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1.
The relative influence of the photoperiod and of periodic ammonium pulses in entraining the cell division cycle in nitrogen-limited cyclostat cultures differs dramatically in Hymenomonas carterae Braarud and Fagerl, Amphidinium carteri Hulburt and Thalassiosira weissflogii Grun. We examined how each species processes an NH4+ pulse at various times during the cell cycle and the L/D cycle. Rates of NH4+ uptake and changes in cellular concentrations of NH4+, free amino acids, and protein were examined after the addition of an NH4+ pulse. Depletion of NH4+ from the medium occurred earlier when the pulse was given at the beginning of the light period than at the beginning of the dark period in H. carterae and A. carteri. Depletion took longer in the T. weissflogii cultures and the kinetics were similar during both stages of the photocycle in this species. Similarly, the temporal phasing and maximum pool sizes varied with timing of the NH4+ pulse in H. carterae and A. carteri but complete assimilation was relatively rapid. More persistent pools of NH4+ and free amino acids accumulated in T. weissflogii, and the patterns of assimilation varied little as a function of the timing of the pulse with respect to the photocycle. Although nitrogen metabolism occurred rapidly in nitrogen-limited H. carterae and A. carteri, the entrainment of the cell division cycle by the photoperiod resulted in a large degree of uncoupling between completion of nitrogen assimilation and cell division. It is hypothesized that the strong entrainment of the cell division cycle of T. weissflogii by NH4+ pulses results from a relatively slow rate of nitrogen metabolism. 相似文献
2.
We examined the energetic dependency of the biochemical and physiological responses of Thalassiosira pseudonana Hasle and Heimdal. Chaetoceros gracilis Schütt, Dunaliella tertiolecta Butcher, and Gymnodinium sanguineum Hirasaka to NH4+, NO3?, and urea by growing them at subsaturating and saturating photon flux (PF). At subsaturating PF, when energy was limiting, NO3? and NH4+ grown cells had similar growth rates and C and X quotas. Therefore, NO3? grown cells used up to 48% more energy than NH4+ grown cells to assimilate carbon and nitrogen. Based on our measurements of pigments, chlorophyll-a-specific in vivo absorption cross-section, and fluorescence-chlorophyll a?1, we suggest that NO3?, grown cells do not compensate for the greater energy requirements of NO3? reduction by trapping more light energy. At saturating PF, when energy is not limiting, the utilization of NO3?, compared to NH4+ resulted in lower growth rates and N quotas in Thalassiosira pseudonana and lower N quotas in Chaetoceros gracilis, suggesting enzymatic rather than energetic limitations to growth. The utilization of urea compared to Nh4+ resulted in lower growth rates in Chaetoceros gracilis and Gymnodinium sanguineum (saturating PF) and in lower N quotas in all species tested at both subsaturating and saturating PF. The high C:N ratios observed in all urea-grown species suggest that nitrogen assimilation may be limited by urea uptake or deamination and that symptoms of N limitation in microalgae may be induced by the nature of the N source in addition to the N supply rate. Our results provide new eridence that the maximum growth rates of microalgae may be limited by enzymatic processes associated with the assimilation of NO3?, or urea. 相似文献
3.
Nitrogen uptake rates were measured as a function of time following saturating additions (15 μMg-at N·?1) of 15N-labelid ammonium, urea, and nitrate to N-starved cultures of the picoflagellate Micromonas pusilla Butcher. Uptake rates were estimated from both the accumulation of 15N into the cells and the disappearance of nitrogen from the medium. Transient elevated (surge) uptake rates of NH4+ and urea were observed after enrichment. During the first 5 min the initial urea and NH4+ uptake rates were 2- and 4-fold greater than the maximum growth rate (μMmax)observed prior to No3? depletion in the cultures. The elevated urea uptake rates declined quickly to a relatively constant value, whereas the initial rates of NH4+ uptake declined rapidly but were followed by a subsequent increase prior to remaining roughly constant. Nitrate was not taken up as readily by N-starved M. pusilla as the reduced N forms. Although NO3+ uptake commenced immediately after enrichment (i.e. no lag period) the N-Specific rate over the next 6 h averaged half the μMmax observed during NO3? replete conditions. 相似文献
4.
COMPARISONS OF NITRATE UPTAKE, STORAGE, AND REDUCTION IN MARINE DIATOMS AND FLAGELLATES 总被引:1,自引:0,他引:1
Diatoms, but not flagellates, have been shown to increase rates of nitrogen release after a shift from a low growth irradiance to a much higher experimental irradiance. We compared NO3 ? uptake kinetics, internal inorganic nitrogen storage, and the temperature dependence of the NO3 ? reduction enzymes, nitrate (NR) and nitrite reductase (NiR), in nitrogen‐replete cultures of 3 diatoms (Chaetoceros sp., Skeletonema costatum, Thalassiosira weissflogii) and 3 flagellates (Dunaliella tertiolecta, Pavlova lutheri, Prorocentrum minimum) to provide insight into the differences in nitrogen release patterns observed between these species. At NO3 ? concentrations <40 μmol‐N·L ? 1, all the diatom species and the dinoflagellate P. minimum exhibited saturating kinetics, whereas the other flagellates, D. tertiolecta and P. lutheri, did not saturate, leading to very high estimated K s values. Above ~60 μmol‐N·L ? 1, NO3 ? uptake rates of all species tested continued to increase in a linear fashion. Rates of NO3 ? uptake at 40 μmol‐N·L ? 1, normalized to cellular nitrogen, carbon, cell number, and surface area, were generally greater for diatoms than flagellates. Diatoms stored significant amounts of NO3 ? internally, whereas the flagellate species stored significant amounts of NH4 + . Half‐saturation concentrations for NR and NiR were similar between all species, but diatoms had significantly lower temperature optima for NR and NiR than did the flagellates tested in most cases. Relative to calculated biosynthetic demands, diatoms were found to have greater NO3 ? uptake and NO3 ? reduction rates than flagellates. This enhanced capacity for NO3 ? uptake and reduction along with the lower optimum temperature for enzyme activity could explain differences in nitrogen release patterns between diatoms and flagellates after an increase in irradiance. 相似文献
5.
Differential effect of ammonium on the induction of nitrate and nitrite reductase activities in roots of barley (Hordeum vulgare) seedlings 总被引:2,自引:0,他引:2
Muhammad Aslam Robert L. Travis D. William Rains Ray C. Huffaker 《Physiologia plantarum》1997,101(3):612-619
The effect of exogenous NH4+ on the induction of nitrate reductase activity (NRA; EC 1.6.6.1) and nitrite reductase activity (NiRA; EC 1.7.7.1) in roots of 8-day-old intact barley (Hordeum vulgare L.) seedlings was studied. Enzyme activities were induced with 0.1, 1 or 10 mM NO3+ in the presence of 0, 1 or 10 mM NH4+, Exogenous NH4+ partially inhibited the induction of NRA when roots were exposed to 0.1 mM, but not to 1 or 10 mM NO3+, In contrast, the induction of NiRA was inhibited by NH4+ at all NO3+ levels. Maximum inhibition of the enzyme activities occurred at 1.0 mM NH4+ Pre-treatment with NH4+ had no effect on the subsequent induction of NRA in the absence of additional NH4+ whereas the induction of NiRA in NH4+-pretreated roots was inhibited in the absence of NH4+ At 10 mM NO3+ L-methionine sulfoximine stimulated the induction of NRA whether or not exogenous NH4+ was present. In contrast, the induction of NiRA was inhibited by L-methionine sulfoximine irrespective of NH4+ supply. During the postinduction phase, exogenous NH4+ decreased NRA in roots supplied with 0.1 mM but not with 1mM NH3+ whereas, NiRA was unaffected by NH4+ at either substrate concentration. The results indicate that exogenous NH4+ regulates the induction of NRA in roots by limiting the availability of NO3+. Conversely, it has a direct effect, independent of the availability of NO3+, on the induction of NiRA. The lack of an NH4+ effect on NiRA during the postinduction phase is apparently due to a slower turnover rate of that enzyme. 相似文献
6.
Ammonium and nitrate uptake rates in the macroalgae Ulva fenestrata (Postels and Ruprecht) (Chlorophyta) and Gracilaria pacifica (Abbott) (Rhodophyta) were determined by 15N accumulation in algal tissue and by disappearance of nutrient from the medium in long‐term (4–13 days) incubations. Nitrogen‐rich algae (total nitrogen> 4% dry weight [dw]) were used to detect isotope dilution by release of inorganic unlabeled N from algal thalli. Uptake of NH4 + was similar for the two macroalgae, and the highest rates were observed on the first day of incubation (45 μmol N·g dw ? 1·h ? 1 in U. fenestrata and 32 μmol N·g dw ? 1·h ? 1 in G. pacifica). A significant isotope dilution (from 10 to 7.9 atom % enrichment) occurred in U. fenestrata cultures during the first day, corresponding to a NH4 + release rate of 11 μmol N·g dw ? 1·h ? 1. Little isotope dilution occurred in the other algal cultures. Concurrently to net NH4 + uptake, we observed a transient free amino acid (FAA) release on the first day in both macroalgal cultures. The uptake rates estimated by NH4 + disappearance and 15N incorporation in algal tissue compare well (82% agreement, defined as the percentage ratio of the lower to the higher rate) at high NH4 + concentrations, provided that isotope dilution is taken into account. On average, 96% of added 15NH4 + was recovered from the medium and algal tissue at the end of the incubation. Negligible uptake of NO3 ? was observed during the first 2–3 days in both macroalgae. The lag of uptake may have resulted from the need for either some N deprivation (use of NO3 ? pools) or physiological/metabolic changes required before the uptake of NO3 ? . During the subsequent days, NO3 ? uptake rates were similar for the two macroalgae but much lower than NH4 + uptake rates (1.97–3.19 μmol N·g dw ? 1·h ? 1). Very little isotope dilution and FAA release were observed. The agreement between rates calculated with the two different methods averaged 91% in U. fenestrata and 95% in G. pacifica. Recovery of added 15NO3 ? was virtually complete (99%). These tracer incubations show that isotope dilution can be significant in NH4 + uptake experiments conducted with N‐rich macroalgae and that determination of 15N atom % enrichment of the dissolved NH4 + is recommended to avoid poor isotope recovery and underestimation of uptake rates. 相似文献
7.
James E. Cloern 《Journal of phycology》1977,13(4):389-395
Specific growth rate of Cryptomonas ovata var. palustris Pringsheim was measured in batch culture at 14 light-temperature combinations. Both the maximum growth rate (μm) and optimum light intensity (Iopt) fit an empirical function that increases exponentially with temperature up to an optimum (Topt), then declines rapidly as temperature exceeds Topt. Incorporation of these functions into Steele's growth equation gives a good estimate of specific growth rate over a wide range of temperature and light intensity. Rates of phosphate, ammonium and nitrate uptake were measured separately at 16 combinations of irradiance and temperature and following a spike addition of all starved cells initially took up nutrient at a rapid rate. This transitory surge was followed by a period of steady, substrate-saturated uptake that persisted until external nutrient concentration fell. Substrate-saturated NO3?-uptake proceeded at very slow rates in the dark and was stimulated by both increased temperature and irradiance; NH4+-uptake apparently proceeded at a basal rate at 8 and l4 C and was also stimulated by increased temperature and irradiance. Rates of NH4?-uptake were much higher than NO3?-uptake at all light-temperature combinations. Below 20 C, PO4?3-uptake was more rapid in dark than in light, but was light enhanced at 26 C. 相似文献
8.
A planktonic alga similar in general morphology and pigments to Aureococcus anophagefferens Hargraves and Sieburth has caused persistent and ecologically damaging blooms along the south Texas coast. Experiments using 100 μM NO3?, NO2?, and NH4+ demonstrated that the alga could not use NO3? for growth but could use NO2? and NH4+. Doubling iron or trace metal concentrations did not permit growth on NO3?. Chemical composition data for cultures grown in excess NO3? or NH4+, respectively, were as follows: N·cell?1 (0.88 vs. 1.3 pg), C:N ratio (25:1 vs. 6.4:1), C:chlorophyll a (chl a) (560:1 vs. 44:1), and chl a·cell?1 (0.033 vs. 0.16 pg). These data imply that cells supplied with NO3? were N-starved. Culture addition of 10 mM final concentration chlorate (a nitrate analog) did not affect the Texas isolate while NO3? utilizing A. anophagefferens was lysed, suggesting that the NO3? reductase of the Texas isolate is nonfunctional. Rates of primary productivity determined during a dense bloom indicated that light-saturated growth rates were ca. 0.45 d?1, which is similar to maximum rates determined in laboratory experiments (0.58 d?1± 0.16). However, chemical composition data were consistent with the growth rate of these cells being limited by N availability (C:N 28, C:chl a 176, chl a·cell?1 0.019). Calculations based on a mass balance for nitrogen suggest that the bloom was triggered by an input of ca. 69 μM NH4+ that resulted from an extensive die-off of benthos and fish. 相似文献
9.
A. Henry Latorella Susan K. Bromberg Kent Lieber James Robinson 《Journal of phycology》1981,17(2):211-214
Fifteen nitrate assimilation-deficient mutants of the euryhaline green alga, Dunaliella tertiolecta Butcher were selected by their chlorate resistance. Ten mutants, unable to grow on NO3? but able to grow on NO2?, had no detectable nitrate reductase activity. Five mutants, unable to grow on either NO3? or NO2?, had depressed levels of both nitrate and nitrite reductase. A method for assaying methyl viologen-nitrate reductase in the presence of nitrite reductase is described. 相似文献
10.
Ammonium uptake in Lemna gibba G 1, related membrane potential changes, and inhibition of anion uptake 总被引:6,自引:0,他引:6
Wolfram R. Ullrich Marie Larsson Carl-Magnus Larsson Sigrid Lesch Anton Novacky 《Physiologia plantarum》1984,61(3):369-376
In N-starved (?N) fronds of Lemna gibba L. G 1, NH4+ uptake rates were several-fold those of NO3?-supplied (+N) fronds. NO3?, uptake in +N-plants was slow and not inhibited by addition of NH4+. However, in ?N-plants with higher NO3? and still higher NH4+ uptake rates, addition of NH4+ immediately reduced the NO3? uptake rates to about one third until the NH4+ was consumed. The membrane potential (Em) decreased immediately upon addition of NH4+ in all fronds, but whereas depolarisation was moderate and transient in +N-plants, it was strong, up to 150 mV, in N-starved plants, where Em remained at the level of the K+ diffusion potential (ED) until NH4+ was removed. In N-starved plants NH4+ uptake and membrane depolarisation showed the same concentration dependence, except for an apparent linear component for uptake. Phosphate uptake was inhibited by NH4+ similarly to NO3? uptake, but only in P- and N-starved plants, not after mere P starvation. Influx of NO3? and H2PO 4? into the negatively charged cells of Lemna is mediated by anion/H+ cotransport, but NH4+ influx can follow the electrochemical gradient. Its saturating component may reflect a carrier-mediated NH4+ uniport, the linear component diffusion of NH4+ or NH3. Inhibition of anion/H+ cotransport by high NH4+ influx rates may be due to loss of the proton-driving force, Δμ?H+, across the plasmalemma. Reversible inhibition by NH4+ of the H+ extrusion pump may contribute to the finding that Δμ?H+ cannot be reconstituted in the presence of higher NH4+ concentrations. 相似文献
11.
Ammonium can stimulate nitrate and nitrite reductase in the absence of nitrate in Clematis vitalba 总被引:3,自引:1,他引:3
R. A. Bungard A. Wingler J. D. Morton M. Andrews M. C. Press & J. D. Scholes 《Plant, cell & environment》1999,22(7):859-866
Nitrogen assimilation was studied in the deciduous, perennial climber Clematis vitalba. When solely supplied with NO3– in a hydroponic system, growth and N-assimilation characteristics were similar to those reported for a range of other species. When solely supplied with NH4+, however, nitrate reductase (NR) activity dramatically increased in shoot tissue, and particularly leaf tissue, to up to three times the maximum level achieved in NO3– supplied plants. NO3– was not detected in plant material that had been solely supplied with NH4+, there was no NO3– contamination of the hydroponic system, and the NH4+-induced activity did not occur in tobacco or barley grown under similar conditions. Western Blot analysis revealed that the induction of NR activity, either by NO3– or NH4+, was matched by NR and nitrite reductase protein synthesis, but this was not the case for the ammonium assimilation enzyme glutamine synthetase. Exposure of leaf disks to N revealed that NO3– assimilation was induced in leaves directly by NO3– and NH4+ but not glutamine. Our results suggest that the NH4+-induced potential for NO3– assimilation occurs when externally sourced NH4+ is assimilated in the absence of any NO3– assimilation. These data show that the potential for nitrate assimilation in C. vitalba is induced by a nitrogenous compound in the absence of its substrate and suggest that NO3– assimilation in C. vitalba may have a significant role beyond the supply of reduced N for growth. 相似文献
12.
Radish (Raphanus sativus L.) seedlings pretreated with different hormones viz. kinetin, gibberellic acid and abscisic acid were subjected to different
N-forms. The seedlings were treated with different concentrations of KNO3, NH4Cl and NH4NO3 and the changes in nitrate reductase activity were seen in light and dark conditions in the cotyledons. Nitrate reductase
activity was affected differently by hormone application. Nitrate increased and ammonia decreased nitrate reductase activity;
in both light and dark-grown seedlings KNO3 induced more in vitro nitrate reductase activity. NH
4
+
when combined with NO
3
−
, however, could level up to some extent, with KNO3 in light, except in kinetin. A transient response of induction of NR activity was evident with decreased levels after a certain
specific ambient N-concentration, despite the presence of high N in the medium. However, the pattern of transition varied
with the hormones applied. Further, hormones are found to affect induction of different isoforms of nitrate reductase by NH
4
+
and NO
3
−
. NH
4
+
induced isoform was prominently promoted by kinetin treatment in dark. The data documents a particular kind of interaction
between controlling factors (light, N-source and phytohormones) which affect nitrate reductase levels. 相似文献
13.
NH4+ and NO3? uptake were measured by continuous sampling with an autoanalyzer. For Hypnea musciformis (Wulfen) Lamouroux, NO3?up take followed saturable kinetics (K2=4.9 μg-at N t?1, Vmax= 2.85 μg- at N, g(wet)?1. h?1. The ammonium uptake data fit a trucatd hyperbola, i.e., saturation was not reach at the concentrations used. NO3? uptake was reduced one-half in the presence of NH4+, but presence of NO3? had no effect on NH4+ uptake. Darkness reduced both NO3? and NH4+ uptake by one-third to one-half. For Macrocystis pyrufera (L) C. Agardh, NO3? uptake followed saturable kinetices: K2=13.1 μg-at N. l?1. Vmax=3.05 μg-at N. g(wet)?1. h?1.NH4+ uptake showed saturable kinetics at concentration below 22 μg-at N l -1 (K2=5.3 μg-at N.1–1, Vmax= 2.38 μg-at N G (wet)?1.h?1: at higher concentration uptake increased lincarly with concentrations. NO3?and NH4+ were taken up simulataneously: presence of one form did not affect uptake of the other. 相似文献
14.
Louise T. Kregting Catriona L. Hurd Conrad A. Pilditch Craig L. Stevens 《Journal of phycology》2008,44(2):320-330
The influence of seawater velocity (1.5–12 cm · s?1) on inorganic nitrogen (N) uptake by the soft‐sediment perennial macroalga Adamsiella chauvinii (Harv.) L. E. Phillips et W. A. Nelson (Rhodophyta) was determined seasonally by measuring uptake rate in a laboratory flume. Regardless of N tissue content, water velocity had no influence on NO3? uptake in either winter or summer, indicating that NO3?‐uptake rate was biologically limited. However, when thalli were N limited, increasing water velocity increased NH4+ uptake, suggesting that mass‐transfer limitation of NH4+ is likely during summer for natural populations. Uptake kinetics (Vmax, Ks) were similar among three populations of A. chauvinii at sites with different mean flow speeds; however, uptake rates of NO3? and NH4+ were lower in summer (when N status was generally low) than in winter. Our results highlight how N uptake can be affected by seasonal changes in the physiology of a macroalga and that further investigation of N uptake of different macroalgae (red, brown, and green) during different seasons is important in determining the relative influence of water velocity on nutrient uptake. 相似文献
15.
Mobilization of the reserve β-1,3-glucan (chrysolaminaran) in N-limited cells of the marine diatom Skeletonema costatum (Grev.) Cleve (Bacillariophyceae) was investigated. The diatom was grown in pH-regulated batch cultures with a 14:10-h light:dark cycle until N depletion. In a pulse-chase experiment, the cells were first incubated in high light (200 μmol photons·m − 2·s − 1) with 14C-bicarbonate until dissolved inorganic carbon was exhausted. Unlabeled bicarbonate (1 mM) was then added, and the cells were incubated in the dark and subsequently in low light (20 μmol photons·m − 2·s − 1) with additions of 40 μM NH4 + . In the 14C pulse phase with high light and N depletion, β-1,3-glucan accumulated and accounted for 85% of incorporated 14C. In the subsequent 14C chase phases, added NH4 + was assimilated at an N-specific rate of 0.11 h − 1 in both the dark and low light, and in both cases it caused a significant mobilization of β-1,3-glucan (dark, 26%; low light, 19%). Biochemical fractionation of organic 14C showed that free amino acids were most rapidly labeled in the early stage of NH4 + assimilation, whereas proteins and polysaccharides were labeled more rapidly after 1.2 h. Analysis of the cellular free amino acids strongly indicated that de novo biosynthesis was occurring, with a Gln:Glu ratio increasing from 0.4 to 10 within 1.2 h. After the NH4 + was exhausted, the cellular pools of glucan and amino acids became constant or slowly decreased. In another experiment, N-limited cells were first incubated in high light until dissolved inorganic carbon was exhausted and were further incubated in high light with 150 μM NH4 + under inorganic carbon limitation. Added NH4 + was assimilated at an N-specific rate of 0.023 h − 1, and cellular β-1,3-glucan decreased by 15% within 6 h. Hence, β-1,3-glucan was mobilized during NH4 + assimilation, even though inorganic carbon was modifying the metabolic rates. The results provide new evidence of β-1,3-glucan supplying essential precursors for biosynthesis of amino acids and other components in S. costatum in both the dark and subsaturating light and even saturating light under inorganic carbon limitation. 相似文献
16.
Miriam Hassidim Nir Keren Itzhak Ohad Leonora Reinhold Aaron Kaplan 《Journal of phycology》1997,33(5):811-817
A CO2 concentrating mechanism has been identified in the phycoerythrin-possessing Synechococcus sp. WH7803 and has been observed to be severely inhibited by short exposure to elevated light intensities. A light treatment of 300–2000 μmol quanta·m?2·s?1 resulted in a considerable decay in the variable fluorescence of PSII with time, suggesting decreased efficiency of energy transfer from the phycobilisomes, direct damage to the reaction center II, or both. Measurements of the activity of PSII and changes in fluorescence emission spectra during a light treatment of 1000 μmol quanta·m?2·s?1 indicated considerable reduction in the energy flow from the phycocyanin to the phycobilisome terminal acceptor and chlorophyll a. Consequently, whereas the maximal photosynthetic rate, at saturating light and Co2 concentration, was hardly affected by a light treatment of 1000 μmol quanta·m?2·s?1 for 2 h, the light intensity required to reach that maximum increased with the duration of the light treatment. 相似文献
17.
The green algaDunaliella salina UTEX 200 was cultured at high (5 percnt;) [CO2] in a medium containing 10 mmol/L of either NO3− or NH4+ as the sole N source. Specific growth rate was 50 percnt; higher for NH4+-grown cells than for their counterparts cultured in the presence of NO3−. Cell size, protein content, Rubisco protein, phosphoenolpyruvate carboxylase (PEPC) activity, and light independent carbon fixation were enhanced by growth in the presence of NH4+. On the other hand, maximal photosynthetic rate and cell glycerol concentration were lower when N was supplied as NH4+. The activity of glutamine synthetase was affected very little by the N-source.D. salina UTEX 200 showed some peculiarities in its mechanism of adaptation to high [N] in comparison to other strains previously used for similar studies. This allowed dissection of the underlying mechanism of the growth response to high [N], highlighting the potential role of PEPC, the main anaplerotic enzyme, as a pivotal player in the adaptation of cells to these conditions. 相似文献
18.
When NH4 + or NO3 ? was supplied to NO3 ? ‐stressed cells of the microalga Dunaliella tertiolecta Butcher, immediate transient changes in chl a fluorescence were observed over several minutes that were not seen in N‐replete cells. These changes were predominantly due to nonphotochemical fluorescence quenching. Fluorescence changes were accompanied by changes in photosynthetic oxygen evolution, indicating interactions between photosynthesis and N assimilation. The magnitude of the fluorescence change showed a Michaelis‐Menten relationship with half‐saturation concentration of 0.5 μM for NO3 ? and 10 μM for NH4 + . Changes in fluorescence responses were characterized in D. tertiolecta both over 5 days of N starvation and in cells cultured at a range of NO3 ? ‐limited growth rates. Variation in responses was more marked in starved than in limited cells. During N starvation, the timing and onset of the fluorescence responses were different for NO3 ? versus NH4 + and were correlated with changes in maximum N uptake rate during N starvation. In severely N‐starved cells, the major fluorescence response to NO3 ? disappeared, whereas the response to NH4 + persisted. N‐starved cells previously grown with NH4 + alone showed fluorescence responses with NH4 + but not NO3 ? additions. The distinct responses to NO3 ? and NH4 + may be due to the differences between regulation of the uptake mechanisms for the two N sources during N starvation. This method offers potential for assessing the importance of NO3 ? or NH4 + as an N source to phytoplankton populations and as a diagnostic tool for N limitation. 相似文献
19.
20.
Two strains of Dunaliella salina (Dunal) Teod., UTEX 1644 and UTEX 200, were cultured under different growth regimes, including 10 mM NO3? or NH4+, 1.5 or 3.0 M NaCl, and low (0.035%) or high (5%) CO2 in air. The release of 14C-labeled dissolved organic carbon (DOC), expressed as a rate and as a percentage of photosynthetic 14CO2 assimilation, was subsequently determined. The percentage of DOC released was inversely related to cell density in the assay medium, but photosynthesis on a per-cell basis was not. Release of DOC was low, in the range of 1–5% of photosynthesis, but during acclimation to growth on NH4+, it rose to 11%. The presence of NH4+ rather than NO3? in the growth medium increased the rate of release by both strains, but the percentage release was stimulated only in UTEX 200 cells, because their photosynthetic rate was depressed by NH4+. For UTEX 1644, high, as compared to low, CO2-grown cells, had somewhat higher rates and percentages of DOC release, but release from UTEX 200 cells was unaffected by the growth-CO2. The rate of DOC release by high CO2-grown cells was not enhanced at a low concentration of dissolved inorganic carbon, indicating that the released material did not originate from the photorespiratory pathway. The effects of NaCl on DOC release varied with strain and growth conditions. For UTEX 200, the cells in NO3?, but not NH4+, exhibited a doubling or more in percentage of release with a doubling in NaCl concentration, irrespective of growth-CO2. With UTEX 1644 the low CO2-grown cells showed the greatest enhancement in 3.0 M NaCl. Organic matter accumulated on the external surface of the cell membrane and constituted a well-defined cell-coat, which was more dense in NH4+ than in NO3?-grown cells. Microtubules, which may play a role in maintaining cell shape, were observed just below the plasma membrane. From a practical viewpoint, the presence of organic material in the hypersaline ponds of salt-works is detrimental to salt production. When D. salina cells become abundant in such ponds, the attendant, continuous release of DOC may make a significant contribution to the problem. 相似文献