PHOTOPHYSIOLOGY IN TWO SOUTHERN OCEAN PHYTOPLANKTON TAXA: PHOTOSYNTHESIS OF PHAEOCYSTIS ANTARCTICA (PRYMNESIOPHYCEAE) AND FRAGILARIOPSIS CYLINDRUS (BACILLARIOPHYCEAE) UNDER SIMULATED MIXED‐LAYER IRRADIANCE1

In the Ross Sea, the prymnesiophyte Phaeocystis antarctica G. Karst. dominates deeply mixed water columns, while diatoms dominate shallower mixed layers. Understanding what controls the dynamics of these two phytoplankton taxa is essential because they dominate virtually all coastal polar waters, have different nutrient utilization characteristics, and support dissimilar food webs. We cultured two strains of P. antarctica and one strain of the diatom Fragilariopsis cylindrus (Grunow) Willi Krieg under three dynamic irradiance regimes that simulated different mixed‐layer depths and measured their photosynthetic characteristics, cellular pigment concentrations, and cellular carbon and nitrogen content. In both species, chl a–normalized maximum carbon uptake rate (P_m^* ) and specific growth rate were highest in the deeply mixed treatment that had a dark period. In all irradiance treatments, both (P_m^* ) and photosynthetic efficiency (α*) were greater for the two P. antarctica strains than for the F. cylindrus strain. In contrast, P. antarctica strains were more susceptible to photoinhibition (β*) than the F. cylindrus strain. When photosynthetic rates of each phytoplankton taxon were normalized by cellular particulate organic carbon (POC), the difference in the maximal photosynthetic rate () was generally reduced. In the dynamic irradiance treatment that simulated the shallowest mixed‐layer irradiance, all three phytoplankton had similar ; however, the diatom had a 2‐fold higher POC‐normalized photosynthetic efficiency (α^C). Finally, we performed calculations using the measured POC‐normalized photosynthetic parameters to show that α^C and can play a greater role than β^C in determining the competitive outcome between P. antarctica and F. cylindrus in both shallow and deep mixed‐layer environments of the Ross Sea.     

STRATEGIES AND RATES OF PHOTOACCLIMATION IN TWO MAJOR SOUTHERN OCEAN PHYTOPLANKTON TAXA: PHAEOCYSTIS ANTARCTICA (HAPTOPHYTA) AND FRAGILARIOPSIS CYLINDRUS (BACILLARIOPHYCEAE)1

We investigated rates and mechanisms of photoacclimation in cultures of Phaeocystis antarctica G. Karst. and Fragilariopsis cylindrus (Grunow) Willi Krieg, phytoplankton taxa that each dominate distinct areas of the Ross Sea, Antarctica. Both P. antarctica and F. cylindrus acclimated to increases in irradiance by reducing the effective size of the pigment antenna (σPSII) via xanthophyll‐cycle activity and reductions in chl. While enhanced photoprotection facilitated increases in specific growth rate and eventually led to higher light‐saturated photosynthetic rates (P^cell_m) in P. antarctica, increases in those variables were much smaller in F. cylindrus. In response to a lower irradiance, relaxation of xanthophyll‐cycle activity led to an increase in σPSII in both taxa, which occurred much more slowly in F. cylindrus. A surprising increase in specific growth rate over the first 36 h of acclimation in P. antarctica may have facilitated the significant reductions in P^cell_m observed in that taxon. In general, P. antarctica acclimated more quickly to changes in irradiance than F. cylindrus, exhibited a wider range in photosynthetic rates, but was more susceptible to photoinhibition. This acclimation strategy is consistent with growth in deeply mixed water columns with variations in irradiance that allow time for repair. In contrast, the slower acclimation rates, extensive photoprotection, and low photoinhibition exhibited by F. cylindrus suggest that it does not require the same period for repair as P. antarctica and is best suited for growth in habitats with relatively uniform irradiance, such as shallow mixed layers or sea ice.     

THE IMPACT OF NONPHOTOCHEMICAL QUENCHING OF FLUORESCENCE ON THE PHOTON BALANCE IN DIATOMS UNDER DYNAMIC LIGHT CONDITIONS1

The nonphotochemical quenching (NPQ) of fluorescence is an important photoprotective mechanism in particular under dynamic light conditions. Its photoprotective potential was suggested to be a functional trait of algal diversity. In the present study, the influence of the photoprotective capacity on the growth balance was investigated in two diatoms, which possess different NPQ characteristics. It was hypothesized that under fluctuating light conditions Cyclotella meneghiniana Kütz. would benefit from its large and flexible NPQ potential, whereas the comparably small NPQ capacity in Skeletonema costatum (Grev.) Cleve should exert an unfavorable impact on growth. The results of the study clearly falsify this hypothesis. Although C. meneghiniana possesses a fast NPQ component, this diatom was not able to recover its full NPQ capacity under fluctuating light. On the other hand, the induction of NPQ at relatively low irradiance in S. costatum resulted in rather small differences in the fraction of energy dissipation by the NPQ mechanism in the comparison of both diatoms. Larger differences were found in the metabolic characteristics. Both diatoms differed in their biomass composition, with a higher content of lipids in C. meneghiniana but higher amounts of carbohydrates in S. costatum. Finally, the lower degree of reduction in the biomass compensated for the higher respiration rates in S. costatum and resulted in a higher quantum efficiency of biomass production. An indirect correlation between the photoprotective and the metabolic capacity is discussed.     

PHOTOPHYSIOLOGICAL RESPONSES OF FRAGILARIOPSIS CYLINDRUS (BACILLARIOPHYCEAE) TO NITROGEN DEPLETION AT TWO TEMPERATURES1

The photosynthetic efficiency and photoprotective capacity of the sea‐ice diatom, Fragilariopsis cylindrus (Grunow) W. Krieg., grown in a matrix of nitrogen repletion and depletion at two different temperatures (?1°C and +6°C) was investigated. Temperature showed no significant effect on photosynthetic efficiency or photoprotection in F. cylindrus. Cultures under nitrogen depletion showed enhanced photoprotective capacity with an increase in nonphotochemical quenching (NPQ) when compared with nitrogen‐replete cultures. This phenomenon was achieved at no apparent cost to the photosynthetic efficiency of PSII (F_V/F_M). Nitrogen depletion yielded a partially reduced electron transport chain in which maximum fluorescence (F_M) could only be obtained by adding 3‐(3,4‐dichlorophenyl)‐1,1‐dimethylurea (DCMU). reoxidation curves showed the presence of Q_B nonreducing PSII centers under nitrogen depletion. Fast induction curves (FICs) and electron transport rates (ETRs) revealed slowing of the electrons transferred from the primary (Q_A) to the secondary (Q_B) quinone electron acceptors of PSII. The data presented show that nitrogen depletion in F. cylindrus leads to the formation of Q_B nonreducing PSII centers within the photosystem. On a physiological level, the formation of Q_B nonreducing PSII centers in F. cylindrus provides the cell with protection against photoinhibition by facilitating the rapid induction of NPQ. This strategy provides an important ecological advantage, especially during the Antarctic spring, maintaining photosynthetic efficiency under high light and nutrient‐limiting conditions.     

CURRENT KNOWLEDGE OF THE LIFE CYCLES OF PHAEOCYSTIS GLOBOSA AND PHAEOCYSTIS ANTARCTICA (PRYMNESIOPHYCEAE)1

Despite continuous efforts since the 1950s and more recent advances in culturing flagellates and nonflagellate cells of the prymnesiophyte Phaeocystis, a number of different life‐cycle models exist today that appear to apply for P. globosa Scherff. and P. antarctica G. Karst., both spherical colony formers. In one such model, this life cycle consists of three different flagellates and one nonmotile cell stage that is embedded in carbohydrate matrix‐forming colonies of different sizes and forms. Recently, noncolonial aggregates of diploid nonmotile cells attached to surfaces of diatoms were put forward as a new stage in the sexual life cycle of P. antarctica. However, it can be discussed that these “attached aggregates” (AAs) are an intermediate between motile diploid flagellates, with their well‐known tendency to adhere to surfaces, and the young spherical colony with its diploid nonmotile cells, which in nature is commonly found attached to diatoms. A life‐cycle model pertaining to both P. globosa and P. antarctica is presented.     

GENERAL FEATURES OF PHOTOPROTECTION BY ENERGY DISSIPATION IN PLANKTONIC DIATOMS (BACILLARIOPHYCEAE)1

Planktonic diatoms (Bacillariophyceae) have to cope with large fluctuations of light intensity and periodic exposure to high light. After a shift to high light, photoprotective dissipation of excess energy characterized by the nonphotochemical quenching of fluorescence (NPQ) and the concomitant deepoxidation of diadinoxanthin to diatoxanthin (DT) were measured in four different planktonic marine diatoms (Bacillariophyceae): Skeletonema costatum (Greville) Cleve, Cylindrotheca fusiformis Reimann et Lewin, Thalassiosira weissflogii (Grunow) Fryxell et Hasle, and Ditylum brightwellii (West) Grunow in comparison to the model organism Phaeodactylum tricornutum Böhlin. Upon a sudden increase of light intensity, deepoxidation was rapid and de novo synthesis of DT also occurred. In all species, NPQ was linearly related to the amount of DT formed during high light. In this report, we focused on the role of DT in the dissipation of energy that takes place in the light‐harvesting complex. In S. costatum for the same amount of DT, less NPQ was formed than in P. tricornutum and as a consequence the photoprotection of PSII was less efficient. The general features of photoprotection by harmless dissipation of excess energy in planktonic diatoms described here partly explain why diatoms are well adapted to light intensity fluctuations.     

EFFECTS OF IRON AND LIGHT STRESS ON THE BIOCHEMICAL COMPOSITION OF ANTARCTIC PHAEOCYSTIS SP. (PRYMNESIOPHYCEAE). I. INTRACELLULAR DMSP CONCENTRATIONS

Iron is essential for phytoplankton growth, as it is involved in many metabolic processes. It controls photosynthesis as well as many enzymatic processes. As such, iron affects the cell's energy supply and contributes to the assimilation of carbon and nitrogen. To determine whether iron limitation would result in energy stress or induced nitrogen deficiency, an Antarctic Phaeocystis sp. (Prymnesiophyceae) strain was studied for its biochemical composition, with the main emphasis on intracellular production of dimethylsulfoniopropionate (DMSP). DMSP is suggested to replace nitrogen containing solutes under conditions of nitrogen deficiency. Batch cultures of Antarctic Phaeocystis sp. were grown under iron-rich and iron-poor conditions and simultaneously subjected to high and low light intensities. Iron depletion induced chlorosis and suppressed growth rates as well as the maximum yield of the cultures; these effects were reinforced by low light intensities. Cell volumes were strongly reduced under iron-limited conditions. However, this reduction in cell volume was accompanied by a reduced DMSP content only in cultures experiencing low light intensities. Under high light conditions, no reduction of DMSP was observed; hence, intracellular DMSP concentrations increased. These observations are discussed relative to carbon and nitrogen metabolism and the biosynthetic pathway of DMSP. It is argued that under high light, low iron conditions, the cells were bordering on nitrogen deficiency induced by iron limitation, whereas under low light, low iron conditions, the cells were energy limited resulting in overall suppressed metabolic rates. Between treatments, DMSP to chlorophyll- a ratios varied by a factor of 5, demonstrating the dependence of this parameter on the physiological state of the cell.     

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.     

STUDIES ON TRANSPARENT EXOPOLYMER PARTICLES (TEP) PRODUCED IN THE ROSS SEA (ANTARCTICA) AND BY PHAEOCYSTIS ANTARCTICA (PRYMNESIOPHYCEAE)1

The distribution and production of transparent exopolymer particles (TEPs) were studied quantitatively both in cultures of Phaeocystis antarctica Karsten (Prymnesiophyceae) and in natural phytoplankton assemblages in the Ross Sea, Antarctica. TEP production in culture was a function of growth rate and photosynthetic activity and was strongly influenced by photon flux density. The concentrations of TEP measured during a bloom, dominated by P. antarctica, were higher than those produced by coastal diatom blooms and were correlated with chlorophyll a (Chl a), being low at Chl a levels below 3 μgL^?1 but increasing rapidly at greater Chl a concentrations. Because higher chlorophyll hek are dominated 4 larger P. antarctica colonies, this relationship suggests that TEP was produced primarily by sloughing and disintegration of the colonial matrix. TEP concentrations (both absolute and relative to Chl a) increased as the bloom's biomass increased. Vertical distributions of TEP and Chl a showed TEP: chlorophyll maxima at the bottom of the water column at most stations. Because TEP and floc formation are tightly coupled, we suggest that mucous flocs derived from TEP, rather than intact P. antarctica colonies, are the dominant component of aggregates and subsequent organic carbon vertical flux.     

VARIABILITY IN CELL SIZE,NUTRIENT DEPLETION,AND GROWTH RATES OF THE SOUTHERN OCEAN DIATOM FRAGILARIOPSIS KERGUELENSIS (BACILLARIOPHYCEAE) AFTER PROLONGED IRON LIMITATION1

Diatoms are the main primary producers in the Southern Ocean, governing the major nutrient cycles. Fragilariopsis kerguelensis (O’Meara) Hust. is the most abundant diatom species in the Southern Ocean and its paleo‐oceanographic record is frequently used to reconstruct the past position and nutrient characteristics of the Antarctic polar front. Here we report on the responses of F. kerguelensis on prolonged exposure to a range of iron concentrations, allowing a characterization of morphological and nutrient‐depletion changes in relation to iron status. Under iron limitation, F. kerguelensis grew slower, cells became smaller, chains became shorter, and the nutrient‐depletion ratios changed. Prolonged exposure to iron limitation caused F. kerguelensis to decrease its surface area and volume 2‐fold, and to increase its surface‐to‐volume ratio by 25%. With the decrease in growth rates, silicon (Si) and phosphorus (P) depletion per cell remained fairly constant, but when normalized per surface area (Si) or per cell volume (P), depletion increased. In contrast, nitrogen (N) depletion per cell decreased significantly together with the decrease in growth rates but was constant when normalized per cell volume. The different response in Si, P, and N depletion resulted in changes in the nutrient‐depletion ratios, most notably in the Si:N ratio, which significantly increased, and in the N:P ratio, which significantly decreased with decreasing growth rates. It is concluded that under iron limitation, variation in cell size and/or nutrient depletion ultimately can cause changes in oceanic biogeochemical nutrient cycles. It enables the use of cell size of F. kerguelensis as a paleo‐oceanographic proxy.     

IMPACT OF IRON LIMITATION ON THE PHOTOSYNTHETIC APPARATUS OF THE DIATOM CHAETOCEROS MUELLERI (BACILLARIOPHYCEAE)

Iron starvation induced marked increases in flavodoxin abundance and decreases in light-saturated and light-limited photosynthesis rates in the diatom Chaetoceros muelleri. Consistent with the substitution of flavodoxin for ferredoxin as an early response to iron starvation, increases of flavodoxin abundance were observed before declines of cell division rate or chl a specific photosynthesis rates. Changes in the abundance of flavodoxin after the addition of iron to iron-starved cells indicated that flavodoxin was not actively degraded under iron-replete conditions. Greater declines in light-saturated oxygen evolution rates than dark oxygen consumption rates indicated that the mitochondrial electron transfer chain was not affected as greatly by iron starvation as the photosynthetic electron transfer chain. The carbon:nitrogen ratio was unaffected by iron starvation, suggesting that photosynthetic electron transfer was a primary target of iron starvation and that reductions in nitrate assimilation were due to energy limitation (the C:N ratio would be expected to rise under nitrogen-limited but energy-replete conditions). Parallel changes were observed in the maximum light-saturated photosynthesis rate and the light-limited initial slope of the photosynthesis-light curve during iron starvation and recovery. The lowest photosynthesis rates were observed in iron-starved cells and the highest values in iron-replete cells. The light saturation parameter, I_k, was not affected by iron starvation, nor was the chl-to-C ratio markedly reduced. These observations were consistent with iron starvation having a similar or greater effect on photochemical charge separation in PSII than on downstream electron transfer steps. Declines of the ratio of variable to maximum fluorescence in iron-starved cells were consistent with PSII being a primary target of iron starvation. The functional cross-section of PSII was affected only marginally (<20%) by iron starvation, with the largest values observed in iron-starved cells. The rate constant for electron transfer calculated from fast repetition rate fluorescence was found to covary with the light-saturated photosynthesis rate; it was lowest in the most severely starved cells.     

COLONY SIZE OF PHAEOCYSTIS ANTARCTICA (PRYMNESIOPHYCEAE) AS INFLUENCED BY ZOOPLANKTON GRAZERS1

The haptophyte Phaeocystis antarctica G. Karst. is a dominant phytoplankton species in the Ross Sea, Antarctica, and exists as solitary cells and mucilaginous colonies that differ by several orders of magnitude in size. Recent studies with Phaeocystis globosa suggest that colony formation and enlargement are defense mechanisms against small grazers. To test if a similar grazer‐induced morphological response exists in P. antarctica, we conducted incubation experiments during the austral summer using natural P. antarctica and zooplankton assemblages. Dialysis bags that allowed exchange of dissolved chemicals were used to separate P. antarctica and zooplankton during incubations. Geometric mean colony size decreased by 35% in the control, but increased by 30% in the presence of grazers (even without physical contact) over the 15 d incubation. The estimated colonial‐to‐solitary cell carbon ratio was significantly higher in the grazing treatment. These results suggest that P. antarctica colonies would grow larger in the presence of indigenous zooplankton and skew the carbon partitioning significantly toward the colonial phase. While these observations show that the colony size of P. antarctica was affected by a chemical signal related to grazers, the detailed nature and ecological significance of this signal remain unknown.     

EFFECT OF SILICATE LIMITATION ON VALVE MORPHOLOGY IN THALASSIOSIRA AND COSCINODISCUS (BACILLARIOPHYCEAE)1

Coscinodiscus radiatus Ehrenb. and Thalassiosira eccentrica (Ehrenb.) Cleve were grown in a silicate-limited chemostat at silicate concentrations below 1 μg-atoms · l^?1. The resulting abnormal valves of C. radiatus lacked a thickened ring around the foramina; their pore membranes were thinner and their loculi shallower than those in normal cells. Abnormal valves of T. eccentrica had a fasciculate areolae pattern; they lacked a silica covering over the foramina and some tangential areolae walls. Neither abnormal valve could be termed a new species.     

THE STORAGE GLUCAN OF PHAEOCYSTIS GLOBOSA (PRYMNESIOPHYCEAE) CELLS1

A non-colony-forming axenic strain of Phaeocystis globosa (Harlot) Lagerheim was shown to produce a water-soluble β-d -glucan. This glucan consisted of about 20 glucose units, mainly (l→3)-linked, with branching at position 6. Therefore, it can be classified as a chrysolaminaran. Glucan production occurred mainly during the stationary growth phase and resulted in concentrations as high as 76 pg glucose per cell. When cultures were deprived of light the glucans were consumed, which supports their possible role as compounds used for temporary storage of energy.     

PHYTOPLANKTON PLASMA MEMBRANE REDOX ACTIVITY: EFFECT OF IRON LIMITATION AND INTERACTION WITH PHOTOSYNTHESIS1

Phytoplankton plasma membrane electron transport activity was determined by monitoring the reduction of the impermeant artificial electron acceptor ferricyanide in a range of diatoms. The results revealed that constitutive plasma membrane electron transport activity of marine diatoms is high compared with chlorophytes and higher plant cells. Diatom plasma membrane electron transport activity was not significantly increased by iron limitation. This lack of induction on iron limitation indicates that diatoms have an iron acquisition strategy that is distinct from chlorophytes and the dicotyledon higher plants that exhibit marked increases in plasma membrane ferricyanide reductase activity on iron limitation. The interaction of the constitutive plasma membrane electron transport with photosynthesis was also investigated. We found that 1) ferricyanide reduction at the plasma membrane was progressively inhibited in response to increasing irradiances; 2) the presence of extracellular ferricyanide, but not the reduced couple ferrocyanide, caused a marked inhibition of carbon fixation at high irradiance; and 3) extracellular electron acceptors ferricyanide and hexachloroiridate (but not ferrocyanide) induced an immediate and reversible decrease in fluorescence yields (F_o and F_m). The extent to which extracellular electron acceptors affected CO₂ fixation, F_o, and F_m was related to the level of constitutive ferricyanide reductase activity, the species with highest ferricyanide reduction rates being most sensitive. The data suggest that consumption of electrons and/or reductant at the plasma membrane by external acceptors may compete directly with CO₂ fixation for electrons, alter cytosolic‐chloroplast redox poise, and/or induce a redox‐signaling cascade that alters photosynthetic metabolism.     

EFFECTS OF IRON DEFICIENCY ON ISOCHRYSIS GALBANA (CHRYSOPHYCEAE) AND PHAEODACTYLUM TRICORNUTUM (BACILLARIOPHYCEAE)1

Cultures of Isochrysis galbana Parks and Phaeodactylum tricornutum Bohlin were grown in iron-limited chemostats. With increasing iron deficiency, photosynthetic rate per cell and assimilation number decreased. The pattern of photosynthesis was also altered; in Fe deficient cells the proportion of ¹⁴C fixed in glycine and serine decreased with an accompanying increase into alanine after 3 min assimilation. Although there was no significant effect of Fe deficiency on the proportion of ¹⁴C incorporated into total amino acids and amides, the percentage of total ¹⁴C fixed in protein increased with increasing Fe deficiency. Cellular levels of chlorophyll a, carotenoids, cytochromes and protein also decreased with increasing Fe deficiency. However, the reduction in chlorophyll a/cell was not as great as that of cytochrorne f₁ and Fe deficient cells therefore showed a marked increase in chlorophyll a:cytochrorne f₁ ratio.     

ANALYSIS OF EXPRESSED SEQUENCE TAGS (ESTS) FROM THE POLAR DIATOM FRAGILARIOPSIS CYLINDRUS1

Analysis of expressed sequence tags (ESTs) was performed to gain insights into cold adaptation in the polar diatom Fragilariopsis cylindrus Grunow. The EST library was generated from RNA isolated 5 days after F. cylindrus cells were shifted from approximately +5° C to ?1.8°C. A total of 1376 ESTs were sequenced from a non‐normalized cDNA library and assembled into 996 tentative unique sequences. About 27% of the ESTs displayed similarity (tBLASTX, e‐value of ≤10^?4) to predicted proteins in the centric diatom Thalassiosira pseudonana Hasle & Heindal. Eleven additional algae and plant data bases were used for annotation of sequences not covered by Thalassiosira sequences (7%). Most of the ESTs were similar to genes encoding proteins responsible for translation, ribosomal structure, and biogenesis (3%), followed by genes encoding proteins for amino acid transport and metabolism and post‐translational modifications. Interestingly, 66% of all the EST sequences from F. cylindrus displayed no similarity ( e ‐value ≤10^?4) to sequences from the 12 non‐redundant databases. Even 6 of the 10 strong to moderately expressed sequences in this EST library could not be identified. Adaptation of F. cylindrus to freezing temperatures of seawater may require a complex protein metabolism and possibly also genes, which were highly expressed but still unknown. However, it could also mean that due to low temperatures, there might have been a stronger pressure to adapt amino acid sequences, making it more difficult to identify these unknown sequences and/or that there are still few protist sequences available for comparison.     

THE CHITINOUS NATURE OF FILAMENTS EJECTED BY PHAEOCYSTIS (PRYMNESIOPHYCEAE)1

Filaments ejected by Phaeocystis globosa Scherffel, organized in star-like structures, were observed and analyzed before and after their discharge from cells. Ultrastructural observations obtained after cryofixation and cryosubstitution led to a model for their storage within the cell and for their ejection from the cell. Electron diffraction analysis on the ejected filaments demonstrated their chitinous composition. This technique indicated without ambiguity that each filament was in fact a whisker-like α-chitin crystal, with the axes of the corresponding polymer chains aligned with the filament's axis. X-ray microanalysis of the mats of filaments indicated that the silica content suggested by earlier workers was an artifact resulting from the filtration procedure.