Effect of nitrate-nitrogen limitation on photosynthesis of the diatom Phaeodactylum tricornutum Bohlin (Bacillariophyceae)

Abstract Nitrate limited growth of the diatom Phaeodactylum tricornutum in chemostat cultures produced marked changes in biochemical composition and a six-fold reduction in the specific growth rate. This was associated with a reduction in the carbon and chlorophyll a specific light saturated rates, with little effect on light limited photosynthesis. Variations in specific growth rate were quantitatively related to carbon specific net photosynthesis and maximum chlorophyll a specific light saturated rates were positively correlated with cell nitrogen contents. The correlation between nitrogen content and photosynthesis for P. tricornutum and the differential effect of nitrogen supply on the light response curve of photosynthesis is qualitatively and quantitatively similar to published results for terrestrial vascular plants. There was little change in the photon (quantum) yield of photosynthesis which was not significantly different from 0.125mol O₂ mol photon^-1 the theoretical upper limit based on the Z scheme, even under severe nitrate deficiency. The capacity to maintain a high photon yield under nitrate limitation is discussed in relation to the nitrogen requirements of the stromal and membrane components of the photosynthetic apparatus.     

Cyclic variations of photosynthetic activity under nitrogen fixing conditions in Synchococcus RF-1

When nitrogen fixing cell cultures of Synechococcus RF-1 were subjected to an alternating lightdark regime (12 h:12 h), a cyclic decrease in the photosynthetic oxygen evolution potential was observed during the dark periods. This rhythm of net photosynthesis rate was maintained for at least two days after transition to continuous light. The decrease in net photosynthesis was accompanied by a stimulation of dark respiration. However, the magnitude of oxygen uptake was considerably smaller than the observed decrease in oxygen evolution. The photosynthetic activity of cells taken from the dark period was characterized by (i) a significantly lower quantum yield and (ii) a strong reduction in the light-saturated rate of photosynthesis. Growing the cultures on nitrate or under continuous light completely suppressed this rhythm. Protein synthesis was not necessary for the recovery of the light-saturated rate of photosynthesis during the light period. The cellular content of chlorophyll a and of phycobiliproteins did not vary between light and dark period, indicating that quantitative changes in the composition of the photosynthetic apparatus are not the basis for the observed oscillations. Regulatory modifications of the photosynthetic efficiency are proposed as an adaptation mechanism to adjust the intracellular oxygen concentration to the needs for nitrogenase activity.Abbreviation Chl chlorophyll     

Photosynthetic responses in Phaeocystis antarctica towards varying light and iron conditions

The effects of iron limitation on photoacclimation to a dynamic light regime were studied in Phaeocystis antarctica. Batch cultures were grown under a sinusoidal light regime, mimicking vertical mixing, under both iron-sufficient and -limiting conditions. Iron-replete cells responded to changes in light intensity by rapid xanthophyll cycling. Maximum irradiance coincided with maximum ratios of diatoxanthin/diadinoxanthin (dt/dd). The maximum quantum yield of photosynthesis (F _v /F _m ) was negatively related to both irradiance and dt/dd. Full recovery of F _v /F _m by the end of the light period suggested successful photoacclimation. Iron-limited cells displayed characteristics of high light acclimation. The ratio of xanthophyll pigments to chlorophyll a was three times higher compared to iron-replete cells. Down-regulation of photosynthetic activity was moderated. It is argued that under iron limitation cells maintain a permanent state of high energy quenching to avoid photoinhibition during exposure to high irradiance. Iron-limited cells could maintain a high growth potential due to an increased absorption capacity as recorded by in vivo absorption, which balanced a decrease in F _v /F _m . The increase in the chlorophyll a-specific absorption cross section was related to an increase in carotenoid pigments and a reduction in the package effect. These experiments show that P. antarctica can acclimate successfully to conditions as they prevail in the Antarctic ocean, which may explain the success of this species.     

The photosynthetic response of nutrient-depleted dilute cultures of Skeletonema costatum to pulses of ammonium and nitrate; the importance of phosphate

Experimental data on changes in carbon fixation rate causedby nutrient pulses in dilute cultures of nutrient-depleted Skeletonemacostatum are reported. Pulses contained single nutrients ormixtures of a nitrogen source and phosphate, concentrationsranging from 0 to 5 µM for ammonium and nitrate and from0 to 1 µM for phosphate. The cultures were incubated overnightafter pulsing to allow the rapid stage of nutrient uptake tocome to completion before measurement of carbon fixation thenext day. Increments in N:C ratios due to nitrogenous nutrientuptake depended upon the P:C ratio in the cells as well as theconcentration of the pulse. When P:C ratios were low (<0.005),increases in nitrogen repressed photosynthesis. Phosphate uptakewas independent of the absence or presence of a nitrogen sourceand had only a small stimulatory effect upon carbon fixation.When added jointly with ammonium or nitrate, however, largeincreases in the rate of photosynthesis resulted. These weredue mainly to rises in chlorophyll a concentrations resultingfrom higher N:C ratios in the cells. Chlorophyll-specific carbonfixation rates were hyperbolic functions of P:C ratios but exhibiteda C-shaped relationship to N:C ratios. The stimulatory and repressiveeffects of the nutrient pulses are shown to be consistent withthe view that the rate of photosynthesis at constant illuminationand carbon dioxide partial pressure is mainly controlled bythe chlorophyll a concentration and phosphate availability inthe phytoplankton.     

EFFECT OF IRON NUTRITION ON THE MARINE CYANOBACTERIUM SYNECHOCOCCUS GROWN ON DIFFERENT N SOURCES AND IRRADIANCES1

The effects of Fe deficiency on the marine cyanobacterium Synechococcus sp. were examined in batch cultures grown on nitrate or ammonium as a sole nitrogen source under two different irradiances. Fe-stressed cells showed lower chlorophyll a content and cellular C and N quotas. Light limitation increased the critical iron concentration below which both suppression of growth rate and changes in cellular composition were observed. At a limiting irradiance (26 μmol.m⁻².s⁻¹), this critical value was ∼10 nM, a 10 times increase compared to high-light cultures. Moreover, at low light the cellular chlorophyll a concentration was higher than at saturating light (110 μmol.m⁻².s⁻¹), this difference being most pronounced under Fe-stressed conditions. Cells grown on ammonium showed a lower half-saturation constant for Fe (K_s) compared to cells grown on nitrate, indicating Synechococcus sp. has the ability to grow faster on ammonium than on nitrate in a low Fe environment at high light. Consequently, in high-nutrient and low-chlorophyll regions where Fe limits new production, cyanobacteria most likely grow on regenerated ammonium, which requires less energy for assimilation. The K_s for growth on Fe at low light was significantly higher than at high light compared with the cells grown on the same N source, suggesting the cells require more Fe at low light. Therefore, if cells that are already Fe-limited also become light-limited, their iron stress level will increase even more. For cyanobacteria this is the first report of a study combining the interactions of Fe limitation, light limitation, and nitrogen source (NO₃⁻ vs. NH₄⁺).     

INFLUENCE OF IRON LIMITATION AND NITROGEN SOURCE ON GROWTH AND SIDEROPHORE PRODUCTION BY CYANOBACTERIA1

To characterize the mobilization and uptake of iron by cyanobacteria, 14 species were screened for ability to scavenge iron in a competitive system. The cyanobacteria exhibited a range of growth responses to iron limitation which could be separated into three groups, and a representative species from each group was chosen for further study. Effects of iron-limitation on growth and siderophore production of Anacystis nidulans R2, Anabaena variabilis ATCC 29413, and Plectonema boryanum UTEX 581 were determined. Both A. nidulans R2 and A. variabilis showed a reduced rate of growth with decreased available iron concentration (PFe 17–19). Growth rates increased with further reduction in the level of available iron (pFe 20 to pFe 21). The increase in growth rate occurred at the same available iron concentration as the initiation of extracellular siderophore production. In contrast, the growth of P. boryanum decreased with decreasing available iron levels. No siderophore production was detected from P. boryanum cultures. The growth kinetics of siderophore-producing species differ from traditional nutrient-limited growth kinetics and clearly reflect the presence of a high affinity, siderophore-mediated iron transport system in A. nidulans R2 and A. variabilis. Iron-limited growth kinetics more similar to traditional nutrient-limited growth kinetics were found in P. boryanum. The available nitrogen source influenced amount of siderophore produced and concentration of available iron which induced siderophore production. Siderophores were produced at high iron concentrations (pFe 18) when A. variablilis cultures were grown in the absence of combined nitrogen source. When nitrate was supplied to the culture, iron concentrations had to be reduced to pFe 20 before siderophores were produced. Cells grown on nitrogen also produced greater than two times the amount of siderophore compared with nitrate grown cells. This may be indicative of an increased demand for iron by nitrogen fixing A. variabilis Cultures.     

PHYSIOLOGICAL AND OPTICAL PROPERTIES OF RHIZOSOLENIA FORMOSA (BACILLARIOPHYCEAE) IN THE CONTEXT OF OPEN-OCEAN VERTICAL MIGRATION1

Cultures of Rhizosolenia formosa H. Peragallo were studied to assess whether or not physiological and optical characteristics of this large diatom were consistent with the ability to migrate vertically in the open ocean. Time-course experiments examined changes in chemical composition and buoyancy of R. formosa during nitrate (N)–replete growth, N starvation, and recovery. Cells could maintain unbalanced growth for at least 53 h after depletion of ambient nitrate. Increases in C:N and carbohydrate: protein ratios observed during N starvation reversed within 24 h of reintroduction of nitrate to culture medium. Buoyancy was related to nutrition: Upon N depletion, the percentage of positively buoyant cells decreased to 4% from 11% but reverted to 9% within 12 h of nitrate readdition. Cells took up nitrate in the dark. Nitrogen-specific uptake rates averaged 0.48 d^?1; these rates were higher than N-specific growth rates (0. 15 d^?1), indicating the potential for luxury consumption of nitrate, which can be stored for later use. Measurements of photosynthesis vs. irradiance, chlorophyll-specific absorption (a_ph*(λ)), and pigment composition showed that cells may be adapted for growth under a wide range of irradiances. Values of a_ph*(λ) were lower for N-depleted cells than for N-replete cells, and N-depleted cells had higher ratios of total carotenoids to chlorophyll a. Aggregation of chloroplasts was more pronounced in N-depleted cells. These are possibly photoprotective mechanisms that would be an advantage to N-depleted cells in surface waters. Compounds that absorb in the ultraviolet region were detected in N-replete cells but were absent in N-depleted cultures. Overall, these results have important implications for migrations of Rhizosolenia in nature. Cells may survive fairly long periods in N-depleted surface waters and will continue to take up carbon; then they can resume nitrate uptake and revert to positive buoyancy upon returning to deep, N-rich water. Uncoupled uptake of carbon and nitrogen during migrations of Rhizosolenia is a form of new production that may result in the net removal of carbon from oceanic surface waters.     

Photosynthetic and stomatal conductance responses to acid mist of red spruce seedlings

Abstract Two-year-old seedlings of Picea rubens, growing in open-top chambers in Scotland were treated twice weekly from July 1987 to December 1987, with mist containing ammonium sulphate and nitric acid at a pH of either 2.5 or 5.0. The response of photosynthesis and stomatal conductance to light flux density and carbon dioxide concentration were measured in March 1989. Leaf chlorophyll a and b contents were also measured. Acid mist (pH 2.5) resulted in several significant changes. First, both the rate of light saturated photosynthesis (A_max) and CO₂- saturated rate of photosynthesis (J) were substantially increased, when expressed per unit leaf area. Second, the apparent quantum yield and chlorophylls a and b content increased. Third, as a consequence of the greater chlorophyll content of the leaves treated with acid mist, the rate of A_max, and J, expressed per unit chlorophyll, was substantially reduced in pH 2.5 treated branches. Stomatal conductance was enhanced at all but the highest light flux densities, and was independent of the CO₂ concentration, remaining high for all values of CO₂ concentration used. These results show that acid mist caused a number of responses in the gas exchange and photosynthetic properties of red spruce.     

Differences in nitrate uptake of species from habitats rich or poor in nitrogen when grown at low nitrate concentrations,using a new growth technique

Summary A method is described for culturing plants at extremely low nutrient concentrations. Using a Braun infusion pump, a fixed amount of nitrate or ammonium was supplied continuously to plants growing in a culture vessel at a rate limiting the uptake of the plants. At a very low nitrogen concentration an equilibrium was established where uptake rate of the plants is equal to the rate of supply by the infusion pump. The nitrogen concentrations reached appeared to be in the order of 1 μM. The method compared the nitrate uptake byHypochaeris radicata L.ssp.radicata, H. radicata ssp.ericetorum Van Soest andUrtica dioica L. and ammonium uptake byH. radicata ssp.radicata andH. radicata ssp.ericetorum. Plants were cultivated in monocultures or in mixed cultures (two species per culture vessel). For the mixed cultures competition for nitrate (or ammonium) between the species was maintained for long periods. The capacities of the uptake systems of two subspecies ofH. radicata from places different in nitrogen supply and pH were adapted equally well to both low nitrate and low ammonium concentrations. Apparently factors other than nitrogen uptake play a part in the distribution of the subspecies. The capacity of the uptake system ofU. dioica, a nitrophilous species, was lower than that ofH. radicata ssp.radicata, a species from places poorer in nitrogen. This difference is related to the different distribution of the two species in the field. The present results are compared with those of previous experiments where K_m and V_max were measured and the significance of both parameters is discussed.     

LIGHT DEPENDENCE OF GROWTH AND PHOTOSYNTHESIS IN PHAEODACTYLUM TRICORNUTUM (BACILLARIOPHYCEAE)1

Phaeodactylum tricornutum Bohlin was maintained in exponential growth over a range of photon flux densities (PFD) from 7 to 230 μmol·m^?2s^?1. The chlorophyll a-specific light absorption coefficient, maximum quantum yield of photosynthesis, and C:N atom ratio were all independent of the PFD to which cells were acclimated. Carbon- and cell-specific, light-satuated, gross photosynthesis rates and dark respiration rates were largely independent of acclimation PFD. Decreases in the chlorophyll a-specific, gross photosynthesis rate and the carbon: chlorophyll ratio and increases of cell- or carbon-specific absorption coefficients were associated with an increase in cell chlorophyll a in cultures acclimated to low PFDs. The compensation PFD for growth was calculated to be 0.5 μmol·m^?2s^?1. The maintenance metabolic rate (2 × 10^?7s^?1), calculated on the basis of the compensation PFD, is an order of magnitude lower than the measured dark respiration rate(2.7 × 10^?6mol O₂·mol C^?1s^?1). Maintenance of high carbon-specific, light-saturated photosynthesis rates in cells acclimated to low PFDs may allow effective use of short exposures to high PFDs in a temporally variable light environment.     

古尔班通古特沙漠白梭梭枝干光合及其影响因素

植物枝干光合（P_g）固定其自身呼吸所释放的CO₂,有效减少植物向大气的CO₂排放量。以古尔班通古特沙漠优势木本植物白梭梭（Haloxylon persicum）为研究对象,利用LI-COR 6400便携式光合仪与特制光合叶室（P-Chamber）相结合,观测白梭梭叶片、不同径级枝干的光响应及光合日变化特征;同时监测环境因子（大气温湿度、光合有效辐射、土壤温度及含水量等）与叶片/枝干性状指标（叶绿素含量、含水量、干物质含量、碳/氮含量等）,揭示叶片/枝干光合的主要影响因子;采用破坏性取样,量化个体水平上叶片与枝干的总表面积,阐明枝干光合对植株个体碳平衡的贡献。研究结果显示：（1）白梭梭叶片叶绿素含量是枝干叶绿素含量的12-16倍,各径级枝干叶绿素含量差异不显著;（2）枝干光饱和点低于叶片,枝干不同径级（由粗至细）,暗呼吸速率和枝干光合逐渐减小;（3）光合有效辐射、土壤含水量和空气温湿度是影响叶片光合的主要因子,对枝干光合无显著影响;（4）枝干光合可以固定其自身呼吸产生CO₂的73%,最高可达90%,枝干光合固定CO₂约占个体水平固碳量的15.4%。研究结果表明,忽视枝干光合的贡献来预测未来气候变化背景下荒漠生态系统碳过程,可能存在根本性缺陷,并且在估算枝干呼吸时,需要考虑枝干是否存在光合作用,以提高枝干呼吸的准确性。     

NITROGEN ASSIMILATION OF AN OCEANIC DIATOM IN NITROGEN-LIMITED CONTINUOUS CULTURE1

The oceanic diatom Thalassiosira pseudonana Hasle and Heimdal (formerly Cyclotella nana) was grown with 12L:12D illumination cycles in nitrogen-limited continuous culture with a mixture of ammonium and nitrate as the N source. Measurements included, at 3 different growth rates (degrees of N limitation), cell concentration, cell carbon, nitrogen, and chlorophyll a contents, cell volume, photosynthetic carbon assimilation vs. irradiance, short-term uptake of ammonium and nitrate vs. their ambient concentrations, and in vitro activities of the assimilatory enzymes nitrate reductase and glutamic dehydrogenase. The various parameters showed either an increase (pattern a) or a decrease (pattern b) with increasing N limitation. Those following pattern a were nitrate reductase activity and the capacity to assimilate nitrate and ammonium. Those following pattern b were glutamic dehydrogenase activity, photosynthetic rate, nitrogen:carbon and chlorophyll a:carbon composition ratios. Results are discussed in terms of the interpretation such measurement for natural phytoplankton and effects of circadian periodicity.     

Photosynthetic properties of an orchid community in central Italy

Abstract

Chlorophyllous Mediterranean orchids share a habitat endangered by climate change and land use change. These orchids are characterized by two mechanisms of carbon assimilation, being autotrophic carbon fixation through photosynthesis supplemented by heterotrophic carbon fixation from mycorrhizal fungi. We investigated whether photosynthesis may sustain autotrophy of several species of orchids co-occurring in the same habitat (the understory of a chestnut forest in the Apennines range) along a vegetative season, and how photosynthesis responds to environmental parameters in the different species. Combined analysis of gas-exchange, chlorophyll fluorescence, optical properties, chlorophylls concentration, and Ribulose 1,5 bisphosphate carboxylase/oxygenase (Rubisco) activity were carried out to characterize the photosynthetic apparatus of the orchid species. Both in vivo and in vitro measurements indicated that in all orchids, in natural conditions and over the entire vegetative season (May to July), a detectable amount of carbon, typical of autotrophic shade leaves, is fixed. It is therefore suggested that these orchids are predominantly autotrophic. As an exception, however, Limodorum abortivum, a co-occurring orchid in the examined habitat, is unable to photosynthesize at rates compatible with autotrophy. At the low light intensity experienced in the understory habitat all orchids exhibited a similar quantum yield, but photosynthesis of Dactylorhiza saccifera and Cephalanthera longifolia was stimulated by light intensities higher than ambient, indicating that these species may better use sunflecks reaching the understory vegetation. Photosynthesis of all orchids, including Limodorum, positively responded to increasing CO₂ concentration and temperature. Whether this will lead to a larger photosynthetic carbon fixation because of present and future climate change needs to be assessed with long-term experiments also including the impacts of climate on mychorrizal activity and host plants.     

Positive effects of continuous low nitrate levels on growth and photosynthesis of Alexandrium tamarense (Gonyaulacales, Dinophyceae)

The growth and photosynthesis of Alexandrium tamarense (Lebour) Balech in different nutrient conditions were investigated. Low nitrate level (0.0882 mmol/L) resulted in the highest average growth rate from day 0 to day 10 (4.58 × 10² cells mL^?1 d^?1), but the lowest cell yield (5420 cells mL^?1) in three nitrate level cultures. High nitrate‐grown cells showed lower levels of chlorophyll a‐specific and cell‐specific light‐saturated photosynthetic rate (P_m^{chl a} and P_m^cell), dark respiration rate (R_d^chla and R_d^cell) and chlorophyll a‐specific apparent photosynthetic efficiency (α^chla) than was seen for low nitrate‐grown cells; whereas the cells became light saturated at higher irradiance at low nitrate condition. When cultures at low nitrate were supplemented with nitrate at 0.7938 mmol/L in late exponential growth phase, or with nitrate at 0.7938 mmol/L and phosphate at 0.072 mmol/L in stationary growth phase, the cell yield was drastically enhanced, a 7–9 times increase compared with non‐supplemented control culture, achieving 43 540 cells mL^?1 and 52 300 cells mL^?1, respectively; however, supplementation with nitrate in the stationary growth phase or with nitrate and phosphate in the late exponential growth phase increased the cell yield by no more than 2 times. The results suggested that continuous low level of nitrate with sufficient supply of phosphate may facilitate the growth of A. tamarense.     

Woody tissue photosynthesis and its contribution to trunk growth and bud development in young plants

Stem photosynthesis can contribute significantly to woody plant carbon balance, particularly in times when leaves are absent or in ‘open’ crowns with sufficient light penetration. We explored the significance of woody tissue (stem) photosynthesis for the carbon income in three California native plant species via measurements of chlorophyll concentrations, radial stem growth, bud biomass and stable carbon isotope composition of sugars in different plant organs. Young plants of Prunus ilicifolia, Umbellularia californica and Arctostaphylos manzanita were measured and subjected to manipulations at two levels: trunk light exclusion (100 and 50%) and complete defoliation. We found that long‐term light exclusion resulted in a reduction in chlorophyll concentration and radial growth, demonstrating that trunk assimilates contributed to trunk carbon income. In addition, bud biomass was lower in covered plants compared to uncovered plants. Excluding 100% of the ambient light from trunks on defoliated plants led to an enrichment in ¹³C of trunk phloem sugars. We attributed this effect to a reduction in photosynthetic carbon isotope discrimination against ¹³C that in turn resulted in an enrichment in ¹³C of bud sugars. Taken together our results reveal that stem photosynthesis contributes to the total carbon income of all species including the buds in defoliated plants.     

Characteristics of Nitrate Reduction in a Mutant of the Blue-Green Alga Agmenellum quadruplicatum

Characteristics of nitrate reduction in terms of nitrite production in an N-methyl-N′-nitro-N-nitrosoguanidine-induced mutant of the blue-green alga Agmenellum quadruplicatum are described. Following induction of nitrate reduction a linear rate of nitrite production proportional to cell concentration was observed. Rate of nitrite production and growth rate showed similar responses to pH, temperature, and light intensity. If required, only trace amounts of carbon dioxide were necessary for nitrite production. Atmospheres of oxygen or nitrogen inhibited production of nitrite. In addition, a low but constant rate of nitrite production was observed in the dark. Nitrite production by mutant AQ-6 was studied in terms of photosynthesis. As nitrite production proceeded, rate of photosynthesis declined. Ultraviolet irradiation and 3-(3,4-dichlorophenyl)-1, 1-dimethylurea poisoning did not prevent nitrite production. The action spectrum of nitrite production was chlorophyll a-like.     

Marine Plankton Algae Grown with Light-Dark Cycles.

The diatoms Ditylum brightwellii and Nitzschia turgidula were grown in semi-continuous culture under various combinations of light intensity, temperature and daylength (photoperiod). Growth was strongly limited by light intensities below 0.03 cal/em². min in both species. Above this intensity, light saturation of growth was rapidly approached in Nitzschia but only gradually so in Ditylum. The growth rate in continuous light was never significantly higher than with 16 hours of light plus 8 hours of dark. In Ditylum, continuous light above 0.03 cal/cm². min caused a strong inhibition of growth at all temperatures. The chlorophyll concentration in the cells was greater the shorter the photopceriod. In cultures synchronised by different combinations of light intensity and photoperiod, cell division generally took place in the light. Synchrony was best under short photoperiods of bright light. Time courses are shown for chlorophyll synthesis and photosynthesis in synchronised cultures.     

CHANGES IN CELL COMPOSITION AND LIPID METABOLISM MEDIATED BY SODIUM AND NITROGEN AVAILABILITY IN THE MARINE DIATOM PHAEODACTYLUM TRICORNUTUM (BACILLARIOPHYCEAE)1

The effects of nitrogen starvation in the presence or absence of sodium in the culture medium were monitored in batch cultures of the marine diatom Phaeodactylum tricornutum Bohlin. During nitrogen starvation in the presence of sodium, cell nitrogen and chlorophyll a decreased, mainly as a consequence of continued cell division. These decreases were accompanied by decreases in the rates of photosynthesis and respiration. There was no change in either cell volume or carbohydrate, but both carbon and lipid increased. During nitrogen starvation in the absence of sodium, cell division ceased. Cell nitrogen and chlorophyll a remained constant, and respiration did not decrease, but the changes in the photosynthetic rate and the lipid content per cell were similar to cultures that were nitrogen-starved in the presence of sodium. The carbon-to-nitrogen ratio increased in both cultures. Nitrogen, in the form of nitrate, and sodium were resupplied to cultures that had been preconditioned in nitrogen- and sodium-deficient medium for 5 d. Control cultures to which neither nitrate or sodium were added remained in a static state with respect to cell number, volume, and carbohydrate but showed slight increases in lipid. Cells in cultures to which 10 mM nitrate alone was added showed a similar response to cultures where no additions were made. Cells in cultures to which 50 mM sodium alone was added divided for 2 d, with concomitant small decreases in all measured constituents. Cell division resumed in cultures to which both sodium and nitrate were added. The lipid content fell dramatically in these cells and was correlated to metabolic oxidation via measured increases in the activity of the glyoxylate cycle enzyme, isocitrate lyase. We conclude that lipids are stored as a function of decreased growth rate and are metabolized to a small extent when cell division resumes. However, much higher rates of metabolism occur if cell division resumes in the presence of a nitrogen source.     

Effects of non-steady-state iron limitation on nitrogen assimilatory enzymes in the marine diatom thalassiosira weissflogii (BACILLARIOPHYCEAE)

Since the recognition of iron‐limited high nitrate (or nutrient) low chlorophyll (HNLC) regions of the ocean, low iron availability has been hypothesized to limit the assimilation of nitrate by diatoms. To determine the influence of non‐steady‐state iron availability on nitrogen assimilatory enzymes, cultures of Thalassiosira weissflogii (Grunow) Fryxell et Hasle were grown under iron‐limited and iron‐replete conditions using artificial seawater medium. Iron‐limited cultures suffered from decreased efficiency of PSII as indicated by the DCMU‐induced variable fluorescence signal (F_v/F_m). Under iron‐replete conditions, in vitro nitrate reductase (NR) activity was rate limiting to nitrogen assimilation and in vitro nitrite reductase (NiR) activity was 50‐fold higher. Under iron limitation, cultures excreted up to 100 fmol NO₂^?·cell^?1·d^?1 (about 10% of incorporated N) and NiR activities declined by 50‐fold while internal NO₂^? pools remained relatively constant. Activities of both NR and NiR remained in excess of nitrogen incorporation rates throughout iron‐limited growth. One possible explanation is that the supply of photosynthetically derived reductant to NiR may be responsible for the limitation of nitrogen assimilation at the NO₂^? reduction step. Urease activity showed no response to iron limitation. Carbon:nitrogen ratios were equivalent in both iron conditions, indicating that, relative to carbon, nitrogen was assimilated at similar rates whether iron was limiting growth or not. We hypothesize that, diatoms in HNLC regions are not deficient in their ability to assimilate nitrate when they are iron limited. Rather, it appears that diatoms are limited in their ability to process photons within the photosynthetic electron transport chain which results in nitrite reduction becoming the rate‐limiting step in nitrogenassimilation.     

Physiological and Biochemical Characteristics of Sugar Beet Plants Grown at an Increased Carbon Dioxide Concentration and at Various Nitrate Doses

Three-week-old sugar beet (Beta vulgaris L.) seedlings were grown for an additional four weeks under controlled conditions: in river sand watered with a modified Knop mixture containing one half-fold (0.5N), standard (1N), and or threefold (3N) nitrate amount, at the irradiance of 90 W/m² PAR, and at the carbon dioxide concentrations of 0.035% (1C treatment) or 0.07% (2C treatment). The increase in the carbon dioxide concentration and in the nitrogen dose resulted in an increase in the leaf area and the leaf and root dry weight per plant. With the increase in the nitrogen dose, morphological indices characterizing leaf growth increased more noticeably in 1C plants than in 2C plants. And vice versa, the effects of increased CO₂ concentration were reduced with the increase in the nitrogen dose. Roots responded to the changes in the CO₂ and nitrate concentrations otherwise than leaves. At a standard nitrate dose (1N), the contents of proteins and nonstructural carbohydrates (sucrose and starch) in leaves depended little on the CO₂ concentration. At a double CO₂ concentration, the content of chlorophyll somewhat decreased, and the net photosynthesis rate (P _n) calculated per leaf area unit increased. An increase in the nitrogen dose did not affect the leaf carbohydrate content of the 1C and 2C plants except the leaves of the 2C-3N plants, where the carbohydrate content decreased. In 1C and 2C plants, an increase in the nitrogen dose caused an increase in the protein and chlorophyll content. Specific P _n values somewhat decreased in 1C-0.5N plants and had hardly any dependence on the nitrate dose in the 2C plants. The carbohydrate content in roots did not depend on the CO₂ concentration, and the content was the highest at 0.5N. Characteristic nitrogen dose-independent acclimation of photosynthesis to an increased carbon dioxide concentration, which was postulated previously [1], was not observed in our experiments with sugar beet grown at doubled carbon dioxide concentration.