EFFECTS OF ULTRAVIOLET‐A RADIATION AND NUTRIENT AVAILABILITY ON THE CELLULAR COMPOSITION OF PHOTOPROTECTIVE COMPOUNDS IN GLENODINIUM FOLIACEUM (DINOPHYCEAE)1

The photoprotective response in the dinoflagellate Glenodinium foliaceum F. Stein exposed to ultraviolet‐A (UVA) radiation (320–400 nm; 1.7 W · m²) and the effect of nitrate and phosphate availability on that response have been studied. Parameters measured over a 14 d growth period in control (PAR) and experimental (PAR + UVA) cultures included cellular mycosporine‐like amino acids (MAAs), chls, carotenoids, and culture growth rates. Although there were no significant effects of UVA on growth rate, there was significant induction of MAA compounds (28 ± 2 pg · cell^?1) and a reduction in chl a (9.6 ± 0.1 pg · cell^?1) and fucoxanthin (4.4 ± 0.1 pg · cell^?1) compared to the control cultures (3 ± 1 pg · cell^?1, 13.3 ± 3.2 pg · cell^?1, and 7.4 ± 0.3 pg · cell^?1, respectively). In a second investigation, MAA concentrations in UVA‐exposed cultures were lower when nitrate was limited (P < 0.05) but were higher when phosphate was limiting. Nitrate limitation led to significant decreases (P < 0.05) in cellular concentration of chls (chl c₁, chl c₂, and chl a), but other pigments were not affected. Phosphate availability had no effect on final pigment concentrations. Results suggest that nutrient availability significantly affects cellular accumulation of photoprotective compounds in G. foliaceum exposed to UVA.     

COMPARATIVE EFFECTS OF LIGHT ON PIGMENTS OF TWO STRAINS OF EMILIANIA HUXLEYI (HAPTOPHYTA)1

Calcifying and a noncalcifying strains of Emiliania huxleyi were cultured in nutrient replete turbidostats under a photon flux density (PFD) gradient from 50 to 600 μmol E·m^?2·s^?1. For both strains, growth was PFD‐saturated at 300 μmol E·m^?2·s^?1. The strains, although with clearly different physiological properties due to the presence or absence of calcification, showed the same trends and magnitude of change in their pigment compliment as a function of PFD. Light‐controlled pigment composition and the trends of change in pigment composition were identical in both strains. Fucoxanthin (Fuco) was the major carotenoid in the calcifying strain, while in the noncalcifying strain this role was assumed by 19′ hexanoyloxyfucoxanthin (19 Hex). The photoprotective pigments and 19 Hex, normalized to chl a, increased with increasing light, while chl a content per cell and chl c's and Fuco, normalized to chl a, decreased with increasing PFD. The sum of all carotenoids normalized to chl a was remarkably similar in all PFDs used. Collectively, our results suggest that 19 Hex was synthesized from Fuco with light as a modulating factor and that the total amount of carotenoids is strain‐specific and synthesized/catabolized in tandem with chl a to a genetically predefined level independent of PFD.     

Conserved V‐ATPase c subunit plays a role in plant growth by influencing V‐ATPase‐dependent endosomal trafficking

In plant cells, the vacuolar‐type H⁺‐ATPases (V‐ATPase) are localized in the tonoplast, Golgi, trans‐Golgi network and endosome. However, little is known about how V‐ATPase influences plant growth, particularly with regard to the V‐ATPase c subunit (VHA‐c). Here, we characterized the function of a VHA‐c gene from Puccinellia tenuiflora (PutVHA‐c) in plant growth. Compared to the wild‐type, transgenic plants overexpressing PutVHA‐c in Arabidopsis thaliana exhibit better growth phenotypes in root length, fresh weight, plant height and silique number under the normal and salt stress conditions due to noticeably higher V‐ATPase activity. Consistently, the Arabidopsis atvha‐c5 mutant shows reduced V‐ATPase activity and retarded plant growth. Furthermore, confocal and immunogold electron microscopy assays demonstrate that PutVHA‐c is mainly localized to endosomal compartments. The treatment of concanamycin A (ConcA), a specific inhibitor of V‐ATPases, leads to obvious aggregation of the endosomal compartments labelled with PutVHA‐c‐GFP. Moreover, ConcA treatment results in the abnormal localization of two plasma membrane (PM) marker proteins Pinformed 1 (AtPIN1) and regulator of G protein signalling‐1 (AtRGS1). These findings suggest that the decrease in V‐ATPase activity blocks endosomal trafficking. Taken together, our results strongly suggest that the PutVHA‐c plays an important role in plant growth by influencing V‐ATPase‐dependent endosomal trafficking.     

CHLOROPHYLL C PIGMENT PATTERNS IN 18 SPECIES (51 STRAINS) OF THE GENUS PSEUDO‐NITZSCHIA (BACILLARIOPHYCEAE)1

The pigment composition of 18 species (51 strains) of the pennate diatom Pseudo‐nitzschia was examined using HPLC. The carotenoid composition was typical for diatoms, with fucoxanthin (the major xanthophyll), diadinoxanthin, diatoxanthin, and β,β‐carotene. However, a diverse array of chl c pigments was observed in the studied strains. All Pseudo‐nitzschia strains contained chl a and chl c₂, traces of Mg‐2,4‐divinyl phaeoporphyrin a₅ monomethyl ester (MgDVP), and traces of a chl c₂–like pigment originally found in the haptophyte Pavlova gyrans. The distribution of chl c₁ and chl c₃ was variable among species (present in seven and 14 species, respectively). Based on chl c distribution, three major pigment types were defined: type 1 (chl c₁ + c₂, four species: P. australis, P. brasiliana, P. multiseries, and P. seriata), type 2 (chl c₁ + c₂ + c₃, three species: P. fraudulenta, P. multistriata, and P. pungens), and type 3 (chl c₂ + c₃, 11 species: P. arenysensis, P. calliantha, P. cuspidata, P. decipiens, P. delicatissima, P. galaxiae, P. mannii, P. pseudodelicatissima, P. subcurvata, P. cf. subpacifica, and a novel Pseudo‐nitzschia species). Type 1 and 2 species also shared the absence of a particular morphological character, the central nodule in the raphe, with the only exception of P. fraudulenta. The implications of such pigment diversity in chemotaxonomy, HAB monitoring, ecology, and phylogeny of Pseudo‐nitzschia species are discussed.     

PIGMENT SIGNATURES AND PHYLOGENETIC RELATIONSHIPS OF THE PAVLOVOPHYCEAE (HAPTOPHYTA)1

Variations in the HPLC‐derived pigment composition of cultured Pavlovophyceae (Cavalier‐Smith) Green et Medlin were compared with phylogenetic relationships inferred from 18S rDNA sequencing, morphological characteristics, and current taxonomy. The four genera described for this haptophyte class (Diacronema Prauser emend. Green et Hibberd, Exanthemachrysis Lepailleur, Pavlova Butcher, and Rebecca Green) were represented by nine different species (one of which with data from GeneBank only). Chlorophylls a, c₁, c₂ and MgDVP (Mg‐[3,8‐divinyl]‐phytoporphyrin‐13²‐methylcarboxylate) and the carotenoids fucoxanthin, diadinoxanthin, diatoxanthin, and β,β‐carotene were detected in all cultures. Species only differed in the content of an unknown (diadinoxanthin‐like) xanthophyll and two polar chl c forms, identified as a monovinyl (chl c₁‐like) and a divinyl (chl c₂‐like) compound. This is the first observation of the monovinyl form in haptophytes. Based on distribution of these two chl c forms, species were separated into Pavlovophyceae pigment types A, B, and C. These pigment types crossed taxonomic boundaries at the generic level but were in complete accordance with species groupings based on molecular phylogenetic relationships and certain ultrastructural characteristics (position and nature of pyrenoid, stigma, and flagella). These results suggest that characterization of the pigment signature of unidentified culture strains of Pavlovophyceae can be used to predict their phylogenetic affinities and vice versa. Additional studies have been initiated to evaluate this possibility for the haptophyte class Prymnesiophyceae.     

LINKING TIME‐DEPENDENT CARBON‐FIXATION EFFICIENCIES IN DUNALIELLA TERTIOLECTA (CHLOROPHYCEAE) TO UNDERLYING METABOLIC PATHWAYS1

The chl‐specific short‐term ¹⁴C‐based production (P^b) measurement is a widely used tool to understand phytoplankton responses to environmental stresses. However, among the metabolic consequences of these stresses is variability in lifetimes of newly fixed carbon that cause P^b to range between chl‐specific net primary production (NPP*) and chl‐specific gross photosynthetic electron flow that is available for carbon reduction () depending on growth rate. To investigate the basis for this discrepancy, photosynthate utilization was characterized in Dunaliella tertiolecta Butcher grown at three different growth rates in N‐limited chemostats. P^b was measured throughout a 2 min to 24 h time course and showed clear growth‐rate‐dependent differences in lifetimes of newly fixed carbon. ¹⁴C pulse‐chase experiments revealed differences in patterns of carbon utilization between growth rates. At high growth rate, the majority of ¹⁴C was initially fixed into polysaccharide and lipid, but the relative contribution of each labeled biochemical pool to the total label changed over 24 h. In fast‐growing cells, labeled polysaccharides decreased 50%, while labeled lipids increased over the first 4 h. At low growth rate, ¹⁴C was initially incorporated primarily into protein, but the contribution of labeled protein to the total label increased over the next 24 h. Together, time‐resolved measurements of P^b and cellular NAD and NADP content suggest an enhanced role for alternative dissipation pathways at very low growth rate. Findings of this study contribute to an integrated understanding of growth‐rate‐dependent shifts in metabolic processes from photosynthesis to net growth.     

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.     

PHOTOSYNTHETIC ADAPTATION OF A BLOOM‐FORMING CYANOBACTERIUM MICROCYSTIS AERUGINOSA (CYANOPHYCEAE) TO PROLONGED UV‐B EXPOSURE1

The bloom‐forming cyanobacterium Microcystis aeruginosa Kütz 854 was cultured with 1.05 W·m^?2 UV‐B for 3 h every day, and its growth, pigments, and photosynthesis were investigated. The specific growth rates represented by chl a concentration and OD₇₅₀ were inhibited 8% and 9% by UV‐B exposure, respectively. Six days of UV‐B treatment significantly reduced cellular contents of phycocyanin and allophycocyanin by 32% and 62%, respectively, and markedly increased the carotenoid content by 27%, but had little effect on the chl a content. The initial values of optimal photosynthetic efficiency for UV‐B treated samples were, respectively, 52%, 87%, and 93% of controls on days 4, 7, and 10 of growth. The light‐saturated photosynthetic rates at day 6 were significantly lower than controls grown without UV‐B. The probability of electron transfer beyond Q_A decreased during UV‐B exposure, and this indicated that the acceptor side of PSII was one of main damage sites. The adaptation of M. aeruginosa 854 to UV‐B radiation could be observed from light‐saturated photosynthetic rates on day 13 and diurnal changes of chl fluorescence during the late growth phase. When both exposed to higher UV‐B, samples cultured under 1.05 W·m^?2 UV‐B for 9 days recovered faster than controls. It is suggested that M. aeruginosa 854 had at least three adaptive strategies to cope with the enhanced UV‐B: increasing the synthesis of carotenoids to counteract reactive oxidants caused by UV‐B exposure, degrading phycocyanin and allophycocyanin to avoid further damage to DNA and reaction centers, and enhancing the repair of UV‐B induced damage to the photosynthetic apparatus.     

Coccolithogenesis In Scyphosphaera apsteinii (Prymnesiophyceae)

Coccolithophores are the most significant producers of marine biogenic calcite, although the intracellular calcification process is poorly understood. In the case of Scyphosphaera apsteinii Lohmann 1902, flat ovoid muroliths and bulky, vase‐shaped lopadoliths with a range of intermediate morphologies may be produced by a single cell. This polymorphic species is within the Zygodiscales, a group that remains understudied with respect to ultrastructure and coccolith ontogeny. We therefore undertook an analysis of cell ultrastructure, morphology, and coccolithogenesis. The cell ultrastructure showed many typical haptophyte features, with calcification following a similar pattern to that described for other heterococcolith bearing species including Emiliania huxleyi. Of particular significance was the reticular body role in governing fine‐scale morphology, specifically the central pore formation of the coccolith. Our observations also highlighted the essential role of the inter‐ and intracrystalline organic matrix in growth and arrangement of the coccolith calcite. S. apsteinii secreted mature coccoliths that attached to the plasma membrane via fibrillar material. Time‐lapse light microscopy demonstrated secretion of lopadoliths occurred base first before being actively repositioned at the cell surface. Significantly, growth irradiance influenced the coccosphere composition with fewer lopadoliths being formed relative to muroliths at higher light intensities. Overall, our observations support dynamic metabolic (i.e., in response to growth irradiance), sensory and cytoskeletal control over the morphology and secretion of polymorphic heterococcoliths. With a basic understanding of calcification established, S. apsteinii could be a valuable model to further study coccolithophore calcification and cell physiological responses to ocean acidification.     

INTERACTIVE EFFECTS OF IRRADIANCE AND TEMPERATURE ON THE PHOTOSYNTHETIC PHYSIOLOGY OF THE PENNATE DIATOM PSEUDO‐NITZSCHIA GRANII (BACILLARIOPHYCEAE) FROM THE NORTHEAST SUBARCTIC PACIFIC1

This study examined how light and temperature interact to influence growth rates, chl a, and photosynthetic efficiency of the oceanic pennate diatom Pseudo‐nitzschia granii Hasle, isolated from the northeast subarctic Pacific. Growth rates were modulated by both light and temperature, although for each irradiance tested, the growth rate was always the greatest at ～14°C. Chl a per cell was affected primarily by temperature, except at the maximum chl a per cell (at 10°C) where the effects of light were noticeable. At both ends of the temperature gradient, cells displayed evidence of chlorosis even at low light intensities. Chl fluorescence data suggested that cells at 8°C were significantly more efficient in their photosynthetic processes than cells at 20°C, despite having comparable concentrations of chl. Cells at low temperature showed photosynthetic characteristics similar to high‐irradiance‐adapted cells. The decline of growth rates beyond the optimum growth temperature coincided with the cell's inability to accumulate chl in response to increasing temperature. The decline in photosynthetic ability at 20°C was likely due to a combination of high‐temperature stress on cellular membranes and a decline in chl. Our results highlight the important interactions between light and temperature and the need to incorporate these interactions into the development of phytoplankton models for the subarctic Pacific.     

MOLECULAR CLONING AND EXPRESSION OF THE PROLIFERATING CELL NUCLEAR ANTIGEN GENE FROM THE COCCOLITHOPHORID PLEUROCHRYSIS CARTERAE (HAPTOPHYCEAE)1

The gene encoding proliferating cell nuclear antigen (PCNA) was isolated from the marine coccolithophorid microalga Pleurochrysis carterae (Braarud et Fagerland) Christensen (Haptophyceae). Two mRNAs (Pcpcna1 and Pcpcna2) were identified and contained an identical coding region for 222 amino acid residues and an untranslated sequence of 302 base pair (Ut1) and 246 base pair (Ut2), respectively. Comparison between PCR‐derived genomic DNA fragments and cDNA sequences revealed five introns. The coding region of Pcpcna is similar to counterparts in other organisms and contains highly conserved functional domains. Phylogenetic analyses indicated clustering of Pcpcna with pcna in its haptophyte relative Isochrysis galbana Parke. A recombinant fusion protein of Pcpcna, overexpressed in Escherichia coli, was recognized by the PC10 antibody against rat PCNA. Using RT‐PCR and Western blotting, Pcpcna was found to be highly transcribed and translated during the exponential growth phase relative to the stationary growth phase, with a positive correlation between gene expression and growth rate. It can be concluded that the pcna is conserved in this coccolithophorid phytoplankton and that its expression is growth stage related.     

DIFFERENTIAL RESPONSES OF ANABAENA SP. PCC 7120 (CYANOPHYCEAE) CULTURED IN NITROGEN‐DEFICIENT AND NITROGEN‐ENRICHED MEDIA TO ULTRAVIOLET‐B RADIATION1

Stratospheric ozone depletion increases the amount of ultraviolet‐B radiation (UVBR) (280–320 nm) reaching the surface of the earth, potentially affecting phytoplankton. In this work, Anabaena sp. PCC 7120, a typically nitrogen (N)‐fixing filamentous bloom‐forming cyanobacterium in freshwater, was individually cultured in N‐deficient and N‐enriched media for long‐term acclimation before being subjected to ultraviolet‐B (UVB) exposure experiments. Results suggested that the extent of breakage in the filaments induced by UVBR increases with increasing intensity of UVB stress. In general, except for the 0.1 W · m^?2 treatment, which showed a mild increase, UVB exposure inhibits photosynthesis as evidenced by the decrease in the chl fluorescence parameters maximum photochemical efficiency of PSII (F_v/F_m) and maximum relative electron transport rate. Complementary chromatic acclimation was also observed in Anabaena under different intensities of UVB stress. Increased total carbohydrate and soluble protein may provide some protection for the culture against damaging UVB exposure. In addition, N‐deficient cultures with higher recovery capacity showed overcompensatory growth under low UVB (0.1 W · m^?2) exposure during the recovery period. Significantly increased (～830%) ATPase activity may provide enough energy to repair the damage caused by exposure to UVB.     

Systemic RNAi of V‐ATPase subunit B causes molting defect and developmental abnormalities in Periplaneta fuliginosa

The vacuolar (H+)-ATPases (V-ATPases) are ATP-driven proton pumps with multiple functions in many organisms. In this study, we performed structural and functional analysis of vha55 gene that encodes V-ATPase subunit B in the smokybrown cockroach Periplaneta fuliginosa (Blattodea). We observed a high homology score of the deduced amino acid sequences between 10 species in seven orders. RNAi of the vha55 gene in R fuliginosa caused nymphal/nymphal molting defects with incomplete shedding of old cuticles, growth inhibition, as well as bent and wrinkled cuticles of thoraxes and abdominal segments. Since growth inhibition caused by vha55 RNAi did not interfere in the commencement of cockroach molting, molting timing and body growth might be controlled by independent mechanism. Our study suggested V-ATPases might be a good candidate molecule for evolutionary and developmental studies of insect molting.     

The signaling lipid phosphatidylinositol‐3,5‐bisphosphate targets plant CLC‐a anion/H+ exchange activity

Phosphatidylinositol‐3,5‐bisphosphate (PI(3,5)P₂) is a low‐abundance signaling lipid associated with endo‐lysosomal and vacuolar membranes in eukaryotic cells. Recent studies on Arabidopsis indicated a critical role of PI(3,5)P₂ in vacuolar acidification and morphology during ABA‐induced stomatal closure, but the molecular targets in plant cells remained unknown. By using patch‐clamp recordings on Arabidopsis vacuoles, we show here that PI(3,5)P₂ does not affect the activity of vacuolar H⁺‐pyrophosphatase or vacuolar H⁺‐ATPase. Instead, PI(3,5)P₂ at low nanomolar concentrations inhibited an inwardly rectifying conductance, which appeared upon vacuolar acidification elicited by prolonged H⁺ pumping activity. We provide evidence that this novel conductance is mediated by chloride channel a (CLC‐a), a member of the anion/H⁺ exchanger family formerly implicated in stomatal movements in Arabidopsis. H⁺‐dependent currents were absent in clc‐a knock‐out vacuoles, and canonical CLC‐a‐dependent nitrate/H⁺ antiport was inhibited by low concentrations of PI(3,5)P₂. Finally, using the pH indicator probe BCECF, we show that CLC‐a inhibition contributes to vacuolar acidification. These data provide a mechanistic explanation for the essential role of PI(3,5)P₂ and advance our knowledge about the regulation of vacuolar ion transport.     

Biolayer interferometry of lipid nanodisc‐reconstituted yeast vacuolar H+‐ATPase

Vacuolar H⁺‐ATPase (V‐ATPase) is a large, multisubunit membrane protein complex responsible for the acidification of subcellular compartments and the extracellular space. V‐ATPase activity is regulated by reversible disassembly, resulting in cytosolic V₁‐ATPase and membrane‐integral V₀ proton channel sectors. Reversible disassembly is accompanied by transient interaction with cellular factors and assembly chaperones. Quantifying protein‐protein interactions involving membrane proteins, however, is challenging. Here we present a novel method to determine kinetic constants of membrane protein–protein interactions using biolayer interferometry (BLI). Yeast vacuoles are solubilized, vacuolar proteins are reconstituted into lipid nanodiscs with native vacuolar lipids and biotinylated membrane scaffold protein (MSP) followed by affinity purification of nanodisc‐reconstituted V‐ATPase (V₁V₀ND). We show that V₁V₀ND can be immobilized on streptavidin‐coated BLI sensors to quantitate binding of a pathogen derived inhibitor and to measure the kinetics of nucleotide dependent enzyme dissociation.     

V- AND P-TYPE Ca2+-STIMULATED ATPases IN A CALCIFYING STRAIN OF PLEUROCHRYSIS SP. (HAPTOPHYCEAE)

Coccolithophorids are marine unicellular algae characterized by their ability to carry out controlled, subcellular calcification. The biochemical and kinetic features of membrane-bound Ca²⁺-stimulated ATPases have been examined. Membranes and organelles from axenic cultures of Pleurochrysis sp. (CCMP299) were isolated by means of sucrose density centrifugation. High levels of Ca²⁺-stimulated ATPase were detected in chloroplasts, Golgi apparatus, plasma membrane, and coccolith vesicles. The sensitivity of the enzyme activity in the organelles and membranes was assessed with pharmacologic agents that are known to be specific for the several isoforms of Ca²⁺-stimulated ATPase. The Ca²⁺-stimulated ATPase activity in the Golgi and coccolith vesicle preparations was sensitive to nitrate, thiocyanate, and sodium azide and insensitive to vanadate, cyclopiazonic acid, and thapsigargin. ATP-dependent H⁺ movement, but not ⁴⁵Ca²⁺ transport, across the coccolith vesicle was demonstrated. The Ca²⁺-stimulated ATPase in the plasma membrane preparation was sensitive to vanadate. ATP-dependent, vanadate-sensitive efflux of ⁴⁵Ca²⁺ was demonstrated for microsomal material derived from gradient-isolated plasma membrane. Polypeptides from isolated Golgi and coccolith vesicle preparations cross-reacted to an antibody raised against a subunit of the oat root proton pump, whereas polypeptides from the chloroplast preparations did not cross-react. These findings show that a V-type Ca²⁺-stimulated ATPase is located on the coccolith vesicle membrane and a P-type Ca²⁺-stimulated ATPase is located on the plasma membrane.     

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.