EFFECTS OF IRON AND LIGHT STRESS ON THE BIOCHEMICAL COMPOSITION OF ANTARCTIC PHAEOCYSTIS SP. (PRYMNESIOPHYCEAE). II. PIGMENT COMPOSITION

A strain of Phaeocystis sp., isolated in the Southern Ocean, was cultured under iron- and light-limited conditions. The cellular content of chlorophyll a and accessory light-harvesting (LH) pigments increased under low light intensities. Iron limitation resulted in a decrease of all light-harvesting pigments. However, this decrease was greatly compensated for by a decrease in cell volume. Cellular concentrations of the LH pigments were similar for both iron-replete and iron-deplete cells. Concentrations of chlorophyll a were affected only under low light conditions, wherein concentrations were suppressed by iron limitation. Ratios of the LH pigments to chlorophyll a were highest for iron-deplete cells under both light conditions. The photoprotective cycle of diato/diadinoxanthin was activated under high light conditions, and enhanced by iron stress. The ratio of diatoxanthin to diadinoxanthin was highest under high light, low iron conditions.   Iron limitation induced synthesis of 19'-hexanoyloxyfucoxanthin and 19'-butanoyloxyfucoxanthin at the cost of fucoxanthin. Fucoxanthin formed the main carotenoid in iron-replete Phaeocystis cells, whereas for iron-deplete cells 19'-hexanoyloxyfucoxanthin was found to be the main carotenoid. This shift in carotenoid composition is of importance in view of the marker function of both pigments, especially in areas where Phaeocystis sp. and diatoms occur simultaneously. A hypothesis is presented to explain the transformation of fucoxanthin into 19'-hexanoyloxyfucoxanthin and 19'-butanoyloxyfucoxanthin, referring to their roles as a light-harvesting pigment.     

Subunit composition and pigmentation of fucoxanthin-chlorophyll proteins in diatoms: evidence for a subunit involved in diadinoxanthin and diatoxanthin binding

Two different fucoxanthin-chlorophyll protein complexes (FCP) were purified from the centric diatom Cyclotella meneghiniana and characterized with regard to their polypeptide and pigment composition. Whereas the oligomeric FCPb complex is most probably composed of fcp5 gene products, the trimeric FCPa has subunits encoded by fcp1-3 and fcp6/7. The amount of the latter polypeptide is enhanced when FCPa is isolated from algae grown under HL conditions. This increase in Fcp6/7 polypeptides is accompanied by an increase in the pool of xanthophyll cycle pigments, diadinoxanthin and diatoxanthin, and a concomitant decrease in fucoxanthin content. In addition, the de-epoxidation ratio, i.e., the amount of diatoxanthin in relation to the pool of xanthophyll cycle pigments, is increased by a factor of 2. With regard to fluorescence yield, HL FCPa was quenched in comparison to LL FCPa. This is in accordance with the larger amount of diatoxanthin that is bound, which is supposed to act as a quencher like zeaxanthin in higher plants. Thus, we conclude that the enhanced content of diatoxanthin in FCPa plays a protective role, which is paralleled by a weakened light harvesting function due to a smaller amount of fucoxanthin.     

PIGMENT TURNOVER IN THE MARINE DIATOM THALASSIOSIRA WEISSFLOGII. II. THE 14CO2-LABELING KINETICS OF CAROTENOIDS1

The biosynthesis and turnover of the pigments fucoxanthin, diadinoxanthin (DD), and diatoxanthin (DT) were studied in exponentially growing cultures of the diatom Thalassiosira weissflogii (Grunow) Fryxell and Hasle to investigate the dependence of pigment turnover on algal growth rates and light intensity. ¹⁴C-bicarbonate was used as a tracer. The labeling kinetics of fucoxanthin and DT were described satisfactorily by a simple precursor-pigment model with two free parameters, the precursor and pigment turnover rate. At growth irradiances < 200 μE · m^?2· s^?1, labeling kinetics of DD indicated the presence of two kinetically distinct DD pools and at least one precursor pool. The average growth rate-normalized pigment turnover rate of fucoxanthin was 0. The growth rate-normalized turnover rate of DT, determined only at high light irradiances (> 200 μE·m^?2·s^?1), was 1.3. At high light irradiances, the growth rate-normalized turnover rate of DD was 1.8. At low light irradiances, the turnover rates of the two DD pools were 3.7 and 0, respectively. The corresponding pigment turnover times were on the order of days to weeks, depending on the growth rate of the cultures. A comparison of pigment pool sizes, pigment turnover rates, and precursor turnover rates suggests that fucoxanthin is synthesized from a pool of DD and that DD and DT are synthesized from a common precursor, possibly β-carotene. No evidence was seen for dynamic xanthophyll cycling. This suggests that the commonly known “xanthophyll cycle” is the simple unidirectional conversion of DD into DT, or of DT into DD, in response to rapid irradiance changes.     

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.     

RESPONSE OF THE PHOTOSYNTHETIC APPARATUS OF PHAEODACTYLUM TRICORNUTUM (BACILLARIOPHYCEAE) TO NITRATE,PHOSPHATE, OR IRON STARVATION1

The effects of nitrate, phosphate, and iron starvation and resupply on photosynthetic pigments, selected photosynthetic proteins, and photosystem II (PSII) photochemistry were examined in the diatom Phaeodactylum tricornutum Bohlin (CCMP 1327). Although cell chlorophyll a (chl a) content decreased in nutrient-starved cells, the ratios of light-harvesting accessory pigments (chl c and fucoxanthin) to chl a were unaffected by nutrient starvation. The chl a-specific light absorpition coefficient (a*) and the functional absorption cross-section of PSII (σ) increased during nutrient starvation, consistent with reduction of intracellular self-shading (i.e. a reduction of the “package effect”) as cells became chlorotic. The light-harvesting complex proteins remained a constant proportion of total cell protein during nutrient starvation, indicating that chlorosis mirrored a general reduction in cell protein content. The ratio of the xanthophylls cycle pigments diatoxanthin and diadinoxanthin to chl a increased during nutrient starvation. These pigments are thought to play a photo-protective role by increasing dissipation of excitation energy in the pigment bed upstream from the reaction centers. Despite the increase in diatoxanthin and diadinoxanthin, the efficiency of PSII photochemistry, as measured by the ration of variable to maximum fluorescence (F_v/F_m) of dark-adapted cells, declined markedly under nitrate and iron starvation and moderately under phosphate starvation. Parallel to changes in F_v/F_m were decreases in abundance of the reaction center protein D1 consistent with damage of PSII reaction centers in nutrient-starved cells. The relative abundance of the carboxylating enzyme, ribulose bisphosphate carboxylase/oxygenase (RUBISCO), decreased in response to nitrate and iron starvation but not phosphate starvation. Most marked was the decline in the abundance of the small subunit of RUBISCO in nitrate-starved cells. The changes in pigment content and fluorescence characteristics were typically reversed within 24 h of resupply of the limiting nutrient.     

The light-harvesting antenna of the diatom Phaeodactylum tricornutum. Evidence for a diadinoxanthin-binding subcomplex

Diatoms differ from higher plants by their antenna system, in terms of both polypeptide and pigment contents. A rapid isolation procedure was designed for the membrane-intrinsic light harvesting complexes (LHC) of the diatom Phaeodactylum tricornutum to establish whether different LHC subcomplexes exist, as well to determine an uneven distribution between them of pigments and polypeptides. Two distinct fractions were separated that contain functional oligomeric complexes. The major and more stable complex ( approximately 75% of total polypeptides) carries most of the chlorophyll a, and almost only one type of carotenoid, fucoxanthin. The minor complex, carrying approximately 10-15% of the total antenna chlorophyll and only a little chlorophyll c, is highly enriched in diadinoxanthin, the main xanthophyll cycle carotenoid. The two complexes also differ in their polypeptide composition, suggesting specialized functions within the antenna. The diadinoxanthin-enriched complex could be where the de-epoxidation of diadinoxanthin into diatoxanthin mostly occurs.     

Evidence for the existence of one antenna-associated, lipid-dissolved and two protein-bound pools of diadinoxanthin cycle pigments in diatoms

We studied the localization of diadinoxanthin cycle pigments in the diatoms Cyclotella meneghiniana and Phaeodactylum tricornutum. Isolation of pigment protein complexes revealed that the majority of high-light-synthesized diadinoxanthin and diatoxanthin is associated with the fucoxanthin chlorophyll protein (FCP) complexes. The characterization of intact cells, thylakoid membranes, and pigment protein complexes by absorption and low-temperature fluorescence spectroscopy showed that the FCPs contain certain amounts of protein-bound diadinoxanthin cycle pigments, which are not significantly different in high-light and low-light cultures. The largest part of high-light-formed diadinoxanthin cycle pigments, however, is not bound to antenna apoproteins but located in a lipid shield around the FCPs, which is copurified with the complexes. This lipid shield is primarily composed of the thylakoid membrane lipid monogalactosyldiacylglycerol. We also show that the photosystem I (PSI) fraction contains a tightly connected FCP complex that is enriched in protein-bound diadinoxanthin cycle pigments. The peripheral FCP and the FCP associated with PSI are composed of different apoproteins. Tandem mass spectrometry analysis revealed that the peripheral FCP is composed mainly of the light-harvesting complex protein Lhcf and also significant amounts of Lhcr. The PSI fraction, on the other hand, shows an enrichment of Lhcr proteins, which are thus responsible for the diadinoxanthin cycle pigment binding. The existence of lipid-dissolved and protein-bound diadinoxanthin cycle pigments in the peripheral antenna and in PSI is discussed with respect to different specific functions of the xanthophylls.     

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.     

BENTHIC AND PLANKTONIC ALGAL COMMUNITIES IN A HIGH ARCTIC LAKE: PIGMENT STRUCTURE AND CONTRASTING RESPONSES TO NUTRIENT ENRICHMENT1

We investigated the fine pigment structure and composition of phytoplankton and benthic cyanobacterial mats in Ward Hunt Lake at the northern limit of High Arctic Canada and the responses of these two communities to in situ nutrient enrichment. The HPLC analyses showed that more than 98% of the total pigment stocks occurred in the benthos. The phytoplankton contained Chrysophyceae, low concentrations of other protists and Cyanobacteria (notably picocyanobacteria), and the accessory pigments chl c₂, fucoxanthin, diadinoxanthin, violaxanthin, and zeaxanthin. The benthic community contained the accessory pigments chl b, chl c₂, and a set of carotenoids dominated by glycosidic xanthophylls, characteristic of filamentous cyanobacteria. The black surface layer of the mats was rich in the UV‐screening compounds scytonemin, red scytonemin‐like, and mycosporine‐like amino acids, and the blue‐green basal stratum contained high concentrations of light‐harvesting pigments. In a first bioassay of the benthic mats, there was no significant photosynthetic or growth response to inorganic carbon or full nutrient enrichment over 15 days. This bioassay was repeated with increased replication and HPLC analysis in a subsequent season, and the results confirmed the lack of significant response to added nutrients. In contrast, the phytoplankton in samples from the overlying water column responded strongly to enrichment, and chl a biomass increased by a factor of 19.2 over 2 weeks. These results underscore the divergent ecophysiology of benthic versus planktonic communities in extreme latitudes and show that cold lake ecosystems can be dominated by benthic phototrophs that are nutrient sufficient despite their ultraoligotrophic overlying waters.     

The chloroplast pigments of the algal classes Eustigmatophyceae and Xanthophyceae. II. Xanthophyceae

The chloroplast pigments of Pleurochloris meiringensis Vischer, Mischococcus sphaerocephalus Vischer and Tribonema aequale Pascher have been analysed by a variety of chromatographic methods. There are significant differences between these xanthophycean algae and the eustigmatophycean algae formerly classified with them. In particular the former contain diadinoxanthin as the major xanthophyll whereas diadinoxanthin is absent from the latter and violaxanthin occurs as the major xanthophyll pigment. The other pigments of the Xanthophyceae sensu stricto include chlorophyll a, β-carotene, vaucheriaxanthin-ester, heteroxanthin, diatoxanthin, cryptoxanthin epoxide and a neoxanthin-like pigment.     

ULTRASTRUCTURE AND PIGMENTS OF TWO STRAINS OF THE PICOPLANKTONIC ALGA PELAGOCOCCUS SUBVIRIDIS (CHRYSOPHYCEAE)1

The organelle ultrastructure and photosynthetic pigments of a new isolate of the picoplanktonic alga Pelagococcus subviridis Norris from the East Australian Current was compared with the North Pacific Ocean type species. No differences in the ultrastructure of the two isolates were observed. Mitosis was studied in detail in the Australian strain, and showed two unusual features: the de novo appearance of centrioles prior to mitosis, and the formation of a small, extra-nuclear spindle. The major carotenoids in both strains were fucoxanthin and a 19′-butanoyloxyfucoxanthin-like pigment, with diadinoxanthin and diatoxanthin as secondary pigments. Several minor carotenoids have not yet been identified. In addition to chlorophylls a and c₂, a new chlorophyll c derivative (chlorophyll c₃), present in both strains, was separated by high-performance thin-layer chromatography (HPTLC). The Australian isolate, unlike the type material, showed no evidence of chlorophyllase activity during cell harvest and extraction. While the pigment composition suggests affinities with certain newly examined prymnesiophytes, organelle ultrastructure indicates Pelagococcus to be a member of the Chrysophyceae. Mitosis is, however, atypical of both Prymnesiophyceae and Chrysophyceae, and if this picoplanktonic alga is to be retained in the Chrysophyceae it must be seen as a most unusual member.     

Oligomerization and pigmentation dependent excitation energy transfer in fucoxanthin-chlorophyll proteins

The ultrafast caroteonid to chlorophyll a energy transfer dynamics of the isolated fucoxanthin-chlorophyll proteins FCPa and FCPb from the diatom Cyclotella meneghiniana was investigated in a comprehensive study using transient absorption in the visible and near infrared spectral region as well as static fluorescence spectroscopy. The altered oligomerization state of both antenna systems results in a more efficient energy transfer for FCPa, which is also reflected in the different chlorophyll a fluorescence quantum yields. We therefore assume an increased quenching in the higher oligomers of FCPb. The influence of the carotenoid composition was investigated using FCPa and FCPb samples grown under different light conditions and excitation wavelengths at the blue (500 nm) and red (550 nm) wings of the carotenoid absorption. The different light conditions yield in altered amounts of the xanthophyll cycle pigments diadinoxanthin and diatoxanthin. Since no significant dynamic changes are observed for high light and low light samples, the contribution of the xanthophyll cycle pigments to the energy transfer is most likely negligible. On the contrary, the observed dynamics change drastically for the different excitation wavelengths. The analyses of the decay associated spectra of FCPb suggest an altered energy transfer pathway. For FCPa even an additional time constant was found after excitation at 500 nm. It is assigned to the intrinsic lifetime of either the xanthophyll cycle carotenoids or more probable the blue absorbing fucoxanthins. Based on our studies we propose a detailed model explaining the different excitation energy transfer pathways in FCPa.     

DIEL CHANGES IN THE COMPOSITION OF PHOTOSYNTHETIC PIGMENTS AND CELLULAR CARBON AND NITROGEN IN CHATTONELLA ANTIQUA (RAPHIDOPHYCEAE)1

An axenic clonal culture of Chattonella antiqua (Hada) Ono was grown on a 12: 12 h LD cycle in a laboratory culture tank containing 1 m³ of f/2 medium. Diel changes in mean cell volume, cellular carbon (carbon content per cell), C/N ratio, cellular Chl a, Chl a/c ratio and carotenoid composition were observed. Mean cell volume and cellular C, N and pigments increased during the light period as a result of photosynthesis and decreased with increase of cell concentration by phased cell division during the dark period. These changes indicated that carbon assimilation and pigment synthesis occurred together during the light period. However, the patterns of increase were not the same since different diel patterns were also found in the ratios of C/N and chl a/c. Photosynthetic pigments were analyzed by reversed-phase high-performance liquid chromatography with ion-pairing solution. This analysis showed that the dominant carotenoids in C. antiqua were fucoxanthin, violaxanthin and β-carotene. Diel patterns of Chls a and c were similar to that of fucoxanthin but different from those of violaxanthin and β-carotene. The cellular contents of Chl a, fucoxanthin and carbon increased in a parallel manner during the light period. On the other hand, the increase of violaxanthin was restricted to only a few hours at the beginning of the light period during cell division cycles.     

The harmful alga Aureococcus anophagefferens utilizes 19'-butanoyloxyfucoxanthin as well as xanthophyll cycle carotenoids in acclimating to higher light intensities

Aureococcus anophagefferens is a picoplanktonic microalga that is very well adapted to growth at low nutrient and low light levels, causing devastating blooms ("brown tides") in estuarine waters. To study the factors involved in long-term acclimation to different light intensities, cells were acclimated for a number of generations to growth under low light (20μmolphotonsm(-2)s(-1)), medium light (60 or 90μmolphotonsm(-2)s(-1)) and high light (200μmolphotonsm(-2)s(-1)), and were analyzed for their contents of xanthophyll cycle carotenoids (the D pool), fucoxanthin and its derivatives (the F pool), Chls c(2) and c(3), and fucoxanthin Chl a/c polypeptides (FCPs). Higher growth light intensities resulted in increased steady state levels of both diadinoxanthin and diatoxanthin. However, it also resulted in the conversion of a significant fraction of fucoxanthin to 19'-butanoyloxyfucoxanthin without a change in the total F pool. The increase in 19'-butanoyloxyfucoxanthin was paralleled by a decrease in the effective antenna size, determined from the slope of the change in F(0) as a function of increasing light intensity. Transfer of acclimated cultures to a higher light intensity showed that the conversion of fucoxanthin to its derivative was a relatively slow process (time-frame of hours). We suggest the replacement of fucoxanthin with the bulkier 19'-butanoyloxyfucoxanthin results in a decrease in the light-harvesting efficiency of the FCP antenna and is part of the long-term acclimative response to growth at higher light intensities.     

Photoacclimation impacts the molecular features of photosystem supercomplexes in the centric diatom Thalassiosira pseudonana

In diatoms, light-harvesting processes take place in a specific group of proteins, called fucoxanthin chlorophyll a/c proteins (FCP). This group includes many members and represents the major characteristic of the diatom photosynthetic apparatus, with specific pigments bound (chlorophyll c, fucoxanthin, diadino- and diatoxanthin besides chlorophyll a). In thylakoids, FCP and photosystems (PS) form multimeric supercomplexes.In this study, we compared the biochemical properties of PS supercomplexes isolated from Thalassiosira pseudonana cells grown under low light or high light conditions, respectively. High light acclimation changed the molecular features of the PS and their ratio in thylakoids. In PSII, no obvious changes in polypeptide composition were observed, whereas for PSI changes in one specific group of FCP proteins were detected. As reported before, the amount of xanthophyll cycle pigments and their de-epoxidation ratio was increased in PSI under HL. In PSII, however, no additional xanthophyll cycle pigments occurred, but the de-epoxidation ratio was increased as well. This comparison suggests how mechanisms of photoprotection might take place within and in the proximity of the PS, which gives new insights into the capacity of diatoms to adapt to different conditions and in different environments.     

SHORT-TERM RESPONSE OF THE DIADINOXANTHIN CYCLE AND FLUORESCENCE YIELD TO HIGH IRRADIANCE IN CHAETOCEROS MUELLERI (BACILLARIOPHYCEAE)1

The relationship between the diadinoxanthin cycle and changes in fluorescence yield in the diatom Chaetoceros muelleri Lemm. (clone CH10, Amorient Aquafarm, Inc., Hawaii) was investigated. High-light-induced changes in fluorescence yield and xanthophyll de-epoxidation occurred very rapidly (first order rate constant 1.60 min^?1). The observed light-induced changes in diatoxanthin and diadinoxanthin concentration were consistent with a two-pool scheme for diadinoxanthin, one of which does not undergo de-epoxidation. Changes in xanthophyll concentration correlated with changes in in vivo absorbance indicating that diadinoxanthin cycle activity in vivo can be monitored spectrophotometrically. However, changes in cell absorbance were small relative to total optical absorption cross section. Increases in the concentration of diatoxanthin were linearly correlated with increases in the rate constant for thermal de-excitation in the antenna of photosystem II (PSII). Antenna quenching produced or mediated by diatoxanthin may, thus, protect the PSII reaction center in diatoms. Changes in the maximum fluorescence yield suggested that changes in the reaction center also contributed to nonphotochemical quenching of fluorescence. Thus, reaction center quenching affected the relationship between antenna quenching and changes in photochemical efficiency producing the effect of a decrease in fluorescence yield without a decrease in photochemical efficiency.     

Pigment composition and photoacclimation as keys to the ecological success of Gonyostomum semen (Raphidophyceae,Stramenopiles)

Aquatic habitats are usually structured by light attenuation with depth resulting in different microalgal communities, each one adapted to a certain light regime by their specific pigment composition. Several taxa contain pigments restricted to one phylogenetic group, making them useful as marker pigments in phytoplankton community studies. The nuisance and invasive freshwater microalga Gonyostomum semen (Raphidophyceae) is mainly found in brown water lakes with sharp vertical gradients in light intensity and color. However, its pigment composition and potential photoadaptations have not been comprehensively studied. We analyzed the photopigment composition of 12 genetically different strains of G. semen by high performance liquid chromatography after acclimation to different light conditions. We confirmed the pigments chl a, chl c1c2, diadinoxanthin, trans‐neoxanthin, cis‐neoxanthin, α and β carotene, which have already been reported for G. semen. In addition, we identified, for the first time, the pigments violaxan‐thin, zeaxanthin, and alloxanthin in this species. Alloxanthin has never been observed in raphidophytes before, suggesting differences in evolutionary plastid acquisition between freshwater lineages and the well‐described marine species. The amount of total chl a per cell generally decreased with increasing light intensity. In contrast, the increasing ratios of the prominent pigments diadinoxanthin and alloxanthin per chl a with light intensity suggest photoprotective functions. In addition, we found significant variation in cell‐specific pigment concentration among strains, grouped by lake of origin, which might correspond to genetic differences between strains and populations.     

Cadmium inhibits epoxidation of diatoxanthin to diadinoxanthin in the xanthophyll cycle of the marine diatom Phaeodactylum tricornutum.

Cd has pleiotropic effects on plant physiology and in particular on photosynthesis. It has not been established yet if Cd alters the functioning of the xanthophyll cycle. To answer this question, an exponentially growing culture of the marine diatom Phaeodactylum tricornutum was incubated with Cd (20 mg/l) for 24 h and irradiated with a light activating the xanthophyll cycle, which in diatoms, consists of the reversible deepoxidation of diadinoxanthin to diatoxanthin. The measurements show that the deepoxidation step is not influenced by Cd. In contrast, the Cd concentration used sharply inhibits the epoxidation of diatoxanthin to diadinoxanthin.     

Relationship between growth irradiance and the xanthophyll cycle pool in the diatom Nitzschia palea

The size and de-epoxidation state of the xanthophyll cycle pool was measured in cultures of Nitzschia palea grown at six fluence rates in continuous light or with a 12 h photoperiod. In both series the size of the pool increased with increasing irradiance. The de-epoxidized form, diatoxanthin, was only present at fluence rates saturating for growth. The portion of diadinoxanthin, which was not readily de-epoxidized in saturating light, was constant and not related to the size of the pool. In the culture grown in a light-dark cycle at 300 μmol photons m^-2 s^-1 (PAR) increasing de-epoxidation took place in the latter half of the photoperiod, when the rate of photosynthesis was decreasing. A rapid, spectrophotometric method for measuring the extent of de-epoxidation of the xanthophyll cycle pool in a culture of diatoms is described. Upon addition of a small volume of hydrochloric acid to an extract of pigments in 90% acetone, the absorbance at 480 nm is reduced. The size of the reduction is a measure of the state of the xanthophyll cycle pool, since the absorbance of diatoxanthin is reduced by 5%, but the absorbance of diadinoxanthin by 87% due to an epoxide-furanoid rearrangement, which causes the absorption spectrum to be shifted by ca 20 nm towards shorter wavelengths.     

Pigment Diversity in Freshwater Phytoplankton. I. A Comparison of Spectrophotometric and Paper Chromatographic Methods

Spectrophotometric and paper chromatographic analyses of the pigments in the phytoplankton were made from early spring till the end of summer in two small Dutch freshwater lakes. It was found that pigment diversity cannot be adequately estimated by MARGALEF'S pigment ratio nor by polychromatic spectrophotometric methods. The pigments detected with the paper chromatographic method were: chlorophyll-a, chlorophyll-b, chlorophyll-c, phaeophytin-a (traces), phaeophorbide-a, Mg-containing chlorophyll-derivatives, carotene, lutein, violaxanthin, neoxanthin (traces), fucoxanthin, diadinoxanthin, diatoxanthin (traces), peridinin and keto-carotenoids (traces). It is suggested to distinguish between a richness-component and an evenness-component of pigment diversity.