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Diatoms are one of the key phytoplankton groups in the ocean, forming vast oceanic blooms and playing a significant part in global primary production. To shed light on the role of redox metabolism in diatom's acclimation to light–dark transition and its interplay with cell fate regulation, we generated transgenic lines of the diatom Thalassiosira pseudonana that express the redox‐sensitive green fluorescent protein targeted to various subcellular organelles. We detected organelle‐specific redox patterns in response to oxidative stress, indicating compartmentalized antioxidant capacities. Monitoring the GSH redox potential (EGSH) in the chloroplast over diurnal cycles revealed distinct rhythmic patterns. Intriguingly, in the dark, cells exhibited reduced basal chloroplast EGSH but higher sensitivity to oxidative stress than cells in the light. This dark‐dependent sensitivity to oxidative stress was a result of a depleted pool of reduced glutathione which accumulated during the light period. Interestingly, reduction in the chloroplast EGSH was observed in the light phase prior to the transition to darkness, suggesting an anticipatory phase. Rapid chloroplast EGSH re‐oxidation was observed upon re‐illumination, signifying an induction of an oxidative signaling during transition to light that may regulate downstream metabolic processes. Since light–dark transitions can dictate metabolic capabilities and susceptibility to a range of environmental stress conditions, deepening our understanding of the molecular components mediating the light‐dependent redox signals may provide novel insights into cell fate regulation and its impact on oceanic bloom successions. [ABSTRACT FROM AUTHOR]

Light drives phytoplankton productivity, so phytoplankton must exploit variable intensities and durations of light exposure, depending upon season, latitude, and depth. We analyzed the growth, photophysiology and composition of small, Thalassiosira pseudonana, and large, Thalassiosira punctigera, centric diatoms from temperate, coastal marine habitats, responding to a matrix of photoperiods and growth light intensities. T. pseudonana showed fastest growth rates under long photoperiods and low to moderate light intensities, while the larger T. punctigera showed fastest growth rates under short photoperiods and higher light intensities. Photosystem II function and content responded primarily to instantaneous growth light intensities during the photoperiod, while diel carbon fixation and RUBISCO content responded more to photoperiod duration than to instantaneous light intensity. Changing photoperiods caused species-specific changes in the responses of photochemical yield (e−/photon) to growth light intensity. These photophysiological variables showed complex responses to photoperiod and to growth light intensity. Growth rate also showed complex responses to photoperiod and growth light intensity. But these complex responses resolved into a close relation between growth rate and the cumulative daily generation of reductant, across the matrix of photoperiods and light intensities. [ABSTRACT FROM AUTHOR]

Diatoms are responsible for a large proportion of global carbon fixation, with the possibility that they may fix more carbon under future levels of high CO2. To determine how increased CO2 concentrations impact the physiology of the diatom Thalassiosira pseudonana Hasle et Heimdal, nitrate-limited chemostats were used to acclimate cells to a recent past (333 ± 6 μatm) and two projected future concentrations (476 ± 18 μatm, 816 ± 35 μatm) of CO2. Samples were harvested under steady-state growth conditions after either an abrupt (15-16 generations) or a longer acclimation process (33-57 generations) to increased CO2 concentrations. The use of un-bubbled chemostat cultures allowed us to calculate the uptake ratio of dissolved inorganic carbon relative to dissolved inorganic nitrogen ( DIC: DIN), which was strongly correlated with f CO2 in the shorter acclimations but not in the longer acclimations. Both CO2 treatment and acclimation time significantly affected the DIC: DIN uptake ratio. Chlorophyll a per cell decreased under elevated CO2 and the rates of photosynthesis and respiration decreased significantly under higher levels of CO2. These results suggest that T. pseudonana shifts carbon and energy fluxes in response to high CO2 and that acclimation time has a strong effect on the physiological response. [ABSTRACT FROM AUTHOR]

Phytochelatin synthase ( PC synthase) is the enzyme that catalyzes the production of phytochelatins, peptides of the structure (γ-Glu-Cys) n-Gly, where n = 2-11, from the sulfhydryl-containing tripeptide glutathione, in response to elevated metal exposure. Biochemical utilization of Cd in the marine diatom Thalassiosira weissfloggi, as well as unusually high ratios of PC to Cd in some Thalassiosira species including T. pseudonana Hasle et Heimdal, motivated the characterization of T. pseudonana PC synthase 1 ( TpPCS1). This enzyme is the product of one of three genes in the T. pseudonana genome predicted to encode for a PC synthase based on its homology to canonical PC synthases previously examined. TpPCS1 was cloned, expressed in Escherichia coli and purified under both aerobic and anaerobic conditions. TpPCS1 exhibits several characteristics that set it distinctly apart from the well-studied PC synthase, Arabidopsis thaliana PCS1 ( AtPCS1). It is extremely sensitive to oxidation, which suppresses activity, and it is readily inhibited by the addition of Cd in the absence of thiolate ligands. TpPCS1 also has significantly greater affinity for one of its key substrates, the bis-glutathionato-Cd complex. TpPCS1 kinetics is best described by a ternary complex model, as opposed to the ping-pong model used to describe AtPCS1 kinetics. The findings indicate that although the function of TpPCS1 is synonymous to that of AtPCS1, its divergent biochemistry suggests adaptation of this enzyme to the distinct trace metal chemistry of the marine environment and the unique physiological needs of T. pseudonana. [ABSTRACT FROM AUTHOR]

The cellular iron (Fe) quota of centric diatoms has been shown to vary in response to the ambient dissolved Fe concentration; however, it is not known how centric diatoms store excess intracellular Fe. Here, we use synchrotron X-ray fluorescence (SXRF) element mapping to identify Fe storage features in cells of Thalassiosira pseudonana Hasle et Heimdal and Thalassiosira weissflogii G. A. Fryxell et Hasle grown at low and high Fe concentrations. Localized intracellular Fe storage features, defined as anomalously high Fe concentrations in regions of relatively low phosphorus (P), sulfur (S), silicon (Si), and zinc (Zn), were twice as common in T. weissflogii cells grown at high Fe compared to low-Fe cells. Cellular Fe quotas of this strain increased 2.9-fold, the spatial extent of the features increased 4.6-fold, and the Fe content of the features increased 14-fold under high-Fe conditions, consistent with a vacuole storage mechanism. The element stoichiometry of the Fe features is consistent with polyphosphate-bound Fe as a potential vacuolar Fe storage pool. Iron quotas increased 2.5-fold in T. pseudonana grown at high Fe, but storage features contained only 2-fold more Fe and did not increase in size compared to low-Fe cells. The differences in Fe storage observed between T. pseudonana and T. weissflogii may have been due to differences in the growth states of the cultures. [ABSTRACT FROM AUTHOR]

Increasing anthropogenic carbon dioxide is causing changes to ocean chemistry, which will continue in a predictable manner. Dissolution of additional atmospheric carbon dioxide leads to increased concentrations of dissolved carbon dioxide and bicarbonate and decreased pH in ocean water. The concomitant effects on phytoplankton ecophysiology, leading potentially to changes in community structure, are now a focus of concern. Therefore, we grew the coccolithophore Emiliania huxleyi (Lohmann) W. W. Hay et H. Mohler and the diatom strains Thalassiosira pseudonana (Hust.) Hasle et Heimdal CCMP 1014 and T. pseudonana CCMP 1335 under low light in turbidostat photobioreactors bubbled with air containing 390 ppmv or 750 ppmv CO2. Increased pCO2 led to increased growth rates in all three strains. In addition, protein levels of RUBISCO increased in the coastal strains of both species, showing a larger capacity for CO2 assimilation at 750 ppmv CO2. With increased pCO2, both T. pseudonana strains displayed an increased susceptibility to PSII photoinactivation and, to compensate, an augmented capacity for PSII repair. Consequently, the cost of maintaining PSII function for the diatoms increased at increased pCO2. In E. huxleyi, PSII photoinactivation and the counter-acting repair, while both intrinsically larger than in T. pseudonana, did not change between the current and high-pCO2 treatments. The content of the photosynthetic electron transport intermediary cytochrome b6/f complex increased significantly in the diatoms under elevated pCO2, suggesting changes in electron transport function. [ABSTRACT FROM AUTHOR]

The cellular iron (Fe) quota of centric diatoms has been shown to vary in response to the ambient dissolved Fe concentration; however, it is not known how centric diatoms store excess intracellular Fe. Here, we use synchrotron X-ray fluorescence (SXRF) element mapping to identify Fe storage features in cells of Thalassiosira pseudonana Hasle et Heimdal and Thalassiosira weissflogii G. A. Fryxell et Hasle grown at low and high Fe concentrations. Localized intracellular Fe storage features, defined as anomalously high Fe concentrations in regions of relatively low phosphorus (P), sulfur (S), silicon (Si), and zinc (Zn), were twice as common in T. weissflogii cells grown at high Fe compared to low-Fe cells. Cellular Fe quotas of this strain increased 2.9-fold, the spatial extent of the features increased 4.6-fold, and the Fe content of the features increased 14-fold under high-Fe conditions, consistent with a vacuole storage mechanism. The element stoichiometry of the Fe features is consistent with polyphosphate-bound Fe as a potential vacuolar Fe storage pool. Iron quotas increased 2.5-fold in T. pseudonana grown at high Fe, but storage features contained only 2-fold more Fe and did not increase in size compared to low-Fe cells. The differences in Fe storage observed between T. pseudonana and T. weissflogii may have been due to differences in the growth states of the cultures. [ABSTRACT FROM AUTHOR]

Increasing anthropogenic carbon dioxide is causing changes to ocean chemistry, which will continue in a predictable manner. Dissolution of additional atmospheric carbon dioxide leads to increased concentrations of dissolved carbon dioxide and bicarbonate and decreased pH in ocean water. The concomitant effects on phytoplankton ecophysiology, leading potentially to changes in community structure, are now a focus of concern. Therefore, we grew the coccolithophore Emiliania huxleyi (Lohmann) W. W. Hay et H. Mohler and the diatom strains Thalassiosira pseudonana (Hust.) Hasle et Heimdal CCMP 1014 and T. pseudonana CCMP 1335 under low light in turbidostat photobioreactors bubbled with air containing 390 ppmv or 750 ppmv CO2. Increased pCO2 led to increased growth rates in all three strains. In addition, protein levels of RUBISCO increased in the coastal strains of both species, showing a larger capacity for CO2 assimilation at 750 ppmv CO2. With increased pCO2, both T. pseudonana strains displayed an increased susceptibility to PSII photoinactivation and, to compensate, an augmented capacity for PSII repair. Consequently, the cost of maintaining PSII function for the diatoms increased at increased pCO2. In E. huxleyi, PSII photoinactivation and the counter-acting repair, while both intrinsically larger than in T. pseudonana, did not change between the current and high-pCO2 treatments. The content of the photosynthetic electron transport intermediary cytochrome b6/f complex increased significantly in the diatoms under elevated pCO2, suggesting changes in electron transport function. [ABSTRACT FROM AUTHOR]

Diel periodicity and effects of inorganic carbon (Ci) and NO3− on the expression of 11 key genes for primary carbon and nitrogen metabolism, including potential C4 photosynthesis, in the marine diatom Thalassiosira pseudonana Hasle et Heimdal were investigated. Target gene transcripts were measured by quantitative reverse transcriptase–PCR, and some of the gene-encoded proteins were analyzed by Western blotting. The diatom was grown with a 12 h photoperiod at two different Ci concentrations maintained by air-equilibration with either 380 μL · L−1 (near-ambient) or 100 μL · L−1 (low) CO2. Transcripts of the principal Ci and NO3− assimilatory genes RUBISCO LSU ( rbcL) and nitrate reductase displayed very strong diel oscillations with peaks at the end of the scotophase. Considerable diel periodicities were also exhibited by the β-carboxylase genes phospho enolpyruvate carboxylase ( PEPC1 and PEPC2) and phospho enolpyruvate carboxykinase ( PEPCK), and the Benson–Calvin cycle gene sedoheptulose–bisphosphatase (SBPase), with peaks during mid- to late scotophase. In accordance with the transcripts, there were substantial diel periodicities in PEPC1, PEPC2, PEPCK, and especially rbcL proteins, although they peaked during early to mid-photophase. Inorganic carbon had some transient effects on the β-carboxylase transcripts, and glycine decarboxylase P subunit was highly up-regulated by low Ci concentration, indicating increased capacity for photorespiration. Nitrogen-starved cells had reduced amounts of carbon metabolic gene transcripts, but the PEPC1, PEPC2, PEPCK, and rbcL transcripts increased rapidly when NO3− was replenished. The results suggest that the β-carboxylases in T. pseudonana play key anaplerotic roles but show no clear support for C4 photosynthesis. [ABSTRACT FROM AUTHOR]

Nitrate transporter genes ( Nrt2) encode high-affinity nitrate transporters in marine phytoplankton, and their transcript levels are potential markers of nitrogen deficiency in eukaryotic phytoplankton. For the proper interpretation of measured Nrt2 transcript abundances, a relative expression assay was proposed and tested in Isochrysis galbana Parke (Prymnesiophyceae) and Thalassiosira pseudonana (Hust.) Hasle et Heimdal (Bacillariophyceae). The minimal transcript levels of Nrt2 genes were achieved by the addition of 100 μM ammonium, which led to a rapid decline in Nrt2 transcripts in 10–30 min. Experiments using a concentration series revealed that the effective dosage of ammonium to create a minimal transcript level of ∼1 μmol · mol−1 18S rRNA was ≥25 μM in both species. On the other hand, the addition ofl-methionine sulfoximine (MSX), an inhibitor of glutamine synthetase, enhanced the Nrt2 transcript level in I. galbana but did not affect that in T. pseudonana. Nitrogen deprivation was used as an alternative means to create maximal Nrt2 transcript levels. By transferring cells into N-free medium for 24 h, Nrt2 transcript levels increased to ∼90 μmol · mol−1 18S rRNA in I. galbana, and to ∼800 μmol · mol−1 18S rRNA in T. pseudonana. The degree of nitrogen deficiency thus can be determined by comparing original Nrt2 transcript levels with the minimal and maximal levels. [ABSTRACT FROM AUTHOR]

While urea has long been recognized as an important form of nitrogen in planktonic ecosystems, very little is known about how many or which phytoplankton and bacteria can use urea as a nitrogen source. We developed a method, targeting the gene encoding urease, for the direct detection and identification of ureolytic organisms and tested it on seven axenic phytoplankton cultures (three diatoms, two prymnesiophytes, a eustigmatophyte, and a pelagophyte) and on three nonaxenic Aureococcus anophagefferens Hargraves et Sieburth cultures (CCMP1784 and two CCMP1708 cultures from different laboratories). The urease amplicon sequences from axenic phytoplankton cultures were consistent with genomic data in the three species for which both were available. Seven of 12 phytoplankton species have one or more introns in the amplified region of their urease gene(s). The 63 urease amplicons that were cloned and sequenced from nonaxenic A. anophagefferens cultures grouped into 17 distinct sequence types. Eleven types were related to α-Proteobacteria, including three types likely belonging to the genus Roseovarius. Four types were related to γ-Proteobacteria, including two likely belonging to the genus Marinobacter, and two types were related to β-Proteobacteria. Terminal restriction fragment length polymorphism (TRFLP) analyses suggested that the sequenced amplicons represented approximately half of the diversity of bacterial urease genes present in the nonaxenic cultures. While many of the bacterial urease sequence types were apparently lab- or culture-specific, others were found in all three nonaxenic cultures, suggesting the possibility of specific relationships between these bacteria and A. anophagefferens. [ABSTRACT FROM AUTHOR]

Long-term growth response to natural solar radiation with enhanced ultraviolet-B (UVB) exposure was examined in two species of dinoflagellates [ Alexandrium tamarense (M. Lebour) Balech, At, and Heterocapsa triquetra (Ehrenb.) F. Stein, Ht], including two strains of A. tamarense, one from Spain and another from UK, and one diatom species ( Thalassiosira pseudonana Hasle et Heimdal). We examined whether variable photoprotection (mycosporine-like amino acids [MAAs] and xanthophyll-cycle pigments) affected photosynthetic performance, phytoplankton light absorption, and growth. Growth rate was significantly reduced under enhanced UVB for the UK strain of At and for Ht (both grew very little) as well as for the diatom (that maintained high growth rates), but there was no effect for the Spanish strain of At. MAA concentration was high in the dinoflagellates, but undetectable in the diatom, which instead used the xanthophyll cycle for photoprotection. The highest cell concentrations of MAAs and photoprotective pigments were observed in the UK strain of At, along with lowest growth rates and Fv/ Fm, indicating high stress levels. In contrast, the Spanish strain showed progressive acclimation to the experimental conditions, with no significant difference in growth between treatments. Increase in total MAAs followed linearly the cumulative UVB of the preceding day, and both total and primary MAAs were maintained at higher constitutive levels in this strain. Acclimation to enhanced UVB in the diatom resulted in an increase in PSII activity and reduction in nonphotochemical quenching, indicating an increased resistance to photoinhibition after a few weeks. All four species showed increased phytoplankton light absorption under enhanced UVB. Large intrastrain differences suggest a need to consider more closely intraspecific variability in UV studies. [ABSTRACT FROM AUTHOR]

Diatoms are unicellular brown algae that likely arose from the endocytobiosis of a red alga into a single-celled heterotroph and that constitute an algal class of major importance in phytoplankton communities around the globe. The first whole-genome sequence from a diatom species, Thalassiosira  pseudonana Hasle et Heimdal, was recently reported, and features that are central to diatom physiology and ecology, such as silicon and nitrogen metabolism, iron uptake, and carbon concentration mechanisms, were described. Following this initial study, the basic cellular systems controlling cell signaling, gene expression, cytoskeletal structures, and response to stress have been cataloged in an attempt to obtain a global view of the molecular foundations that sustain such an ecologically successful group of organisms. Comparative analysis with several microbial, plant, and metazoan complete genome sequences allowed the identification of putative membrane receptors, signaling proteins, and other components of central interest to diatom ecophysiology and evolution. Thalassiosira  pseudonana likely perceives light through a novel phytochrome and several cryptochrome photoreceptors; it may lack the conserved RHO small-GTPase subfamily of cell-polarity regulators, despite undergoing polarized cell-wall synthesis; and it possesses an unusually large number of heat-shock transcription factors, which may indicate the central importance of transcriptional responses to environmental stress. The availability of the complete gene repertoire will permit a detailed biochemical and genetic analysis of how diatoms prosper in aquatic environments and will contribute to the understanding of eukaryotic evolution. [ABSTRACT FROM AUTHOR]

Temperature is expected to modify the effects of ultraviolet radiation (UVR) on photosynthesis by affecting the rate of repair. We studied the effect of short-term (1 h) and long-term (days) acclimation to temperature on UVR photoinhibition in the diatom Thalassiosira pseudonana Hasle et Heimdal. Photosynthesis was measured during 1 h exposures to varying irradiances of PAR and UVR + PAR at 15, 20, and 25°C, the latter corresponding to the upper temperature limit for optimal growth in T. pseudonana. The exposures allowed the estimation of photosynthesis–irradiance (P–E) curves and biological weighting functions (BWFs) for photoinhibition. For the growth conditions used, temperature did not affect photosynthesis under PAR. However, photoinhibition by UVR was highly affected by temperature. For cultures preacclimated to 20°C, the extent of UVR photoinhibition increased with decreasing temperature, from 63% inhibition of PAR-only photosynthesis at 25°C to 71% at 20°C and 85% at 15°C. These effects were slightly modified after several days of acclimation: UVR photoinhibition increased from 63% to 75% at 25°C and decreased from 85% to 80% at 15°C. Time courses of photochemical efficiency (ΦPSII) under UVR + PAR were also fitted to a model of UVR photoinhibition, allowing the estimation of the rates of damage ( k) and repair ( r). The r/ k values obtained for each temperature treatment verified the responses observed with the BWF ( R2 = 0.94). The results demonstrated the relevance of temperature in determining primary productivity under UVR exposures. However, the results suggested that temperature and UVR interact mainly over short (hours) rather than long (days) timescales. [ABSTRACT FROM AUTHOR]

The effects of nitrogen source, temperature, and light level on transcription of five genes from three pathways of carbon and nitrogen metabolism were determined for the centric diatom Thalassiosira pseudonana (Hustedt) Hasle et Heimdal. The targeted genes were NR (nitrate reductase) and GSII (glutamine synthetase II), both required for nitrate utilization; PGP (phosphoglycolate phosphatase) and GDCT (glycine decarboxylase T-protein), both required for photorespiration; and SBP (sedoheptulose 1,7-bisphosphatase), required for carbon fixation (Calvin cycle). Quantitative reverse transcriptase PCR was used to compare transcript abundances for these genes from T. pseudonana cells grown at two light intensities (50 or 300 μmol photons·m−2·s−1), at two temperatures (12 or 22°C), and on two different nitrogen sources (nitrate or ammonium). Transcript abundance (normalized to 18S rRNA) was higher for all five genes at the higher light intensity. At 12°C, fewer transcripts were detected for the Calvin cycle gene, whereas more transcripts were detected for the photorespiration and nitrate utilization genes. Nitrogen source affected transcript accumulation of the photorespiration and nitrate utilization genes but not the Calvin cycle gene. Extracellular concentrations of glycolate, a photorespiratory-specific product, were determined under the high light conditions and were found to decrease at 12°C and when cells were grown on ammonium. A hypothetical model is presented to explain the patterns of transcript accumulation and glycolate release in terms of energy balance within the cell. [ABSTRACT FROM AUTHOR]

The elemental composition and the cell cycle stages of the marine diatom Thalassiosira pseudonana Hasle and Heimdal were studied in continuous cultures over a range of different light- (E), nitrogen- (N), and phosphorus- (P) limited growth rates. In all growth conditions investigated, the decrease in the growth rate was linked with a higher relative contribution of the G2+M phase. The other phases of the cell cycle, G1 and S, showed different patterns, depending on the type of limitation. All experiments showed a highly significant increase in the amount of biogenic silica per cell and per cell surface with decreasing growth rates. At low growth rates, the G2+M elongation allowed an increase of the silicification of the cells. This pattern could be explained by the major uptake of silicon during the G2+M phase and by the independence of this process on the requirements of the other elements. This was illustrated by the elemental ratios Si/C and Si/N that increased from 2- to 6-fold, depending of the type of limitation, whereas the C/N ratio decreased by 10% (E limitation) or increased by 50% (P limitation). The variations of the ratios clearly demonstrate the uncoupling of the Si metabolism compared with the C and N metabolisms. This uncoupling enabled us to explain that in any of the growth condition investigated, the silicification of the cells increased at low growth rates, whereas carbon and nitrogen cellular content are differently regulated, depending of the growth conditions. [ABSTRACT FROM AUTHOR]

The elemental composition and the cell cycle stages of the marine diatom Thalassiosira pseudonana Hasle and Heimdal were studied in continuous cultures over a range of different light- (E), nitrogen- (N), and phosphorus- (P) limited growth rates. In all growth conditions investigated, the decrease in the growth rate was linked with a higher relative contribution of the G2+M phase. The other phases of the cell cycle, G1 and S, showed different patterns, depending on the type of limitation. All experiments showed a highly significant increase in the amount of biogenic silica per cell and per cell surface with decreasing growth rates. At low growth rates, the G2+M elongation allowed an increase of the silicification of the cells. This pattern could be explained by the major uptake of silicon during the G2+M phase and by the independence of this process on the requirements of the other elements. This was illustrated by the elemental ratios Si/C and Si/N that increased from 2- to 6-fold, depending of the type of limitation, whereas the C/N ratio decreased by 10% (E limitation) or increased by 50% (P limitation). The variations of the ratios clearly demonstrate the uncoupling of the Si metabolism compared with the C and N metabolisms. This uncoupling enabled us to explain that in any of the growth condition investigated, the silicification of the cells increased at low growth rates, whereas carbon and nitrogen cellular content are differently regulated, depending of the growth conditions. [ABSTRACT FROM AUTHOR]

Experiments on a marine diatom, Thalassiosira pseudonana (Hustedt) clone 3H, demonstrate that under moderate photon flux densities (75 μmol quanta·m[sup-2]·s[sup-1]) of visible light the inhibition of photosynthesis by supplemental ultraviolet (UV) radiation (UV-B: 280-320 nm) is for time scales of 0.5-4 h. Results are consistent with predictions of a recently developed model of photosynthesis under the influence of UV and visible irradiance. Although net destruction of chlorophyll occurs during a 4-h exposure to UV-B, and the effect is a function of exposure, the principal effect of UV-B is a decreased in chlorophyll-specific photosynthetic rate. The dependence of photoinhibition on dosage rate, rather than cumulatve dose, and the hyperbolic shape of the relationship are consistent with net photoinhibition being an equilibrium between damage and repair. The ratio of damage to repair is estimated by a mathematical analysis of the inhibition of the photosynthesis during exposures to UV-B. A nitrate-limited culture was much more sensitive to UV-B than were the nutrient-replete cultures, but the kinetics of photoinhibition were similar. The analysis suggests that the nutrient-limited culture was more sensitive than the nutrient-replete cultures because repair or turnover of critical proteins associated with photosynthesis is inhibited. An inhibitor of chloroplast protein synthesis was used to suppress repair processes. Photoinhibition by UV-B was enhanced, and inhibition was a function of cumulative dose, as would be expected if damage were not countered by repair. The fundamental importance of repair processes should be considered in the design of field experiments and models of UB-B effects in the environment, especially in the context of vertical mixing. Repair processes must also be considered whenever biological weighting functions are developed. [ABSTRACT FROM AUTHOR]

The influence of temperature on the biochemical composition of eight species of marine phytoplankton was investigated. Thalassiosira pseudonana Hasle and Heimdal, Phaeodactylum tricornutum Bohlin and, Pavlova lutheri Droop (three of eight species studied) had minimum values of carbon and nitrogen quotas at intermediate temperatures resulting in a broad U-shaped response in quotas over the temperature range of 10 to 25&deg, C. Protein per cell also had minimum values at intermediate temperatures for six species. For T. pseudonana, P. tricornutum, and P. lutheri, patterns of variation in carbon, nitrogen, and protein quotas as a function of temperature were similar. Over all species, lipid and carbohydrate per cell showed no consistent trends with temperature. Only chlorophyll a quotas and the carbon : chlorophyll a ratios (&theta,) showed consistent trends across all species. Chlorophyll a quotas were always lower at 10&deg, C than at 25&deg, C. Carbon : chlorophyll a ratios (&theta,) were always higher at 10&deg, C than at 25&deg, C. We suggest that although &theta, consistently increases at lower temperatures, the relationship between temperature and &theta, ranges from linear to exponential and is species specific. Accordingly, the interspecific variance in &theta, that results from species showing a range of possible responses to temperature increases as temperature declines and reaches a maximum at low temperatures. High photon flux densities appear to increase the potential interspecific variance in the carbon : chlorophyll a ratio and therefore exacerbate these trends. [ABSTRACT FROM AUTHOR]

Two species of marine diatoms [Skeletonema costatum (Greville) Cleve and Thalassiosira pseudonana (Hustedt) Hasle and Heimdal] were grown in glass and polycarbonate containers. S. costatum exhibited a significantly lower exponential growth rate and maximal yield and a significantly longer log phase when grown in polycarbonate. Exponential growth rate and maximal yield of T. pseudonana was significantly reduced (P < 0.05 in all cases). This study suggests that a difference in diatom growth between glass and polycarbondate containers might arise in certain cases. However, such a difference may not be detectable with all biomass measurement techniques or with detectale with all biomass measurement techniques or with low within-treatment replication. [ABSTRACT FROM AUTHOR]

An obligate requirement for selenium is demonstrated in axenic culture of the coastal marine diatom Thalassiosira pseudonana (clone 3H) (Hust.) Hasle and Heimdal grown in artificial seawater medium. Selenium deftciency was characterized by a reduction in growth rate and eventually by a cessation of cell division. The addition of 10[SUP-10] M Na[SUB2]O[SUB3] to nutrient enriched artificial seawater resulted in excellent growth of T. pseudonana and only a slight inhibition of growth occurred at Na[SUB2]SeO[SUB3] concentrations of 10[SUP-3] and 10[SUP-2] M. By contrast, Na[SUB2]SeO[SUB4] failed to support growth of T. pseudonana when supplied at concentrations less than 10[SUP-7] M and the growth rate at this concentration was only one quarter of the maximum growth rate. The addition of 10[SUP-3] and 10[SUP-2] M Na[SUB2]SeO[SUB4] to the culture medium was toxic and cell growth was completely inlhibited. Eleven trace elements were tested for their ability to replace the selenium requirement by this alga and all were without effect. In selenium-deficient and selenium-starved cultures of T. pseudonana cell volume increased as much as 10-fold as a result of an increase in cell length (along the pervalvar axis) but cell width was constant. It is concluded that selenium is an indispensable element for the growth of T. pseudonana and it should be included as a nutrient enrichment to artificial seawater medium when culturing this alga. [ABSTRACT FROM AUTHOR]

The effects of selenium deficiency on the siliceous and nonsiliceous components of the planktonic marine diatom Thalassiosira pseudonana (Hust.) Hasle and Heimdal (clone 31-H) are examined using light and electron microscopy. Selenium deficiency induces elongation along the pervalvar axis initially as a result of chain formation caused by the failure of sibling cells to separate and subsequently by cell elongation via the production of hyaline girdle bands. In Se-deficient cultures cell elongation involves the blockage of both mitotic and cytokinetic components of cell division. Selenium deficiency results in ultra structural alterations in the reticular membrane system and in mitochondrial and chloroplast membranes. Various types of inclusions are seen in vacuolar areas and the accumulation of lipid reserves is evident in Se-deficient cells. These results provide indirect evidence for a metabolic Se requirement in this algal species. [ABSTRACT FROM AUTHOR]

Growth rate cellular manganese concentration, and manganese uptake kinetics were measured as functions of the free manganese ion concentration in the medium for two centric diatoms. Thalassiosira oceanica Hasle, isolated from a low-manganese oceanic environment, was capable of better growth at lea, manganese ion concentrations than T. pseudonana Hasle and Heimdal, isolated from high-manganese estuarine waters. This was due to the oceanic species' greater ability to accumulate manganese at low free ion concentrations and to grow well at lower cellular manganese concentrations. Manganese uptake in both species followed classical saturation kinetics, and V[SUBmax] but not K, varied with the manganese ion concentration in the growth medium. The K, of manatganese uptake for T. oceanica (10[SUP-793] M) was one-seventh of that for T. pseudonana, accounting for T. oceanica's greater ability to accumulate manganese at low free ion concentrations. V[SUBmax] appeared to be under negative feedback control in both species and six- to twelve-fold variations in this parameter allowed cells to regulate cellular manganese at nearly constant values at manganese ion concentrations in the vicinity of and somewhat below K[SUBr] The range in free manganese ion concentrations over which regulation occurred was different for the two species, and coincided with the range in free manganese ion concentrations of the natural habitat of each species. [ABSTRACT FROM AUTHOR]

Concentrated stock solutions of sodium metasilicate (Na[sub2]SiO[sub3]·9H[sub2]O). prepared in distilled-distilled water and acidified to pH 2 as suggested by McLachlan (1973), were found to be polymerized; stocks kept at their alkaline pH of dissolution were not polymerized. Depolymerization studies using the acidified stock added to deionized-dis. tilled water, 3% NaCl or artificial saltwater medium showed that complete depolymerization took longer than 2 days. Studies of Si uptake, using the marine diatom Tha-lassiosira pseudonana (Hust.) Hasle and Heimdal. showed that use of an acidified Si stock for experiments can lead to anomolous results. It is recommended that concentrated Si stocks be stored at their pH of dissolution and not be acidified prior to addition to media. In instances where the pH must be adjusted prior to addition (e.g. in some Si-uptake experiments), it may be necessary to take into account polymerization/depolymeriztion effects. [ABSTRACT FROM AUTHOR]

The acute toxity of Cr(VI) to the diatom Thalassiosira pseudonana (Hasle and Heimdal) clone 3H was determined in artificial media of 3.2 and 0.32 ppt salinity and with variations of sulfate concentration in the media independent of salinity. Inhibitory concentrations of Cr(VI) ranged from 6.6 μM for growth rate and 4.9 μM for cell yield at 3.2 ppt salinity and 2.8 μM sulfate to 0.04 μM for growth rate and 0.02 μM for cell yield at 0.32 ppt salinity and 0.019 mM sulfate. Inhibition occurred when this ratio exceeded about 5.00:1, It is suggested that the mechanism for the toxicity of Cr(VI) to diatoms and perhaps other aquatic organisms involves a site at which sulfate and chromate compete. [ABSTRACT FROM AUTHOR]

Light-limited rates of photosynthesis normalized for chlorophyll a, (α), and actual photon absorption (quantum efficiency, φ were determined/hr six eponentially growing algal species grown under identical conditions. The same parameters, et and φ, were also monitored for a single diatom speries, Thalassiosira pseudonana Hasle & Heimdal. through its growth cycle in batch culture. Statistical differences in a could be demonstrated among the six different exponentially growing species while no differences could be showm for φ. Statistical differences among the six species were minimized when photosynthetic rates were normalized for in vivo fluorescence rather than extracted chlorophyll a. Both α and φ were constant while T. pseudonana was in the exponential phase of growth, but both declined as the culture entered stationary phase. While cells were in exponential growth, differences in a were attributed to varying rates of in vivo light absorption per chlorophyll a, thus providing experimental evidence that the in vivo chlorophyll a extinction coefficient, k[SUBc]. (m[SUP2] mg Chl a[SUP-1]), cannot be assumed constant. [ABSTRACT FROM AUTHOR]

The toxicity of Cu to Thalassiosira pseudonana (Hustedt) Hasle and Heimdal was investigated by examining both short and long term effects of Cu on cellular processes. Toxic levels of Cu (pCu* < 13) were found to inhibit short term Si(OH)[SUB4] uptake rates with kinetics characteristic of irreversible inhibition at a hypothetical Cu-sensitive Si(OH)[SUB4] transport site. Residual Si(OH)[SUB4] concentrations (those below which no uptake could occur) were found to increase with increasing levels of Cu, and the toxic effects of Cu could be reversed by increasing the concentrations of Si(OH)[SUB4] in the medium. The actual uptake of Cu by the cells was found to vary inversely with the ambient Si(OH)[SUB4] concentration. Copper did not inhibit the uptake of NO[SUP-][SUB3] or PO[SUP3-][SUB4]. The long term inhibition of growth rate by Cu in this species was shown not to be a result of Si deficiency caused by the inhibition of Si(OH)[SUB4] uptake. Cu inhibited cells were found to have higher Si cell quotas (including a sizable soluble pool) than the control cultures. The were, however, observed to have aberrant frustules, significantly larger than the control cells, suggesting interference with the silification process as a possible mechanism for inhibition of growth by Cu. A conceptual model is proposed for the Cu-Si(OH)[SUB4]- growth relationship. It includes a Cu sensitive Si(OH)[SUB4] transport site that may also serve to transport Cu into the cell, and growth inhibition mediated by intracellular Cu concentrations which may block cell division or cause general cellular disfunction. [ABSTRACT FROM AUTHOR]

Stressful environmental conditions can induce many different acclimation mechanisms in marine phytoplankton, resulting in a range of changes in their photophysiology. Here we characterize the common photophysiological stress response of the model diatom Thalassiosira pseudonana to ten environmental stressors and identify diagnostic responses to particular stressors. We quantify the magnitude and temporal trajectory of physiological parameters including the functional absorption cross-section of PSII (σ PSII ), quantum efficiency of PSII, non-photochemical quenching (NPQ), cell volume, Chl a, and carotenoid (Car) content in response to nutrient starvation (nitrogen (N), phosphorus (P), silicon (Si), and iron (Fe)), changes in temperature, irradiance, pH, and reactive oxygen species (ROS) over 5 time points (0, 2, 6, 24, 72 h). We find changes in conditions: temperature, irradiance, and ROS, often result in the most rapid changes in photophysiological parameters (<2 h), and in some cases are followed by recovery. In contrast, nutrient starvation (N, P, Si, Fe) often has slower (6-72 h) but ultimately larger magnitude effects on many photophysiological parameters. Diagnostic changes include large increases in cell volume under Si-starvation, very large increases in NPQ under P-starvation, and large decreases in the σ PSII under high light. The ultimate goal of this analysis is to facilitate and enhance the interpretation of fluorescence data and our understanding of phytoplankton photophysiology from laboratory and field studies., (© 2020 Phycological Society of America.)