INTRASPECIFIC GENETIC DIVERSITY IN THE MARINE COCCOLITHOPHORE EMILIANIA HUXLEYI (PRYMNESIOPHYCEAE): THE USE OF MICROSATELLITE ANALYSIS IN MARINE PHYTOPLANKTON POPULATION STUDIES1

Using primer pairs for seven previously described microsatellite loci and three newly characterized microsatellite loci from the coccolithophore Emiliania huxleyi (Lohm.) Hay and Mohler, we assessed genetic variation within this species. Analysis of microsatellite length variants (alleles) was conducted for 85 E. huxleyi isolates representative of different ocean basins. These results revealed high intraspecific genetic variability within the E. huxleyi species concept. Pairwise comparison of a 1992 Coastal Fjord group (FJ92) (n=41) and a North East Atlantic (NEA) group (n=21), using F_ST as an indicator of genetic differentiation, revealed moderate genetic differentiation (F_ST=0.09894; P=0; significance level=0.05). Gene flow between the FJ92 and NEA groups was estimated to be low, which is in agreement with the moderate levels of genetic differentiation revealed by the microsatellite data. A genetic assignment method that uses genotype likelihoods to draw inference about the groups to which individuals belong was tested. Using FJ92 and NEA as reference groups, we observed that all the E. huxleyi groups tested against the two reference groups were unrelated to them. On a global biogeographical scale, E. huxleyi populations appear to be highly genetically diverse. Our findings raise the question of whether such a high degree of intraspecific genetic diversity in coccolithophores translates into variability in ecological function.     

MOLECULAR CHARACTERIZATION AND ANTIBODY DETECTION OF A NITROGEN‐REGULATED CELL‐SURFACE PROTEIN OF THE COCCOLITHOPHORE EMILIANIA HUXLEYI (PRYMNESIOPHYCEAE)1

Dissolved organic nitrogen (DON) can account for a significant portion of total nitrogen in some aquatic environments, and many species of phytoplankton are able to scavenge nitrogen from this pool especially when inorganic nitrogen is limiting. Emiliania huxleyi (Lohmann) H. W. Hay et H. Mohler is able to use various forms of DON for growth, including several amino acids, purines, and pyrimidines. A cell‐surface protein up‐regulated in the absence of inorganic nitrogen, NRP1, is hypothesized to play a role in the metabolism of one or more of these organic nitrogen forms. Here, the genomic and cDNA sequence of NRP1 is reported. Structural predictions based on the amino acid sequence suggest a pyridoxal‐5′‐phosphate‐dependent enzyme that may have a role in acquiring nitrogen from amino acids. Further evidence for the function of NRP1 is measured in spent media from nitrogen‐limited cultures, which contain NRP1 and have glutaminase and formamidase activity. Field studies using an antibody to NRP1 show that it is expressed in E. huxleyi during bloom conditions in a Norwegian fjord.     

MOLECULAR CHARACTERIZATION OF A PHOSPHATE‐REGULATED CELL‐SURFACE PROTEIN FROM THE COCCOLITHOPHORID,EMILIANIA HUXLEYI (PRYMNESIOPHYCEAE)1

Emiliania huxleyi (Lohmann) Hay et Mohler is a cosmopolitan coccolithophorid that is known to be an excellent competitor for phosphate. A previous survey of cell‐surface proteins induced by phosphorus limitation in strain CCMP 374 yielded three abundant proteins. Using CCMP 1516, the strain chosen for genome sequence determination, we report the cDNA, genomic, and amino acid sequence of one cell‐surface phosphorus‐limitation induced protein and evidence that a second protein is highly similar. The introns within the genomic DNA encoding this cell‐surface protein as well as those defined by other phosphate‐regulated expressed sequence tags are analyzed. As these proteins are the most abundant cell‐surface proteins present under phosphorus limitation, they likely have a role in the ability of this organism to compete for phosphate.     

A GENETIC MARKER TO SEPARATE EMILIANIA HUXLEYI (PRYMNESIOPHYCEAE) MORPHOTYPES1

Emiliania huxleyi (Lohm.) Hay and Mohler is a ubiquitous unicellular marine alga surrounded by an elaborate covering of calcite platelets called coccoliths. It is an important primary producer involved in oceanic biogeochemistry and climate regulation. Currently, E. huxleyi is separated into five morphotypes based on morphometric, physiological, biochemical, and immunological differences. However, a genetic marker has yet to be found to characterize these morphotypes. With the use of sequence analysis and denaturing gradient gel electrophoresis, we discovered a genetic marker that correlates significantly with the separation of the most widely recognized A and B morphotypes. Furthermore, we reveal that the A morphotype is composed of a number of distinct genotypes. This marker lies within the 3′ untranslated region of a coccolith associated protein mRNA, which is implicated in regulating coccolith calcification. Consequently, we tentatively termed this marker the coccolith morphology motif.     

NEW EVIDENCE FOR MORPHOLOGICAL AND GENETIC VARIATION IN THE COSMOPOLITAN COCCOLITHOPHORE EMILIANIA HUXLEYI (PRYMNESIOPHYCEAE) FROM THE COX1b‐ATP4 GENES1

Emiliania huxleyi (Lohmann) W. W. Hay et H. Mohler is a cosmopolitan coccolithophore occurring from tropical to subpolar waters and exhibiting variations in morphology of coccoliths possibly related to environmental conditions. We examined morphological characters of coccoliths and partial mitochondrial sequences of the cytochrome oxidase 1b (cox1b) through adenosine triphosphate synthase 4 (atp4) genes of 39 clonal E. huxleyi strains from the Atlantic and Pacific Oceans, Mediterranean Sea, and their adjacent seas. Based on the morphological study of culture strains by SEM, Type O, a new morphotype characterized by coccoliths with an open central area, was separated from existing morphotypes A, B, B/C, C, R, and var. corona, characterized by coccoliths with central area elements. Molecular phylogenetic studies revealed that E. huxleyi consists of at least two mitochondrial sequence groups with different temperature preferences/tolerances: a cool‐water group occurring in subarctic North Atlantic and Pacific and a warm‐water group occurring in the subtropical Atlantic and Pacific and in the Mediterranean Sea.     

A PHYTOL-SUBSTITUTED CHLOROPHYLL C FROM EMILIANIA HUXLEYI (PRYMNESIOPHYCEAE)1

A nonpolar chlorophyll c-like pigment was isolated by semi-preparative high performance liquid chromatography from the coccolithophorid Emiliania huxleyi (Lohm.) Hay and Mohler. The visible absorption spectrum of this pigment was similar to that of chlorophyll c₂. However, its nonpolar chromatographic properties were quite different from those of the relatively polar chlorophylls c₂ and c₃ in E. huxleyi and similar to those of chlorophyll a. Analyses by gas chromatography and gas chromatography-mass spectrometry showed that the nonpolar character of this new chlorophyll c was due to the presence of phytol, most likely on the acid side chain of the porphyrin macro-cycle. This is the first example of a phytol-substituted chlorophyll c-like pigment.     

ACCLIMATION OF EMILIANIA HUXLEYI (PRYMNESIOPHYCEAE) TO PHOTON FLUX DENSITY1

Growth rate, pigment composition, and noninvasive chl a fluorescence parameters were assessed for a noncalcifying strain of the prymnesiophyte Emiliania huxleyi Lohman grown at 50, 100, 200, and 800 μmol photons·m^?2·s^?1. Emiliania huxleyi grown at high photon flux density (PFD) was characterized by increased specific growth rates, 0.82 d^?1 for high PFD grown cells compared with 0.38 d^?1 for low PFD grown cells, and higher in vivo chl a specific attenuation coefficients that were most likely due to a decreased pigment package, consistent with the observed decrease in cellular photosynthetic pigment content. High PFD growth conditions also induced a 2.5‐fold increase in the pool of the xanthophyll cycle pigments diadinoxanthin and diatoxanthin responsible for dissipation of excess energy. Dark‐adapted maximal photochemical efficiency (F_v/F_m) remained constant at around 0.58 for cells grown over the range of PFDs, and therefore the observed decline, from 0.57 to 0.33, in the PSII maximum efficiency in the light‐adapted state, (F_v′/F_m′), with increasing growth PFD was due to increased dissipation of excess energy, most likely via the xanthophyll cycle and not due to photoinhibition. The PSII operating efficiency (F_q′/F_m′) decreased from 0.48 to 0.21 with increasing growth PFD due to both saturation of photochemistry and an increase in nonphotochemical quenching. The changes in the physiological parameters with growth PFD enable E. huxleyi to maximize rates of photosynthesis under subsaturating conditions and protect the photosynthetic apparatus from excess energy while supporting higher saturating rates of photosynthesis under saturating PFDs.     

EMILIANIA HUXLEYI (PRYMNESIOPHYCEAE): NITROGEN‐METABOLISM GENES AND THEIR EXPRESSION IN RESPONSE TO EXTERNAL NITROGEN SOURCES1

The availability and composition of dissolved nitrogen in ocean waters are factors that influence species composition in natural phytoplankton communities. The same factors affect the ratio of organic to inorganic carbon incorporation in calcifying species, such as the coccolithophore Emiliania huxleyi (Lohman) W. W. Hay et H. Mohler. E. huxleyi has been shown to thrive on various nitrogen sources, including dissolved organic nitrogen. Nevertheless, assimilation of dissolved nitrogen under nitrogen‐replete and ‐limited conditions is not well understood in this ecologically important species. In this study, the complete amino acid sequences for three functional genes involved in nitrogen metabolism in E. huxleyi were identified: a putative formamidase, a glutamine synthetase (GSII family), and assimilatory nitrate reductase. Expression patterns of the three enzymes in cells grown on inorganic as well as organic nitrogen sources indicated reduced expression levels of nitrate reductase when cells were grown on NH₄⁺ and a reduced expression level of the putative formamidase when growth was on NO₃^?. The data reported here suggest the presence of a nitrogen preference hierarchy in E. huxleyi. In addition, the gene encoding for a phosphate repressible phosphate permease was more highly expressed in cells growing on formamide than in cells growing on inorganic nitrogen sources. This finding suggests a coupling between phosphate and nitrogen metabolism, which might give this species a competitive advantage in nutrient‐depleted environments. The potential of using expression of genes investigated here as indicators of specific nitrogen‐metabolism strategies of E. huxleyi in natural populations of phytoplankton is discussed.     

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.     

LIPID BIOMARKER DIVERSITY IN THE COCCOLITHOPHORID EMILIANIA HUXLEYI (PRYMNESIOPHYCEAE) AND THE RELATED SPECIES GEPHYROCAPSA OCEANICA1

Twenty-four strains of Emiliania huxleyi and two strains of Gephyrocapsa oceanica were grown at 15°C under identical culture conditions to assess genetic variability in key lipid biomarker profiles (C₃₇-C₃₉ alkenones, C₃₆ and C₃₇ alkyl alkenoates, and C₃₁-C₃₈ alkenes). Under our culture conditions, little divergence an biomarker composition was detected between E. huxleyi strains from different oceanic regions or between E. huxleyi and G. oceanica even though the strains originated from biogeographical regions as diverse as the subpolar North Atlantic and subtropical Western Pacific. The major differences observed were in tetraunsaturated alkenone abundance and alkene profiles, which tended to separate neritic from open ocean strains. Different strains from the same locality were as different as strains originating from widely separated ocean basins, indicating extreme genotypic diversity within a population. Replicate cultures of the same strain showed significant variability in their biomarker profiles even though the culture temperature varied by only ±0.3°C, indicating that their synthesis ratios are influenced by environmental and/or physiological variable(s), as yet unidentified, in addition to temperature. Strong covariance in C₃₇ and C₃₈ methyl alkenone unsaturation ratios (U^k₃₇ and U^k_38Mρ respectively) and, in coastal strains, C₃₃, alkene and alkenone unsaturation ratios indicates that these compounds are biochemically linked.     

GPA, A CALCIUM-BINDING PROTEIN IN THE COCCOLITHOPHORID EMILIANIA HUXLEYI (PRYMNESIOPHYCEAE)

Intracellular polysaccharide fractions were isolated from calcifying B-type cells of Emiliania huxleyi and separated by electrophoretic fractionation. In all fractions, the polysaccharide was immunologically related to the polysaccharide of (extracellular) B-type coccoliths (CP-B) and not to polysaccharides of A-type coccoliths (CP-A). Most polysaccharide fractions also contained protein material. The fraction with the largest proportion of protein was used to raise antibodies. The resulting antiserum, α-BP, contained antibodies against both CP-B- and protein-epitopes. The antibodies specific for polysaccharide-epitopes reacted with intracellular polysaccharide fractions of B-type cells only. In contrast, the antibodies specific for protein-epitopes reacted with the intracellular fractions of B-type as well as A-type cells. With immunolocalization, the presence of protein antigen in a layer surrounding both types of cells was demonstrated. A cDNA library of E. huxleyi was screened with α-BP, and a gene called gpa was isolated. The open reading frame of gpa was found to encode a protein (GPA) of 36,608 D, containing, inter alia, 24% acidic residues (18% glutamic acid and 6% aspartic acid), 12% proline, and 23% alanine. GPA has two repeats, one containing a sequence resembling the Ca²⁺-binding loop of EF-hands. Overproduction of GPA in a prokaryotic system yielded a dimeric product capable of binding Ca²⁺. The possible role of GPA in the formation of coccoliths in E. huxleyi is discussed.     

INORGANIC CARBON TRANSPORT IN RELATION TO CULTURE AGE AND INORGANIC CARBON CONCENTRATION IN A HIGH-CALCIFYING STRAIN OF EMILIANIA HUXLEYI (PRYMNESIOPHYCEAE)1

The relationships among inorganic carbon transport, bicarbonate availability, intracellular pH, and culture age were investigated in high-calcifying cultures of Emiliania huxleyi (Lohmann) Hay & Mohler. Measurement of inorganic carbon transport by the silicone-oil centrifugation technique demonstrated that gadolinium, a potential Ca²⁺ channel inhibitor, blocked intracellular inorganic carbon uptake and photosynthetic ¹⁴CO₂₊ fixation in exponential-phase cells. In stationary-phase cells, the intracellular inorganic carbon concentration was unaffected by gadolinium. Gadolinium was also used to investigate the link between bicarbonate and Ca²⁺ transport in high-calcifying cells of E. huxleyi. Bicarbonate availability had significant and rapid effects on pH_i in exponential- but not in stationary-phase cells. 4′, 4′-Diisothiocyanostilbene-2,2′-disulfonic acid did not block bicarbonate uptake from the external medium by exponential-phase cells. Inorganic carbon utilization by exponential- and stationary-phase cells of Emiliania huxleyi was investigated using a pH drift technique in a closed system. Light-dependent alkalization of the medium by stationary-phase cells resulted in a final pH of 10.1 and was inhibited by dextran-bound sulphonamide, an inhibitor of external carbonic anhydrase. Exponential-phase cells did not generate a pH drift. Overall, the results suggest that for high-calcifying cultures of E. huxleyi the predominant pathway of inorganic carbon utilization differs in exponential and stationary phase cells of the same culture.     

VIRAL INFECTION OF EMILIANIA HUXLEYI (PRYMNESIOPHYCEAE) LEADS TO ELEVATED PRODUCTION OF REACTIVE OXYGEN SPECIES1

The effect of viral infection of Emiliania huxleyi (Lohman) Hay and Mohler on the concentration of intracellular reactive oxygen species (ROS), hydrogen peroxide (H₂O₂) excretion and cell photosynthetic capacity (CPC) was examined. During the crash of an E. huxleyi culture induced by viruses intracellular ROS concentrations were generally elevated and reached levels of approximately double those observed in non‐infected control cultures. H₂O₂ concentrations also increased in the media of the infected cultures from background levels of around 130 nM to approximately 580 nM while levels in the controls decreased. These data suggest that oxidative stress is elevated in infected cells. Although the precise mechanism for ROS production was not identified, a traditional defense related oxidative burst was ruled out, as no evidence of a rapid intracellular accumulation of ROS following addition of the virus was found. CPC declined substantially in the infected culture from a healthy 0.6–0 arbitrary units. Clearly infection disrupted normal photosynthetic processes, which could lead to the production of ROS via interruption of the electron transport chain at the PSII level. Alternatively, ROS may also be a necessary requirement for viral replication in E. huxleyi, possibly due to a link with viral‐induced cell death or associated with general death processes.     

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.     

DISTRIBUTION OF TWO TYPES OF EMILIANIA HUXLEYI (PRYMNESIOPHYCEAE) IN THE NORTHEAST ATLANTIC REGION AS DETERMINED BY IMMUNOFLUORESCENCE AND COCCOLITH MORPHOLOGY1

Culture strains of Emiliania huxleyi (Lohmann 1902) Hay et al. 1967 were placed into two groups designated E. huxleyi type A and type B on the basis of coccolith morphology and immunological properties of the coccolith polysaccharide. We studied the distribution of these types in the North Atlantic region using an indirect immunofluorescence assay with antisera directed against the coccolith polysaccharide of E. huxleyi type A and type B and epifluorescence microscopy. In field samples taken in the Northeast Atlantic Ocean, E. huxleyi type A was found exclusively. In contrast, type B was dominant in the North Sea. Scanning electron microscopy of the samples revealed the same unequal distribution of the two types as found with the immunofluorescent-labelling assay.     

COMPARISON OF GROWTH AND SINKING RATES OF NON-COCCOLITH- AND COCCOLITH-FORMING STRAINS OF EMILIANIA HUXLEYI(PRYMNESIOPHYCEAE) GROWN UNDER DIFFERENT IRRADIANCES AND NITROGEN SOURCES1

We examined the effect of the presence or absence of coccoliths on the growth and sinking rates of an oceanic isolate of the coccolithophore Emiliania huxleyi (Lohmann) Hay et Mohler isolated from the northeastern subarctic Pacific. Coccolith-forming and non-coccolith-forming (i.e. naked, nonmotile) strains were obtained from the same isolate and grown under both saturating and limiting irradiance levels with either nitrate or ammonium as the primary nitrogen source. Sinking rate, growth rate, and cell volume (excluding coccoliths) were measured for each culture. Under saturating irradiance, coccolith-forming cells grew significantly slower than naked cells, had significantly higher sinking rates, and had larger cell volumes than naked cells. Under limiting irradiance levels, growth rates of the two strains were identical, sinking rates were higher for coccolith-forming cells in stationary-phase cultures only, and cell volumes remained greater for coccolith-forming cells. The sinking rates achieved for this ubiquitous coccolithophore ranged from <0.1 to 0.5 m · d^?1. Sinking rates were not statistically different between coccolith-forming and naked strains of E. huxleyi under limiting irradiance conditions for log-phase cultures, but sinking rates were greater for coccolith-forming cells under some of the other conditions tested. However, the average sinking rate was never more than twice as great as for coccolith-forming cells, with the exception of nitrate-grown, senescent cells under limiting irradiance (3.4 times greater). Cell volumes (excluding coccoliths) were consistently ca. 1.5 times greater for coccolith-forming cells than for naked cells. Nitrogen source had an effect on growth rate and cell volume, with ammonium-grown cultures growing faster and having larger cell volumes than nitrate-grown cultures of both strains. However, despite the difference in growth rate and cell volume, nitrogen source had little if any effect on sinking rate.     

IMPROVED DETECTION AND CHARACTERIZATION OF FUCOXANTHIN‐TYPE CAROTENOIDS: NOVEL PIGMENTS IN EMILIANIA HUXLEYI (PRYMNESIOPHYCEAE)1

A new atmospheric pressure chemical ionization mass spectrometry (APCI‐LC/MS) method improved detection and aided characterization of fucoxanthin related carotenoids, revealing the coccolithophorid Emiliania huxleyi (Lohm.) Hay et Mohler (strain MBA 92, Plymouth) to contain a wider range of acyloxyfucoxanthins than reported previously. The diversity is confirmed as arising from differences in the length of the alkanoic acid substituent esterified at position C‐19′. Acyloxyfucoxanthins with substituents of between four and eight carbons at the C‐19′ position have been detected in a culture of Emiliania huxleyi, where previously only 19′‐butanoyloxyfucoxanthin and 19′‐hexanoyloxyfucoxanthin have been reported in the literature. Novel fucoxanthinol derivatives were also found. The detection of these novel carotenoids in Emiliania huxleyi permits detailed studies of the impact of environmental factors on individual components of the complex pool of fucoxanthin‐type carotenoids in this organism.     

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.     

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.