CHANGES IN INTRACELLULAR NITROGEN POOLS AND FEEDBACK CONTROLS ON NITROGEN UPTAKE IN CHAETOMORPHA LINUM (CHLOROPHYTA)1

Changes in the size of intracellular nitrogen pools and the potential feedback by these pools on maximum N uptake (NH₄⁺ and NO₃^?) rates were determined for Chaetomorpha linum (Müller) Kützing grown sequentially under nutrient-saturating and nutrient-limiting conditions. The size of individual pools in N-sufficient algae could be ranked as residual organic N (RON) comprised mainly of amino acids and amino compounds > protein N > NO₃^? > NH₄⁺ > chlorophyll N. When the external N supply was removed, growth rates remained high and individual N pools were depleted at exponential rates that reflected both dilution of existing pools by the addition of new biomass from growth and movement between the pools. Calculated fluxes between the tissue N pools showed that the protein pool increased throughout the N depletion period and thus did not serve a storage function. RON was the largest storage reserve; nitrate was the second largest, but more temporary, storage pool that was depleted within 10 days. Upon N resupply, the RON pool increased 3 × faster than either the inorganic or protein pools, suggesting that protein synthesis was the rate-limiting step in N assimilation and caused a buildup of intermediate storage compounds. Maximum uptake rates for both NH₄⁺ and NO₃^? varied inversely with macroalgal N status and appeared to be controlled by changes in small intracellular N pools. Uptake of NO₃^? showed an initial lag phase, but the initial uptake of NH₄⁺ was enhanced and was present only when the intracellular NH₄⁺ pool was depleted in the absence of an external N supply. A strong negative correlation between the RON pool size and maximum assimilation uptake rates for both NH₄⁺ and NO₃^? suggested a feedback control on assimilation uptake by the buildup and depletion of organic compounds. Enhanced uptake and the accumulation of N as simple organic compounds or nitrate both provide a temporary mechanism to buffer against the asynchrony of N supply and demand in C. linum.     

NITROGEN UPTAKE BY FUCUS SPIRALIS (PHAEOPHYCEAE)1,2

Ammoniun, nitrate and nitrite update by Fucus spiralis L. from the Massachusetts coast was examined. Uptake of all appeared to follow saturation type nutrient uptake kinetics, with uptake often restricted at ambient nutrient concentrations. Although only relatively large difference in K₈ values could be easily distinguished, K₈ values for NO_3? and NH₄₊ were generally similar and low compared with NO_2?. There was also some suggestion that K₈ was reduced at lower temperatures. At 15 C. V_max for light and dark uptake for both NH₄+ and NO_3?, and light uptake of N0_2? were similar, suggesting comparable potential use at higher concentrations. Ammonium and NO_3?uptake decreased at lower temperatures giving Q_ro values of 1.8 and 1.6, respectively, between 5 and 15°C. Nitrate and NH₄+ were taken up together and high levels of NH₄+ did not inhibit NO_3? uptake. Light did not affect uptake of either but did stimulate NO_2? uptake. Ammonium and NO_3? uptake were highest in apical frond and whole young plants, and lowest in slower growing, older frond and stipe. On a relative basis. NO_3?, NH₄₊ and NO_2? were estimated to have contributed ca. 59, 39 and 2% respectively, to the yearly N uptake by apical frond. During winter, NO_3? would provide ca. twice the N to F. spiralis as would, NH₄+. From summer to early fall, when NO_3? levels are lower, NO_3? and NH₄+ would be used in comparable amounts.     

UPTAKE AND RELEASE OF NITROGEN BY THE MACROALGAE GRACILARIA VERMICULOPHYLLA (RHODOPHYTA)1

Macroalgae, often the dominant primary producers in shallow estuaries, can be important regulators of nitrogen (N) cycling. Like phytoplankton, actively growing macroalgae release N to the water column; yet little is known about the quantity or nature of this release. Using ¹⁵N labeling in laboratory and field experiments, we estimated the quantity of N released relative to assimilation and gross uptake by Gracilaria vermiculophylla (Ohmi) Papenfuss (Rhodophyta, Gracilariales), a non‐native macroalgae. Field experiments were carried out in Hog Island Bay, a shallow back‐barrier lagoon on the Virginia coast where G. vermiculophylla makes up 85%–90% of the biomass. There was good agreement between laboratory and field measurements of N uptake and release. Daily N assimilation in field experiments (32.3±7.2 μ mol N·g dw^?1·d^?1) was correlated with seasonal and local N availability. The average rate of N release across all sites and dates (65.8±11.6 μ mol N·g dw^?1·d^?1) was 67% of gross daily uptake, and also varied among sites and seasons (range=33%–99%). Release was highest when growth rates and nutrient availability were low, possibly due to senescence during these periods. During summer biomass peaks, estimated N release from macroalgal mats was as high as 17 mmol N·m^?2·d^?1. Our results suggest that most estimates of macroalgal N uptake severely underestimate gross N uptake and that N is taken up, transformed, and released to the water column on short time scales (minutes–hours).     

INTERSPECIFIC VARIATION IN DARK NITROGEN UPTAKE BY DINOFLAGELLATES1

Seven species of marine dinoflagellates were grown in nitrogen-sufficient media under a 12:12 h L:D cycle, and then tested for their ability to take up nitrate and ammonium in the light and in the dark in short-term experiments with ¹⁵N-labelled substrate. The effect of the N substrate chosen, and the effect of sampling time in the L:D cycle, on the relative nitrogen content (the C:N ratio) was investigated at the same time. The physiological extremes in the material were represented by Prorocentrum minimum (Pav.) J. Schiller, which took up and presumably assimilated nitrate equally fast in the light and in the dark, and Gyrodinium aureolum Hulburt, which did not take up nitrate in the dark when in a state of nitrogen sufficiency. A strong coupling between nitrate assimilation and photosynthetic carbon assimilation in the latter species was suggested by the close similarity of the light saturation curves of ¹⁵NO₃^? and ¹⁴CO₂ incorporation, and by a complete blocking of ¹⁵NO₃^? incorporation by 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU). Nitrogen starvation for 24 h induced a capacity in G. aureolum for taking up nitrate in the dark, or in the light in the presence of DCMU, a phenomenon that might be useful for identifying nitrogen limitation in this species in the field. Our study emphasizes the variability of dinoflagellate nitrogen nutrition and illustrates the difficulty of associating mass occurrences of dinoflagellates in nature with any particular nutritional mode.     

UPTAKE OF INORGANIC NITROGEN BY CODIUM FRAGILE SUBSP. TOMENTOSOIDES (CHLOROPHYTA)1

The uptake of nitrate, nitrite and ammonium by Codium fragile subsp. tomentosoides (van Goor) Silva was measured at different combinations of temperature (6–30 C) and irradiance (0–140 μEin.m-^2. s^-1). Uptake of all three forms of N was greater at 12–24 C than at 6 and 30 C. Although uptake was stimulated by light, saturation occurred at relatively low irradiance (7–28 μEin m^-2 s^-1, depending on the N source and temperature). The Michaelis-Menten uptake constants (V_max K)varied with temperature. V_max was greatest at intermediate temperatures and K was lowest at lower temperatures. The V_maxfor NH₄⁺ was higher and the K, for NH₄⁺was lower than those for NO₃^-_- and NO₂^-_-. Codium was capable of simultaneously taking up all three forms of inorganic N although the presence of NH₄⁺ reduced the uptake of both NO₃^-_- and NO₂^-_-. The results of this study indicate that part of the ecological success of Codium in a N-limited environment may be due to its N uptake capabilities.     

AMMONIUN AND NITRATE UPTAKE BY THE MARINE MACROPHYTES HYPNEA MUSVUFORMIS (RHODOPHYTA) AND MACROCYSTIS PYRIFERA (PHAEOPHYTA)1, 2

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, V_max= 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. V_max=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, V_max= 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.     

INORGANIC NITROGEN AND PHOSPHORUS UPTAKE KINETICS IN PALMARIA PALMATA (RHODOPHYTA)1

The N and P uptake responses were studied in a northern Spanish population of the edible red seaweed Palmaria palmata (Linnaeus) Kuntze. The fronds were incubated at different concentrations, and the nutrient depletion in the medium was measured at successive times to calculate uptake rates. Palmaria palmata uptake response was biphasic and nonsaturable for inorganic P. This would allow the species to exploit transient pulses of high P concentration in natural and fertilized conditions. Such a response is a common feature of algae avoiding nutrient deficiency. At average concentrations measured in the ocean, the response was nonsaturable for inorganic N sources, except for ammonium in autumn and winter when it is not the major N source. In contrast to the general rule of ammonium being taken at a higher rate than nitrate, we found similar affinity for both nutrients corresponding to the minor role of ammonium as N source for field populations over the year.     

OLISTHODISCUS LUTEUS (CHRYSOPHYCEAE). III. UPTAKE AND UTILIZATION OF NITROGEN AND PHOSPHORUS1

Uptake and assimilation of nitrogen and phosphorus were studied in Olisthodiscus luteus Carter. A diel periodicity in nitrate reductase activity was observed in log and stationary phase cultures; there was a 10-fold difference in magnitude between maximum and minimum rates, but other cellular features such as chlorophyll a, carbon, nitrogen, C:N ratio (atoms) · cell^?1 were less variable. K_s values (~2 μM) for uptake of nitrate-N and ammonium-N were observed. Phosphorus assimilated · cell^?1· day^?1 varied with declining external phosphorus concentrations; growth rates <0.5 divisions · day^?1 were common at <0.5 μM PO_4-P. Phosphate uptake rates (K_s= 1.0–1.98 μM) varied with culture age and showed multiphasic kinetic features. Alkaline phosphatase activity was not detected. Comparisons of the nutrient dynamics of O. luteus to other phytoplankton species and the ecological implications as related to the phytoplankton community of Narragansett Bay (Rhode Island) are discussed.     

15N MEASUREMENTS OF AMMONIUM AND NITRATE UPTAKE BY ULVA FENESTRATA (CHLOROPHYTA) AND GRACILARIA PACIFICA (RHODOPHYTA): COMPARISON OF NET NUTRIENT DISAPPEARANCE,RELEASE OF AMMONIUM AND NITRATE,AND 15N ACCUMULATION IN ALGAL TISSUE 1

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.     

TRANSIENT AMMONIUM UPTAKE IN THE MACROALGA ULVA LACTUCA (CHLOROPHYTA): NATURE,REGULATION, AND THE CONSEQUENCES FOR CHOICE OF MEASURING TECHNIQUE1

The nature of transient ammonium uptake by the macroalga Ulva lactuca L. was studied from the depletion of ammonium after single additions of ammonium to batch cultures. The experiments were carried out by the application of two different experimental setups: the “multiple flask” and the “perturbation” techniques. Uptake rate was nonlinear with time, and three distinct, succeeding phases of uptake were identified: 1) “surge” uptake, i.e. transiently enhanced uptake that lasted for a few hours only, followed by 2) “internally” controlled uptake, a relatively constant phase occurring at high substrate concentrations, and finally 3) the “externally” controlled uptake phase, which was substrate-dependent and occurred at low substrate concentrations. Surge uptake occurred over a broad range of substrate concentrations but was concentration-dependent and, so, equalled externally controlled uptake rates at substrate concentrations below 3–10 μM. The transient nature of ammonium uptake rate seemed related to rapid changes in small intracellular pools of inorganic nitrogen or amino acids rather than to changes in total N content of the algae. The transient nature of ammonium uptake has important implications for the measurement of uptake rates when either of the two standard methods, the multiple flask and the perturbation technique, are used, and I recommend that a combination of the two methods be used for future uptake experiments.     

METABOLIC REGULATION OF AMMONIUM UPTAKE BY ULVA RIGIDA (CHLOROPHYTA): A COMPARTMENTAL ANALYSIS OF THE RATE-LIMITING STEP FOR UPTAKE1

Non-linear time courses of ammonium (NH₄⁺) depletion from the medium and internal accumulation of soluble nitrogen (N) in macroalgae imply that the rate-limiting step for ammonium uptake changes over time. We tested this hypothesis by measuring the time course of N accumulation in N-limited Ulva rigida C. Agardh. Total uptake was measured as removal of NH₄⁺ from medium. Rates for the component processes (transport of NH₄⁺ across the membrane = R_v assimilation of tissure NH₄⁺ into soluble N compounds = R_a, assimilation of tissue NH₄⁺ into soluble N compounds = R_a and incorporation of soluble N compounds into macromolecules = R₁) were determined by measuring the rate of labelling of the major tissue N pools after the addition of ¹⁵N-ammonium. The results indicate that nitrogen-specific rates (mass N taken up / mass N present / unit time) are ranked in the order of R_t < R_a < R₁ Absolute uptake rates (μmol N. mg dry wt^?1. h^?1) showed a different relationship. Membrane transport appears to be inhibited when NH₄⁺ accumulates in the tissue. Maximum uptake rates occur when assimilation of NH₄⁺ into soluble N compounds begins. Assimilation of NH₄⁺ into soluble N compounds was initially faster than incorporation of soluble N compounds into macromolecules. Implications of rate limitations caused by differences in maximal rates and maximal pool sizes are discussed.     

不同抗旱性冬小麦品种抽穗期根系垂直分布及单根活力的研究

应用氚水示踪技术测定了不同抗旱性小麦品种抽穗期垂直分布的单根吸水进度,并应用常规方法对地下部根系垂直分布和地上部生物产量进行了研究,发现抗旱品种的相对根活力较高,根估算吸水量较低,并且每株总根量较少,深层根的吸水活力也不抗旱品种贞一个数量级。     

METABOLIC AND ECOLOGICAL CONSTRAINTS IMPOSED BY SIMILAR RATES OF AMMONIUM AND NITRATE UPTAKE PER UNIT SURFACE AREA AT LOW SUBSTRATE CONCENTRATIONS IN MARINE PHYTOPLANKTON AND MACROALGAE1

Marine phytoplankton and macroalgae acquire important resources, such as inorganic nitrogen, from the surrounding seawater by uptake across their entire surface area. Rates of ammonium and nitrate uptake per unit surface area were remarkably similar for both marine phytoplankton and macroalgae at low external concentrations. At an external concentration of 1 μM, the mean rate of nitrogen uptake was 10±2 nmol·cm^?2·h^?1 (n=36). There was a strong negative relationship between log surface area:volume (SA:V) quotient and log nitrogen content per cm² of surface (slope=?0.77), but a positive relationship between log SA:V and log maximum specific growth rate (μ_max; slope=0.46). There was a strong negative relationship between log SA:V and log measured rate of ammonium assimilation per cm² of surface, but the slope (?0.49) was steeper than that required to sustain μ_max (?0.31). Calculated rates of ammonium assimilation required to sustain growth rates measured in natural populations were similar for both marine phytoplankton and macroalgae with an overall mean of 6.2±1.4 nmol·cm^?2·h^?1 (n=15). These values were similar to maximum rates of ammonium assimilation in phytoplankton with high SA:V, but the values for algae with low SA:V were substantially less than the maximum rate of ammonium assimilation. This suggests that the growth rates of both marine phytoplankton and macroalgae in nature are often constrained by rates of uptake and assimilation of nutrients per cm² surface area.     

WATER MOTION AND MORPHOLOGY IN CHONDRUS CRISPUS (RHODOPHYTA)1

Morphological variability of intertidal Chondrus crispus Stackh. fronds along a small open rocky coast was related to wave exposure and emersion. Cluster analysis revealed two well-defined morphologies: filiform and planiform, named the N morphotype and B morphotype, respectively. We propose a rapid method of classifying fronds based on the morphology of the cross section at half the height on the thallus. The N morphotype is characterized by fewer dichotomies per unit length, a circular cross section with a large inner cortex, and narrow fronds. It is abundant at low intertidal and exposed sites. The B morphotype is characterized by more dichotomies, smaller sizes, a subelliptical or flattened cross section, and broad fronds. It is abundant at high intertidal sites in sheltered areas. Regression analysis revealed a major effect of water movements on frond morphology with respect to tidal level, which was more evident at high intertidal levels. No relationships were observed between morphology and life history phases.     

UPTAKE OF NITRATE,AMMONIUM, AND UREA BY NITROGEN-STARVED CULTURES OF MICROMONAS PUSILLA (PRASINOPHYCEAE): TRANSIENT RESPONSES1

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 (μM_max)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 μM_max observed during NO₃^? replete conditions.     

源库比改变对小麦氮磷吸收积累量及利用效率的影响

以８个小麦品种为试材,研究了源库比改变对氮磷吸收积累量及利用效率的影响。试验结果表明,去除部分小穗,使某些小麦品种的氮磷吸收积累量增加,另一些品种则降低或基本不受影响。但各品种的籽粒含氮量均因部分去穗而增加。     

NITROGEN UPTAKE AND ASSIMILATION KINETICS IN ALEXANDRIUM MINUTUM (DYNOPHYCEAE): EFFECT OF N‐LIMITED GROWTH RATE ON NITRATE AND AMMONIUM INTERACTIONS1

Uptake and assimilation kinetics of nitrate and ammonium were investigated along with inhibition of nitrate uptake by ammonium in the harmful dinoflagellate Alexandrium minutum Halim at different nitrogen (N)–limited growth rates. Alexandrium minutum had a strong affinity for nitrate and ammonium (K_s=0.26±0.03 and 0.31±0.04 μmol·L^?1, respectively) whatever the degree of N deficiency of the cells. Ammonium was always the preferred form of nitrogen taken up (=0.42–0.50). In the presence of both forms, nitrate uptake was inhibited by ammonium, and inhibition was particularly marked in N‐sufficient cells (I_max～0.9 and K_i=0.31–0.56 μmol·L^?1). In the case of N assimilation, ammonium was also the preferred form in N‐deficient cells (=0.54–0.72), whereas in N‐sufficient cells, both N sources were equally preferred (=0.90–1.00). The comparison of uptake and assimilation rates highlighted the ability of A. minutum to significantly store in 1 h nitrate and ammonium in amounts sufficient to supply twice the daily N requirements of the slowest‐growing N‐deficient cells. Nitrogen uptake kinetic parameters of A. minutum and their ecological implications are discussed.