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Uta Passow - One of the best experts on this subject based on the ideXlab platform.

  • Recovery of Thalassiosira weissflogii from nitrogen and silicon starvation
    2015
    Co-Authors: La Rocha, Uta Passow
    Abstract:

    The ecological success of marine diatoms comes despite their unusual additional requirement for silicon, a nutrient that often limits their growth in the ocean. There may be, however, some physiological and ecological advantages to silicon limitation. A model of nitrogen and silicon metabolism in diatoms (Flynn et al. 1997; Flynn and Martin-Jézéquel 2000) suggests that silicon-starved diatoms will recover more quickly than nitrogen-starved (but otherwise identical) diatoms on the resupply of nutrients. In culture, following 24 h of stationary phase induced by nutrient exhaustion, silicon-starved Thalassiosira weissflogii displayed maximum growth rates within a few hours of the readdition of silicon. But 37 h elapsed before nitrate-starved cultures began dividing again and nitrate-starved cultures did not reattain maximum growth rates over the period investigated. The model suggests that, in terms of number, cells descended from the nitrate starved will never catch up to cells descended from the silicon starved if both populations are simultaneously supplied with a pulse of nutrients. The longer the lag between nutrient supply and the resumption of growth in the nitrate-starved cells, the greater the proportion of the supplied nutrients that ends up in the descendants of the silicon starved. In the model run, the silicon-starved cells acquired 75 % of the added nutrients and the nitrate starved acquired only 25%. Silicon limitation may thus be a mechanism by which diatoms ensure they respond more quickly than other phytoplankton to the upwelling of nutrients into the euphotic zone. Diatoms are astoundingly successful phytoplankton de-spite somewhat uniquely requiring silicon for growth in an ocean largely depleted of silicic acid. Silicon limitation of diatom growth in the ocean has been observed in locations ranging from high-nutrient, upwelling zones in the eastern equatorial Pacific (Dugdale et al. 1995) to the Mississippi River plume (Nelson and Dortch 1996) and the oligotrophi

  • Aggregation and Sedimentation of Thalassiosira weissflogii (diatom) in a Warmer and More Acidified Future Ocean
    PloS one, 2014
    Co-Authors: Shalin Seebah, Matthias S. Ullrich, Caitlin Fairfield, Uta Passow
    Abstract:

    Increasing Transparent Exopolymer Particle (TEP) formation during diatom blooms as a result of elevated temperature and pCO2 have been suggested to result in enhanced aggregation and carbon flux, therewith potentially increasing the sequestration of carbon by the ocean. We present experimental results on TEP and aggregate formation by Thalassiosira weissflogii (diatom) in the presence or absence of bacteria under two temperature and three pCO2 scenarios. During the aggregation phase of the experiment TEP formation was elevated at the higher temperature (20°C vs. 15°C), as predicted. However, in contrast to expectations based on the established relationship between TEP and aggregation, aggregation rates and sinking velocity of aggregates were depressed in warmer treatments, especially under ocean acidification conditions. If our experimental findings can be extrapolated to natural conditions, they would imply a reduction in carbon flux and potentially reduced carbon sequestration after diatom blooms in the future ocean.

  • Series 4: Aggregation of Thalassiosira weissflogii as a function of pCO2, temperature and bacteria
    Biological and Chemical Oceanography Data Management Office (BCO-DMO), 2013
    Co-Authors: Uta Passow, Shalin Seebah
    Abstract:

    The Series 4 Experiment, Aggregation of Thalassiosira weissflogii as a function of pCO2, temperature and bacteria, is made up of 2 phases. The Acclimatisation Phase and the Aggregation Phase. The Acclimatisation Phase has two components – The Carbonate System data and the Cell Counts Data. The Aggregation Phase also has two components – The Carbonate System + TEP data and the Sinking Velocity data. Note: For a complete list of measurements, refer to the supplemental document ' Series4_Field_names.pdf' .

  • Effects of Marinobacter adhaerens HP15 on polymer exudation by Thalassiosira weissflogii at different N:P ratios
    Marine Ecology Progress Series, 2012
    Co-Authors: Astrid Gardes, Hans-peter Grossart, Uta Passow, Yannic Ramaye, Matthias S. Ullrich
    Abstract:

    In the ocean, exopolymer accumulation and the resulting aggregation of marine phytoplankton drive the flux of sinking organic matter. Little is known about the contribution of bacteria to the release and build-up of such exudates. We describe an in-depth investigation of exopolymer production under differing nutrient conditions using a diatombacteria model system. Responses of the marine diatom Thalassiosira weissflogii to different nutrient re gimes and the impact of the co-incubated bacterium Marinobacter adhaerens on algal exudation were evaluated by ana- lyzing quantity and quality of exudation products. Cul- tures of T. weissflogii were grown at nutrient-balanced conditions (Redfield ratio N:P = 16) as well as at nitro- gen- or phosphorus-depleted conditions (N:P = 1 and N:P = 95, respectively) in the presence or absence of M. adhaerens. The impact of M. adhaerens on the con- centration of dissolved organic carbon and transparent exopolymer particles (TEP), as well as on the composi- tion of amino acids and carbohydrates, depended on the nutrient regime. Under nutrient-balanced con - ditions, M. adhaerens stimulated both T. weissflogii growth and TEP production. Under nutrient-depleted conditions, TEP production was enhanced regardless of whether bacteria were present. Phosphorus limita- tion resulted in an appreciable change in the %mol composition of dissolved amino acids in xenic and axenic treatments. Differential lec tin staining revealed that the presence of M. adhae rens enhanced and modified the pro duction of specific extracellular sub- stances. A better understan ding of the effects of diatombacteria interactions on the accumulation of exudation products is crucial in modelling and predict- ing consequences of environmental changes on oceanic organic matter and nutrient cycling.

  • diatom associated bacteria are required for aggregation of Thalassiosira weissflogii
    The ISME Journal, 2011
    Co-Authors: Astrid Gardes, Hans-peter Grossart, Uta Passow, Morten Hvitfeldt Iversen, Matthias S. Ullrich
    Abstract:

    Aggregation of algae, mainly diatoms, is an important process in marine systems leading to the settling of particulate organic carbon predominantly in the form of marine snow. Exudation products of phytoplankton form transparent exopolymer particles (TEP), which acts as the glue for particle aggregation. Heterotrophic bacteria interacting with phytoplankton may influence TEP formation and phytoplankton aggregation. This bacterial impact has not been explored in detail. We hypothesized that bacteria attaching to Thalassiosira weissflogii might interact in a yet-to-be determined manner, which could impact TEP formation and aggregate abundance. The role of individual T. weissflogii-attaching and free-living new bacterial isolates for TEP production and diatom aggregation was investigated in vitro. T. weissflogii did not aggregate in axenic culture, and striking differences in aggregation dynamics and TEP abundance were observed when diatom cultures were inoculated with either diatom-attaching or free-living bacteria. The data indicated that free-living bacteria might not influence aggregation whereas bacteria attaching to diatom cells may increase aggregate formation. Interestingly, photosynthetically inactivated T. weissflogii cells did not aggregate regardless of the presence of bacteria. Comparison of aggregate formation, TEP production, aggregate sinking velocity and solid hydrated density revealed remarkable differences. Both, photosynthetically active T. weissflogii and specific diatom-attaching bacteria were required for aggregation. It was concluded that interactions between heterotrophic bacteria and diatoms increased aggregate formation and particle sinking and thus may enhance the efficiency of the biological pump.

Claudiu T. Supuran - One of the best experts on this subject based on the ideXlab platform.

  • Inhibition survey with phenolic compounds against the δ- and η-class carbonic anhydrases from the marine diatom Thalassiosira weissflogii and protozoan Plasmodium falciparum
    Journal of enzyme inhibition and medicinal chemistry, 2019
    Co-Authors: Siham A. Alissa, Hanan A. Al-ghulikah, Zeid A. Alothman, Sameh M. Osman, Sonia Del Prete, Clemente Capasso, Alessio Nocentini, Claudiu T. Supuran
    Abstract:

    AbstractThe inhibition of δ- and η-class carbonic anhydrases (CAs; EC 4.2.1.1) was poorly investigated so far. Only one δ-CA, TweCA from the diatom Thalassiosira weissflogii, and one η-CA, PfCA, fr...

  • The zinc - but not cadmium - containing ζ-carbonic from the diatom Thalassiosira weissflogii is potently activated by amines and amino acids.
    Bioorganic chemistry, 2018
    Co-Authors: Andrea Angeli, Simona Maria Monti, William A. Donald, Martina Buonanno, Claudiu T. Supuran
    Abstract:

    Abstract The activation of the ζ-class carbonic anhydrase (CAs, EC 4.2.1.1) from the diatom Thalassiosira weissflogii (TweCAζ) incorporating both Zn(II) and Cd(II) at the active site, was investigated for the first time, using a panel of natural and non-natural amino acids and amines. CdTweCAζ was completely insensitive to activation, whereas all these compounds were effective activators of the zinc-containing enzyme ZnTweCAζ, with activation constants ranging between 92 nM and 37.9 µM. The most effective ZnTweCAζ activators were l -adrenaline, 1-(2-aminoethyl)-piperazine and 4-(2-aminoethyl)-morpholine, with K A s in the range of 92–150 nM. l -His, l - and d -Tyr and some pyridyl-alkylamines, had K A s in the range of 0.62–0.98 µM, whereas l -/ d -DOPA, d -Trp, histamine, serotonin and l -Asn were the next most efficient activators, with K A s in the range of 1.27–3.19 µM. The least effective activators were l -Phe (K A of 15.4 µM) and l -Asp (K A of 37.9 µM). This in vitro study may be useful for a more complete understanding of the activation processes of various CA enzyme families, of which the ζ-class was scarcely investigated.

  • Mono- and di-thiocarbamate inhibition studies of the δ-carbonic anhydrase TweCAδ from the marine diatom Thalassiosira weissflogii
    Journal of enzyme inhibition and medicinal chemistry, 2018
    Co-Authors: Silvia Bua, Sonia Del Prete, Clemente Capasso, Murat Bozdag, Fabrizio Carta, William A. Donald, Claudiu T. Supuran
    Abstract:

    AbstractThe inhibition of the δ-class carbonic anhydrase (CAs, EC 4.2.1.1) from the diatom Thalassiosira weissflogii, TweCAδ, was investigated using a panel of 36 mono- and di-thiocarbamates chemotypes that have recently been shown to inhibit mammalian and pathogenic CAs belonging to the α- and β-classes. TweCAδ was not significantly inhibited by most of such compounds (KI values above 20 µM). However, some aliphatic, heterocyclic, and aromatic mono and di-thiocarbamates inhibited TweCAδ in the low micromolar range. For some compounds incorporating the piperazine ring, TweCAδ was effectively inhibited (KIs from 129 to 791 nM). The most effective inhibitors identified in this study were 3,4-dimethoxyphenyl-ethyl-mono-thiocarbamate (KI of 67.7 nM) and the R-enantiomer of the nipecotic acid di-thiocarbamate (KI of 93.6 nM). Given that the activity and inhibition of this class of enzyme have received limited attention until now, this study provides new molecular probes and information for investigating the ro...

  • The first activation study of a δ-carbonic anhydrase: TweCAδ from the diatom Thalassiosira weissflogii is effectively activated by amines and amino acids.
    Journal of enzyme inhibition and medicinal chemistry, 2018
    Co-Authors: Andrea Angeli, Zeid A. Alothman, Sameh M. Osman, Sonia Del Prete, Clemente Capasso, William A. Donald, Fatmah A.s. Alasmary, Claudiu T. Supuran
    Abstract:

    The activation of the δ-class carbonic anhydrase (CAs, EC 4.2.1.1) from the diatom Thalassiosira weissflogii (TweCAδ) was investigated using a panel of natural and non-natural amino acids and amines. The most effective activator of TweCAδ was d-Tyr (KA of 51 nM), whereas several other amino acids and amines, such as L-His, L-Trp, d-Trp, dopamine and serotonin were submicromolar activators (KAs from 0.51 to 0.93 µM). The most ineffective activator of TweCAδ was 4-amino-l-Phe (18.9 µM), whereas d-His, l-/d-Phe, l-/d-DOPA, l-Tyr, histamine, some pyridyl-alkylamines, l-adrenaline and aminoethyl-piperazine/morpholine were moderately potent activators (KAs from 1.34 to 8.16 µM). For any δ-CA, there are no data on the crystal structure, homology modelling and the amino acid residues that are responsible for proton transfer to the active site are currently unknown making it challenging to provide a detailed rational for these findings. However, these data provide further evidence that this class of underexplored CA deserves more attention.

  • CDCA1 From Thalassiosira weissflogii as Representative Member of ζ-Class CAs: General Features and Biotechnological Applications
    Carbonic Anhydrases as Biocatalysts, 2015
    Co-Authors: Simona Maria Monti, Giuseppina De Simone, Claudiu T. Supuran, Vincenzo Alterio
    Abstract:

    Abstract Carbonic anhydrases (CAs) are metalloenzymes present in both prokaryotes and eukaryotes, which play important physiological roles in all life kingdoms. In phytoplankton, CAs are essential for the acquisition of inorganic carbon for photosynthesis, being involved in the carbon-concentrating mechanism (CCM). Recently, CDCA1, a new CA from the marine diatom Thalassiosira weissflogii, has been isolated and classified as the first member of a new CA class, namely, ζ-CA class. CDCA1 is a ∼67.8 kDa enzyme, consisting of three almost identical repeats (R1, R2, and R3) that can functionally substitute Zn2+ or Cd2+ at their active site, an adaptive advantage for diatoms that grow fast in metal-poor environments of the ocean. The high affinity of CDCA1 active site for cadmium makes this enzyme a good candidate for the development of CA-based biosensors for determination of free metal ions in the seawater. Moreover, considering that this enzyme presents a very high catalytic activity for the physiological reaction, potential biotechnological applications in CO2 capture processes may also be envisaged.

Francois M M Morel - One of the best experts on this subject based on the ideXlab platform.

  • The effects of pH and pCO_2 on photosynthesis and respiration in the diatom Thalassiosira weissflogii
    Photosynthesis Research, 2017
    Co-Authors: Johanna A. L. Goldman, Michael L. Bender, Francois M M Morel
    Abstract:

    The response of marine phytoplankton to the ongoing increase in atmospheric p CO_2 reflects the consequences of both increased CO_2 concentration and decreased pH in surface seawater. In the model diatom Thalassiosira weissflogii , we explored the effects of varying p CO_2 and pH, independently and in concert, on photosynthesis and respiration by incubating samples in water enriched in H_2 ^18O. In long-term experiments (~6-h) at saturating light intensity, we observed no effects of pH or p CO_2 on growth rate, photosynthesis or respiration. This absence of a measurable response reflects the very small change in energy used by the carbon concentrating mechanism (CCM) compared to the energy used in carbon fixation. In short-term experiments (~3 min), we also observed no effects of p CO_2 or pH, even under limiting light intensity. We surmise that in T. weissflogii , it is the photosynthetic production of NADPH and ATP, rather than the CO_2-saturation of Rubisco that controls the rate of photosynthesis at low irradiance. In short-term experiments, we observed a slightly higher respiration rate at low pH at the onset of the dark period, possibly reflecting the energy used for exporting H^+ and maintaining pH homeostasis. Based on what is known of the biochemistry of marine phytoplankton, our results are likely generalizable to other diatoms and a number of other eukaryotic species. The direct effects of ocean acidification on growth, photosynthesis and respiration in these organisms should be small over the range of atmospheric p CO_2 predicted for the twenty-first century.

  • Structural and inhibition insights into carbonic anhydrase CDCA1 from the marine diatom Thalassiosira weissflogii
    Biochimie, 2012
    Co-Authors: Vincenzo Alterio, Francois M M Morel, Emma Langella, Giuseppina De Simone, Claudiu T. Supuran, Daniela Vullo, Francesca Viparelli, Giuseppina Ascione, Nina A. Dathan, Simona Maria Monti
    Abstract:

    Carbonic anhydrases (CAs) catalyze with high efficiency the reversible hydration of carbon dioxide, an essential reaction for many biological processes, such as photosynthesis, respiration, renal tubular acidification, and bone resorption. Diatoms, which are one of the most common types of phytoplankton and are widespread in oceans, possess CAs fundamental for acquisition of inorganic carbon. Recently, in the marine diatom Thalassiosira weissflogii a novel enzyme, CDCA1, naturally using Cd in its active site, has been isolated and categorized in a new CA class, namely zeta-CA. This enzyme, which consists of three repeats (R1, R2 and R3), is a cambialistic carbonic anhydrase that can spontaneously exchange Zn or Cd at its active centre, presumably an adaptative advantage for diatoms that grow fast in the metal-poor environment of the surface ocean. In this paper we completed the characterization of this enzyme, reporting the X-ray structure of the last repeat, CDCA1–R3 in its cadmium-bound form, and presenting a model of the full length protein obtained by docking approaches. Results show that CDCA1 has a quite compact not symmetric structure, characterized by two covalently linked R1–R2 and R2–R3 interfaces and a small non-covalent R1–R3 interface. The three dimensional arrangement shows that most of the non-conserved aminoacids of the three repeats are located at the interface regions and that the active sites are far from each other and completely accessible to the substrate. Finally, a detailed inhibition study of CDCA1–R3 repeat in both cadmium- and zinc- bound form has been performed with sulfonamides and sulfamates derivatives. The results have been compared with those previously reported for other CA classes, namely alpha- and beta-classes, and correlated with the structural features of these enzymes.

  • Inhibition of the R1 fragment of the cadmium-containing ζ-class carbonic anhydrase from the diatom Thalassiosira weissflogii with anions
    Bioorganic & medicinal chemistry letters, 2010
    Co-Authors: Francesca Viparelli, Francois M M Morel, Giuseppina De Simone, Simona Maria Monti, Andrea Scozzafava, Alessio Innocenti, Claudiu T. Supuran
    Abstract:

    We investigated the catalytic activity and inhibition of both the zinc and cadmium-containing R1 fragment of the zeta-class carbonic anhydrase (CA, EC 4.2.1.1) from the marine diatom Thalassiosira weissflogii. Our data prove that these enzymes are not only very efficient catalysts for the physiological reaction, but also sensitive to sulfonamide and anion inhibitors, with inhibition constants from the nanomolar to millimolar range. Acetazolamide inhibited the two enzymes with K(I)s in the range of 58-92 nM. The best anion inhibitors of Cd-R1 were thiocyanate, sulfamate and sulfamide, with K(I)s of 10-89 microM, whereas the best Zn-R1 anion inhibitors were sulfamate and sulfamide with K(I)s of 60-72 microM. These enzymes were only weakly inhibited by chloride, bromide or sulfate, main anion components of sea water, with inhibition constants in the range of 0.24-0.85 mM. Thus, similarly to CAs belonging to other classes, the zeta-class CA (with either cadmium or zinc ions at the active site) was inhibited by both anions and sulfonamides.

  • Acquisition of inorganic carbon by the marine diatom Thalassiosira weissflogii
    Functional plant biology : FPB, 2002
    Co-Authors: Francois M M Morel, Elizabeth H. Cox, Anne M. L. Kraepiel, Todd W. Lane, Allen J. Milligan, Irene Schaperdoth, John R. Reinfelder, Philippe D. Tortell
    Abstract:

    Recent data on the physiology of inorganic carbon acquisition by the model marine diatom Thalassiosira weissflogii (Grunow) demonstrate the importance of the catalytic equilibration of HCO3-and CO2by carbonic anhydrases located in the periplasm and in the cytoplasm. These enzymes can use Zn, Co or Cd as their metal centre, and their activity increases at low ambient CO2. The silica frustule provides buffering for extracellular CA activity, The transmembrane transport of CO2 may occur by passive diffusion. Under CO2 limitation, the cytoplasmic HCO3–is used to form malate and oxaloacetic acid via phosphoenolpyruvate carboxylase. It appears that subsequent decarboxylation of these compounds in the chloroplast regenerates CO2 near the site of Rubisco, and thus provides the organism with an effective unicellular C4 photosynthetic pathway. These results, together with other published data, bring up two major questions regarding inorganic carbon acquisition in diatoms: What is the major species of inorganic carbon (CO2 or HCO3–) transported across the membrane under natural conditions? And what is the form of carbon (inorganic or organic) accumulated by the cells?

  • The active site structure of Thalassiosira weissflogii carbonic anhydrase 1.
    Biochemistry, 2000
    Co-Authors: Elizabeth H. Cox, Francois M M Morel, Todd W. Lane, George Mclendon, Roger C. Prince, Ingrid J. Pickering, Graham N. George
    Abstract:

    X-ray absorption spectroscopy at the Zn K-edge indicates that the active site of the marine diatom Thalassiosira weissflogii carbonic anhydrase is strikingly similar to that of mammalian α-carbonic...

Lei Wang - One of the best experts on this subject based on the ideXlab platform.

  • Physiological responses of the diatoms Thalassiosira weissflogii and Thalassiosira pseudonana to nitrogen starvation and high light.
    Marine environmental research, 2021
    Co-Authors: Hongjin Qiao, Zang Shasha, Yan Fang, Lei Wang
    Abstract:

    Abstract As oceans warm, the depth of the upper mixed layer is predicted to decrease, resulting in insufficient nutrient supply and higher solar radiation for phytoplankton. In order to understand the photophysiological responses of the key eukaryotic phytoplankton diatoms to high light and nutrient limitation, we grew two diatoms, Thalassiosira weissflogii and Thalassiosira pseudonana under N starvation conditions and exposed them to high visible light. It showed that the large-sized diatom T. weissflogii can maintain photosynthetic activity for a longer period of time under nitrogen starvation as compared with the small-sized diatom T. pseudonana. The electron transfer reaction was inhibited in both diatoms and the fast closing of reaction centers promoted the development of QB non-reducing PSII centers, thus facilitated the rapid induction of NPQ, however, the induction of NPQ depended on the degree of N starvation. N starvation exacerbated the photoinhibition caused by high light. The smaller-sized T. pseudonana had a higher σi value and was more sensitive to high-light, but its PSII repair rate was also higher. In contrast, T. weissflogii was more tolerant to high light with a lower σi value, but the tolerance was severely reduced under N-starvation. This study provides helpful insight into how climate change variables impact diatom’s photosynthetic physiology.

Antonio Cruzado - One of the best experts on this subject based on the ideXlab platform.

  • Diatom Thalassiosira weissflogii in oligotrophic versus eutrophic culture: models and ultrastructure
    Ecological Modelling, 2003
    Co-Authors: E. Bayraktaroğlu, T. Legović, Zoila Velásquez, Antonio Cruzado
    Abstract:

    Abstract Populations of marine diatom Thalassiosira weissflogii were grown in continuous cultures enriched with f/2 medium. One of the two contrasting cultures (‘eutrophic’) received 5.6 times more nutrients than the other (‘oligotrophic’). Two mathematical models are analyzed to estimate eutrophication differences. The second model based on the Michaelis–Menten uptake and Droop growth shows that cells in the eutrophic culture should have about 56% higher content of silica which is the limiting nutrient. Diatom samples were prepared for the transmission electron microscopy after cells have been kept in chemostats for 37 days. The structure of diatom cells was investigated and a comparison is made between cells grown in oligotrophic and eutrophic conditions. In eutrophic culture, dividing cells were encountered more frequently while cell concentration was approximately equal in both chemostats. The central vacuole of cells in eutrophic culture accumulated dispersed and compact material from amorphous to spherical shape. In some cells the large central vacuole had fibrilar and peppered dense materials in addition to translucent granules, vesicules and multivesicular bodies. In the cytoplasm we found increased number of multivesicular bodies, dense and lucent granules some of which enclose membrane particles and lucent vesicules. Dense material depositions observed in the vacuole are also seen in the cytoplasm associated with organelles, mitochondria and plasmalemma. Cells have well-developed, active and slightly increased number of dictyosomes (5–6). Some dictyosomes with dense secretory material in the cistern are apparently engaged in a granule formation process. Functional significance of dense material in the central vacuole, which has not been observed in cells grown in oligotrophic condition, is discussed.

  • Diatom Thalassiosira weissflogii in oligotrophic versus eutrophic culture: models and ultrastructure
    Ecological Modelling, 2003
    Co-Authors: E. Bayraktaroğlu, T. Legović, Z.r Velasquez, Antonio Cruzado
    Abstract:

    7 páginas, 1 tabla, 1 figuraPopulations of marine diatom Thalassiosira weissflogii were grown in continuous cultures enriched with f/2 medium. One of\ud the two contrasting cultures (‘eutrophic’) received 5.6 times more nutrients than the other (‘oligotrophic’). Two mathematical\ud models are analyzed to estimate eutrophication differences. The second model based on the Michaelis–Menten uptake and Droop\ud growth shows that cells in the eutrophic culture should have about 56% higher content of silica which is the limiting nutrient.\ud Diatom samples were prepared for the transmission electron microscopy after cells have been kept in chemostats for 37 days.\ud The structure of diatom cells was investigated and a comparison is made between cells grown in oligotrophic and eutrophic\ud conditions. In eutrophic culture, dividing cells were encountered more frequently while cell concentration was approximately\ud equal in both chemostats. The central vacuole of cells in eutrophic culture accumulated dispersed and compact material from\ud amorphous to spherical shape. In some cells the large central vacuole had fibrilar and peppered dense materials in addition\ud to translucent granules, vesicules and multivesicular bodies. In the cytoplasm we found increased number of multivesicular\ud bodies, dense and lucent granules some of which enclose membrane particles and lucent vesicules. Dense material depositions\ud observed in the vacuole are also seen in the cytoplasm associated with organelles, mitochondria and plasmalemma. Cells have\ud well-developed, active and slightly increased number of dictyosomes (5–6). Some dictyosomes with dense secretory material\ud in the cistern are apparently engaged in a granule formation process. Functional significance of dense material in the central\ud vacuole, which has not been observed in cells grown in oligotrophic condition, is discussed.Peer reviewe