁵Ca as a tracer. Cells exposed to a repeating cycle of 16 h of light and 8 h of darkness incorporated calcium in extracellular coccoliths at a more or less constant rate throughout a cycle. The cells divided during the dark periods with a concomitant decrease in size. Their size increased during the light periods. Coccolith formation in cells incubated in continuous darkness was greatly reduced and finally ceased. These cells did not divide and did not increase in size. Removal of extracellular coccoliths prior to the calcium incorporation experiments stimulated coccolith formation both in dark-incubated cells and in cells exposed to a repeating light-dark cycle. Cells in the stationary phase of growth ceased producing coccoliths. Calcification could be induced in these cells by removal of the extracellular coccoliths. Based on these findings we suggest that cells of Pleurochrysis carterae tend to produce a complete cover of coccoliths and that the available cell surface is a factor controlling coccolith formation.     

COCCOLITH PRODUCTION AND DETACHMENT BY EMILIANIA HUXLEYI (PRYMNESIOPHYCEAE)1

Laboratory experiments were performed with the prymnesiophyte Emiliania huxleyi (Lohm.) Hay and Mohler, strain 88E, to quantify calcification per cell, coccolith detachment, and effects of coccolith production on optical scattering of individual cells. ¹⁴C incorporation into attached and detached coccoliths was measured using a bulk filtration technique. ¹⁴C-labeled cells also were sorted using a flow cytometer and analyzed for carbon incorporation into attached coccoliths. The difference between the bulk and flow cytometer analyses provided a ¹⁴C-based estimate of the rate of production of detached coccoliths. Coccolith production and detachment were separated in time in batch cultures, with most detachment happening well after calcification had stopped. Accumulation of coccoliths was maximum at the end of logarithmic growth with 50–80 coccoliths per cell (three to five complete layers of coccoliths around the cells). Net accretion rates of coccoliths were on the order of 7 coccoliths· cell^?1·d^?1 while net detachment rates were as high as 15 coccoliths· cell^?1·d^?1 for stationary phase cells. Equal numbers of coccoliths were attached and detached early in logarithmic growth, and as cells aged, the numbers of detached coccoliths exceeded the attached ones by a factor of 6. Our results demonstrate pronounced charges of forward angle light scatter and 90° light scatter of cells as they grow logarithmically and enter stationary phase. Counts of loose coccoliths in batch cultures are consistent with the detachment of coccoliths in layers rather than individual coccoliths.     

Light-dependent coccolith formation in the two forms of Coccolithus pelagicus

Summary Two methods were employed for measuring coccolith formation and photosynthesis in coccolithophorids. The first method was based on measurements of ¹⁴C radioactivity of cells on membrane filters before and after acid treatment. The second method involved a conversion of ¹⁴C in coccoliths or whole cells to BaCO₃ prior to counting. It was observed that in determinations of photosynthetic (or total) ¹⁴C by the first method, the count rate produced by a given amount of the isotope was 30–40% lower in the non-motile and motile forms of Coccolithus pelagicus than in C. huxleyi. There was no similarly great discrepancy in determinations of coccolith ¹⁴C.Light-dependent coccolith formation was demonstrated in both forms of C. pelagicus. The non-motile form may deposit several times more carbon in its coccoliths than it assimilates photosynthetically. In the motile form, coccolith carbon amounts to less than 2% of photosynthetic carbon.     

Action Spectrum of Coccolith Formation

The action spectrum of coccolith formation in Coccolithus huxleyi was determined by measuring the uptake of carbon-14 in coccoliths in four-hour experiments as a function of light intensity at each of seven wavelengths. An action spectrum | of photosynthetic carbon assimilation was obtained at the same time. The coccolith action spectrum had peaks at wavelengths of about 440 nm and 670 nm. probably corresponding to the regions of maximum cellular absorption and carbon assimilation. However, blue light appeared to be relatively more efficient in coccolith formation than in carbon assimilation. The results suggest that light-dependent coccolith formation may be catalyzed by two photochemical reactions, one mediated by chloroplast pigments and the other by some pigment absorbing specifically in the blue part of the spectrum.     

Increased coccolith production by Emiliania huxleyi cultures enriched with dissolved inorganic carbon

Cells of Emiliania huxleyi grown on Eppley's medium enriched with dissolved inorganic carbon (DIC) developed multiple layers of coccoliths. The maximum diameter of cells grown in the presence of 13.2 mM DIC was 12.3 m, whereas that of cells grown in the presence of 1.5 mM DIC was 8.0 m. Although enrichment of Eppley's medium with DIC increased both coccolith production and cell growth, coccolith production was enhanced to a greater extent than cell growth. The enrichment of Eppley's medium with DIC was used to enhance production of coccolith particles by E. huxleyi. Repeated-batch culture, in which DIC, Ca²⁺, nitrate and phosphate concentrations in the medium were maintained by replacing the culture medium, was carried out in a closed photobioreactor. During repeated-batch culture, a maximum coccolith yield of 560 mg/l for 2 days and a maximum biomass yield of 810 mg/l for 2 days were achieved. Enrichment and maintenance of DIC is therefore an efficient method for the production of large quantities of coccoliths.     

NO MECHANISTIC DEPENDENCE OF PHOTOSYNTHESIS ON CALCIFICATION IN THE COCCOLITHOPHORID EMILIANIA HUXLEYI (HAPTOPHYTA)1

There is still considerable uncertainty about the relationship between calcification and photosynthesis. It has been suggested that since calcification in coccolithophorids is an intracellular process that releases CO₂, it enhances photosynthesis in a manner analogous to a carbon‐concentrating mechanism (CCM). The ubiquitous, bloom‐forming, and numerically abundant coccolithophorid Emiliania huxleyi (Lohmann) W. W. Hay et H. Mohler was studied in nutrient‐replete, pH and [CO₂] controlled, continuous cultures (turbidostats) under a range of [Ca²⁺] from 0 to 9 mM. We examined the long‐term, fully acclimated photosynthesis‐light responses and analyzed the crystalline structure of the coccoliths using SEM. The E. huxleyi cells completely lost their coccosphere when grown in 0 [Ca²⁺], while thin, undercalcified and brittle coccoliths were evident at 1 mM [Ca²⁺]. Coccoliths showed increasing levels of calcification with increasing [Ca²⁺]. More robust coccoliths were noted, with no discernable differences in coccolith morphology when the cells were grown in either 5 or 9 mM (ambient seawater) [Ca²⁺]. In contrast to calcification, photosynthesis was not affected by the [Ca²⁺] in the media. Cells showed no correlation of their light‐dependent O₂ evolution with [Ca²⁺], and in all [Ca²⁺]‐containing turbidostats, there were no significant differences in growth rate. The results show unequivocally that as a process, photosynthesis in E. huxleyi is mechanistically independent from calcification.     

Wigwamma scenozonion sp.nov. (Prymnesiophyceae) from West Greenland

Wigwamma scenozonion sp. nov. (Prymnesiophyceae) is described on the basis of electron microscopy of shadowcast whole mounts prepared from water samples collected in the vicinity of Godhavn (West Greenland) in July and August 1977. This nanoplanktonic coccolithophorid possesses two smooth flagella and a shorter coiling haptonema. Coccoliths of one type cover the whole cell. Each coccolith is composed of a ring of rod-like crystallites joined end to end and arranged parallel to the edge of the oval coccolith base-plates. A single enlarged crystallite is found on most coccoliths. W. scenozonion is distinguished from the two previously described Wigwamma species by the lack of coccolith superstructures and by having one, rather than two rings of crystallites along the base-plate edge. In addition to the West Greenland specimens a single W. scenozonion cell has been encountered in a water sample from Denmark.     

CALCIFICATION BY COCCOLITHOPHORIDS: EFFECTS OF pH AND Sr1

Cells of Coccolithus huxleyi which fail to deposit CaCO₃ and form coccoliths often occur as unwanted components in cultures used for studies of calcification. Non-calcified cells generally cannot be made to recalcify, but they can be removed from cultures by treatment at elevated pH or by a method based on faster sinking of calcified cells. Lowering the concentrations of nitrate, phosphate, or trace metals in the medium did not restore calcifying ability of non-calcified cells. However, addition of strontium did promote recalcification of decalcified Cricosphaera carterae grown under calcium limitation. Strontium seemed to promote coccolith attachment to cells rather than to affect calcium uptake or coccolith formation itself.     

Structural and physiological effects of calcium and magnesium in Emiliania huxleyi (Lohmann) Hay and Mohler

In organisms which perform both photosynthesis and calcification, the fact that calcification proceeds faster in the light than in the dark has led to the long-established view that photosynthesis and calcification are closely coupled. It is now clear that calcification does not promote photosynthesis, but an enhancement of calcification by photosynthesis could still explain why calcification is faster in the light. To test this, the kinetics of the two processes were monitored over a wide range of calcium concentrations (0-50 mM) in the coccolithophore Emiliania huxleyi. The addition of 50 mM calcium strongly inhibited both processes, but when incubated in lower concentrations, rates of calcification increased up to 20 mM calcium whilst those of photosynthesis remained constant over the same range of calcium concentrations. So, rates of calcification are able to rise without a concomitant increase in photosynthetic rates. In addition, calcification rate and coccolith morphology responded similarly to changes in calcium concentrations; low calcification rates were associated with poor coccolith structure (undercalcification) and high calcification rates with perfectly formed coccoliths. Calcium concentration thus strongly influences calcification affecting both crystal structure and rate of calcite deposition. A similar structural analysis of coccoliths from cells grown in different magnesium concentrations showed that this ion is also essential for calcification, since strong signs of coccolith malformation and undercalcification were apparent at both low and high magnesium concentrations. In contrast with the calcium results, coccoliths were flawless only in the normal seawater concentration of 58 mM magnesium. We conclude that photosynthesis and calcification are not closely coupled and that calcification depends on a precise balance of both calcium and magnesium concentrations.     

Coccolith volume of the Southern Ocean coccolithophore Emiliania huxleyi as a possible indicator for palaeo‐cell volume

Coccolithophores are a key functional phytoplankton group and produce minute calcite plates (coccoliths) in the sunlit layer of the pelagic ocean. Coccoliths significantly contribute to the sediment record since the Triassic and their geometry have been subject to palaeoceanographic and biological studies to retrieve information on past environmental conditions. Here, we present a comprehensive analysis of coccolith, coccosphere and cell volume data of the Southern Ocean Emiliania huxleyi ecotype A, subject to gradients of temperature, irradiance, carbonate chemistry and macronutrient limitation. All tested environmental drivers significantly affect coccosphere, coccolith and cell volume with driver‐specific sensitivities. However, a highly significant correlation emerged between cell and coccolith volume with V_coccolith = 0.012 ± 0.001 * V_cell + 0.234 ± 0.066 (n = 23, r² = .85, p < .0001, σ_est = 0.127), indicating a primary control of coccolith volume by physiological modulated changes in cell volume. We discuss the possible application of fossil coccolith volume as an indicator for cell volume/size and growth rate and, additionally, illustrate that macronutrient limitation of phosphorus and nitrogen has the predominant influence on coccolith volume in respect to other environmental drivers. Our results provide a solid basis for the application of coccolith volume and geometry as a palaeo‐proxy and shed light on the underlying physiological reasons, offering a valuable tool to investigate the fossil record of the coccolithophore E. huxleyi.     

Coccolith production inHymenomonas carterae

Summary The formation of coccoliths inHymenomonas carterae was studied. It was found that the Golgi body was directly involved in the production of the baseplate scale, the organic matrix membrane and the deposition of calcium carbonate to produce the coccolith. The ICP body is no longer regarded as being involved in coccolith production but more with the getting rid of excess coccoliths within the cell. Because the ICP body contains acid hydrolases it is suggested that this organelle now be referred to as a residual body.Financial assistance from the Council for Scientific and Industrial Research is gratefully acknowledged.     

Polyanion-mediated mineralization — assembly and reorganization of acidic polysaccharides in the Golgi system of a coccolithophorid alga during mineral deposition

Summary Immunolocalization of two highly acidic polysaccharides (PS-1 and PS-2) in a calcifying algaPleurochrysis carterae is described throughout the mineralization process, from before crystal nucleation through the cessation of crystal growth. This unicellular coccolithophorid alga is a useful model for mineralization because it produces calcified scales known as coccoliths in homogeneous cell culture. PS-1 and PS-2 were localized in the crystal coats of mature coccoliths and in electron dense Golgi particles. The polyanions are synthesized in medial Golgi cisternae and co-aggregate with calcium ions into discrete 25 nm particles. Particle-laden vesicles bud from cisternal margins and fuse with a coccolith-forming saccule containing an organic oval-shaped scale which forms the base of the future coccolith. The particles are localized on the base before the onset of mineral deposition and are present in the coccolith saccule throughout the period of crystal (CaCO₃) nucleation and growth. During the final phase of coccolith formation, the particles disappear, and the mature crystals acquire an amorphous coat containing PS-1 and PS-2 polysaccharides which remain with the mineral phase after the coccoliths are extruded from the cell. Postulated mechanisms of polyanion-mediated mineralization are reviewed and their relevance to the calcification of coccoliths is addressed.Abbreviations PS-1 polysaccharide one - PS-2 polysaccharide two - BSA bovine serum albumin - SDS sodium dodecyl sulfate - MES 2-(N-morpholino)-ethanesulfonic acid - EDTA ethylenediaminetetraacetic acid - DHA 3-deoxy-lyxo-2-heptulosaric acid - TCA trichloroacetic acid     

The Effect of cytoskeleton inhibitors on coccolith morphology in Coccolithus braarudii and Scyphosphaera apsteinii

The calcite platelets of coccolithophores (Haptophyta), the coccoliths, are among the most elaborate biomineral structures. How these unicellular algae accomplish the complex morphogenesis of coccoliths is still largely unknown. It has long been proposed that the cytoskeleton plays a central role in shaping the growing coccoliths. Previous studies have indicated that disruption of the microtubule network led to defects in coccolith morphogenesis in Emiliania huxleyi and Coccolithus braarudii. Disruption of the actin network also led to defects in coccolith morphology in E. huxleyi, but its impact on coccolith morphology in C. braarudii was unclear, as coccolith secretion was largely inhibited under the conditions used. A more detailed examination of the role of actin and microtubule networks is therefore required to address the wider role of the cytoskeleton in coccolith morphogenesis. In this study, we have examined coccolith morphology in C. braarudii and Scyphosphaera apsteinii following treatment with the microtubule inhibitors vinblastine and colchicine (S. apsteinii only) and the actin inhibitor cytochalasin B. We found that all cytoskeleton inhibitors induced coccolith malformations, strongly suggesting that both microtubules and actin filaments are instrumental in morphogenesis. By demonstrating the requirement for the microtubule and actin networks in coccolith morphogenesis in diverse species, our results suggest that both of these cytoskeletal elements are likely to play conserved roles in defining coccolith morphology.     

Involvement of Acidic Polysaccharide Ph-PS-2 and Protein in Initiation of Coccolith Mineralization,as Demonstrated by In Vitro Calcification on the Base Plate

Coccolithophorids, unicellular marine microalgae, have calcified scales with elaborate structures, called coccoliths, on the cell surface. Coccoliths generally comprise a base plate, CaCO₃, and a crystal coat consisting of acidic polysaccharides. In this study, the in vitro calcification conditions on the base plate of Pleurochrysis haptonemofera were examined to determine the functions of the base plate and acidic polysaccharides (Ph-PS-1, -2, and -3). When EDTA-treated coccoliths (acidic polysaccharide-free base plates) or low pH-treated coccoliths (whole acidic polysaccharide-containing base plates) were used, mineralization was not detected on the base plate. In contrast, in the case of coccoliths which were decalcified by lowering of the pH and then treated with urea (Ph-PS-2-containing base plates), distinct aggregates, probably containing CaCO₃, were observed only on the rim of the base plates. Energy dispersive X-ray spectroscopy (EDS) confirmed that the aggregates contained Ca and O, although X-ray diffraction analysis did not reveal any evidence of crystalline materials. Also, in vitro mineralization experiments performed on EDTA-treated coccoliths using isolated acidic polysaccharides demonstrated that the Ca-containing aggregates were markedly formed only in the presence of Ph-PS-2. Furthermore, in vitro mineralization experiments conducted on protein-extracted base plates suggested that the coccolith-associated protein(s) are involved in the Ca deposition. These findings suggest that Ph-PS-2 associated with the protein(s) on the base plate rim initiates Ca²⁺ binding at the beginning of coccolith formation, and some other factors are required for subsequent calcite formation.     

Effects of Ca and Mg on Growth and Calcification of the Coccolithophorid Pleurochrysis haptonemofera: Ca Requirement for Cell Division in Coccolith-Bearing Cells and for Normal Coccolith Formation with Acidic Polysaccharides

The effects of Ca²⁺ and Mg²⁺ on cellular growth and calcification in Pleurochrysis haptonemofera were investigated. In the presence of a normal concentration of Mg²⁺, coccolith-bearing cells (C-cells) required more than 0.5 mM Ca²⁺ for growth, while naked cells could grow even with 0.5 mM Ca²⁺. The calcification rate of C-cells, which was determined using decalcified cells, was significantly repressed with less than or equal to 0.5 mM Ca²⁺. Although the calcification rate did not change so much with 5–30 mM Ca²⁺, it decreased with higher concentrations of Ca²⁺, as well as C-cell-specific growth repression. Under these conditions, Ca²⁺ affected the rate of coccolith formation, but neither the coccolith morphology nor total amounts and ratios of divalent cations and acidic polysaccharides (Ph-PS-1, -2, and -3) were included in coccoliths. These findings suggest that sufficient calcification is required for the division of C-cells. Under low Ca²⁺ and high Mg²⁺ conditions, coccoliths with an abnormal morphology, having immature shield elements, were synthesized. Composition analysis of the coccoliths revealed high Mg/Ca and low Ph-PS-2/(Ph-PS-1 and -3) ratios, as compared with those under low Ca²⁺ and normal Mg²⁺ conditions, suggesting that the abnormal morphology is due to a change in the crystal type and/or acidic polysaccharide composition.     

ON THE ROLE OF THE CYTOSKELETON IN COCCOLITH MORPHOGENESIS: THE EFFECT OF CYTOSKELETON INHIBITORS1

The coccolithophore Emiliania huxleyi (Lohmann) W. W. Hay et H. Mohler was cultured in natural seawater with the addition of either the microtubule‐inhibitor colchicine, the actin‐inhibitor cytochalasin B, or the photosynthesis inhibitor 3‐(3,4 dichlorophenyl)‐1,1‐dimethyl‐urea (DCMU). Additionally, E. huxleyi was cultured at different light intensities and temperatures. Growth rate was monitored, and coccolith morphology analyzed. While every treatment affected growth rate, the percentage of malformed coccoliths increased with colchicine, cytochalasin B, and at higher than optimal temperature. These results represent the first experimental evidence for the role of microtubules and actin microfilaments in coccolith morphogenesis.     

Photosynthetic Activity during the Life Cycle of Synchronous Skeletonema Cells

A culture of Skeletonema costatum grown at a light intensity of 3 klux and at 20°C was synchronized in diurnally intermittent illumination of 12 hour light and 12 hour dark. The culture was hardly fully synchronous as the cell division period lasted about 9 hours. The cell division started in the middle of the light period. The concentration of the pigments: chlorophyll a, chlorophyll 6 and fucoxanthin and the rate of light-saturated photosynthesis were followed every hour during the 24 hour period. Both the concentration of pigments and the photosynthetic activity showed a rhythmical variation. The concentration per cell of all three pigments examined increased during the development of the cells and decreased automatically during the period of cell division. An increase in the pigment concentration was found only in the light period. The rate of light-saturated photosynthesis calculated per unit of cell number increased during the cell development and decreased during the division period. The increase in the photosynthetic activity at light-saturation started about 4 hours after the end of cell division, which was 4 hours before the light was turned on while the increase in the concentration of chlorophyll a first started 1–2 hours after this moment. The variation in photosynthetic activity was compared with that found by other workers. The results found with Chlorella ellipsoidea by Japanese scientists (Nihci et al.) was explained as an inhibition phenomenon because the cells were not adapted to the experimental conditions.     

Formation and mosaicity of coccolith segment calcite of the marine algae Emiliania huxleyi

Coccolithophores belong to the most abundant calcium carbonate mineralizing organisms. Coccolithophore biomineralization is a complex and highly regulated process, resulting in a product that strongly differs in its intricate morphology from the abiogenically produced mineral equivalent. Moreover, unlike extracellularly formed biological carbonate hard tissues, coccolith calcite is neither a hybrid composite, nor is it distinguished by a hierarchical microstructure. This is remarkable as the key to optimizing crystalline biomaterials for mechanical strength and toughness lies in the composite nature of the biological hard tissue and the utilization of specific microstructures. To obtain insight into the pathway of biomineralization of Emiliania huxleyi coccoliths, we examine intracrystalline nanostructural features of the coccolith calcite in combination with cell ultrastructural observations related to the formation of the calcite in the coccolith vesicle within the cell. With TEM diffraction and annular dark‐field imaging, we prove the presence of planar imperfections in the calcite crystals such as planar mosaic block boundaries. As only minor misorientations occur, we attribute them to dislocation networks creating small‐angle boundaries. Intracrystalline occluded biopolymers are not observed. Hence, in E. huxleyi calcite mosaicity is not caused by occluded biopolymers, as it is the case in extracellularly formed hard tissues of marine invertebrates, but by planar defects and dislocations which are typical for crystals formed by classical ion‐by‐ion growth mechanisms. Using cryo‐preparation techniques for SEM and TEM, we found that the membrane of the coccolith vesicle and the outer membrane of the nuclear envelope are in tight proximity, with a well‐controlled constant gap of ~4 nm between them. We describe this conspicuous connection as a not yet described interorganelle junction, the “nuclear envelope junction”. The narrow gap of this junction likely facilitates transport of Ca²⁺ ions from the nuclear envelope to the coccolith vesicle. On the basis of our observations, we propose that formation of the coccolith utilizes the nuclear envelope–endoplasmic reticulum Ca²⁺‐store of the cell for the transport of Ca²⁺ ions from the external medium to the coccolith vesicle and that E. huxleyi calcite forms by ion‐by‐ion growth rather than by a nanoparticle accretion mechanism.