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.     

Ammonium uptake by Chlorella

The preincubation of Chlorella cells with glucose caused a tenfold increase of the maximal uptake rate of ammonium without change in the K _m (2 M). A similar stimulation of ammonium uptake was found when the cells were transferred to nitrogen-free growth medium. The time-course of uptake stimulation by glucose revealed a lag period of 10–20 min. The turnover of the ammonium transport system is characterized by a half-life time of 5–10 h, but in the presence of light 30% of uptake activity stayed even after 50 h. 6-Deoxyglucose was not able to increase the ammonium uptake rate. These data together were interpreted as evidence for induction of an ammonium transport system by a metabolite of glucose. Mechanistic studies of the ammonium transport system provided evidence for the electrogenic uptake of the ammonium ion. The charge compensation for NH ₄ ⁺ entry was achieved by immediate K⁺ efflux from the cells, and this was followed after 1 min by H⁺ extrusion. Ammonium accumulated in the cells; the rate of uptake was sensitive to p-trifluoromethoxy-carbonylcyanide-phenylhydrazon and insensitive to methionine-sulfoxime. Uptake studies with methylamine revealed that methylamine transport is obviously catalyzed by the ammonium transport system and, therefore, also increased in glucose-treated Chlorella cells.Abbreviation p.c. packed cells     

Regulation of NH4 + transport pathways in Pisum arvense roots

The effects of metabolic and protein synthesis inhibitors on NH₄ ⁺ uptake by Pisum arvense plants at low (0.05 mM) and high (1 mM) external ammonium concentration were studied. In short-time experiments cycloheximide decreased the ammonium uptake rate at low level of NH₄ ⁺ and increased the absorption of NH₄ ⁺ from uptake medium containing high ammonium concentration. Arsenate and azide supplied into uptake solutions at low ammonium concentration strongly decreased or completely suppressed the NH₄ ⁺ uptake rate, respectively. When the experiments were carried out at high level of ammonium only azide decreased the uptake rate of NH₄ ⁺ and arsenate stimulated this process. Dinitrophenol very strongly repressed the uptake rate of NH₄ ⁺ at both ammonium concentrations. After removing dinitrophenol from both solutions, neither at low nor high external ammonium level the recovery of NH₄ ⁺ uptake rate was achieved within 150 min or 3 h, respectively. The recovery of NH₄ ⁺ uptake rate after removing azide was observed within 90 min and 3 h at low and high ammonium concentrations, respectively. The regulation of NH₄ ⁺ uptake by some inhibitors at low external ammonium level was investigated using plasma membrane vesicles isolated from roots by two-phase partitioning. Orthovanadate completely suppressed the uptake of NH₄ ⁺ by vesicles and quinacrine decreased the NH₄ ⁺ uptake which 55 suggests that ammonium uptake depends on activities of plasma membrane-bound enzymes. On the other hand, it was found that dinitrophenol completely reduced the NH₄ ⁺ uptake by vesicles. The various effects of inhibitors on ammonium uptake dependent on external ammonium concentration suggest the action of different ammonium transport systems in Pisum arvense roots. The ammonium transport into root cells at low NH₄ ⁺ level requires energy and synthesis of protein in the cytoplasm. The research was supported by grant of KBN No. 6PO4C 068 08     

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.     

SHORT TERM UPTAKE OF NUTRIENTS BY ENTEROMORPHA PROLIFERA (CHLOROPHYCEAE)1

Rates of NH₄⁺ and NO₃^? uptake were determined by accumulation of ¹⁵N in plant tissue and by disappearance of nutrient from the medium. Agreement between rates calculated by the two methods was good, averaging 82.7% (SD = 15.8%) and 91.2% (SD = 13.7%) for NH₄⁺ and NO₃^? uptake, respectively. An average of 93.4 and 96.0% of added ¹⁵NH₄⁺ and ¹⁵NO₃^? was recovered from the medium and /or plant tissue at the end of the incubations. Both bacterial uptake and regeneration of NH₄⁺ may contribute to discrepancies between NH₄⁺ uptake rates calculated by ¹⁵N accumulation and disappearance of NH₄⁺ from the medium. The influence of tissue composition on uptake of NH₄⁺, NO₃^? and PO₄^3- by Enteromorpha prolifera (Müller) J. Agardh was examined. For NH₄⁺ uptake, V_max was 188 μmol NH₄^+. g dry wt^?1. h^?1 and K_s ranged from 9.3 to 13.4 μM, but there was no correlation between kinetic parameters and tissue nitrogen content. For NO₃^?, both kinetic parameters were higher for plants with low tissue nitrogen than for plants with high tissue nitrogen. Maximum rates were 169 and 75.4 μmol NO₃^?. g dry wt^?1. h^?1, and K_s was 13.3 and 2.31 μM for low and high tissue nitrogen plants, respectively. Estimates of uptake in the field suggested that NH₄⁺ accounted for 65% and NO₃^? for up to 35% of total nitrogen uptake during the summer. Nutrient uptake rates of field-collected plants also indicated that E. prolifera in Yaquina Bay, Oregon was not likely to have been nitrogen-limited, but may have been phosphorus-limited.     

Evidence for an ammonium transport system in the N2-fixing phototrophic bacteriumRhodospirillum rubrum

An ammonium transport system in the phototrophic N₂-fixing bacteriumRhodospirillum rubrum was characterized by using the uptake of¹⁴C-methylamine as a probe.Uptake showed saturation kinetics with an apparentK _m =110 M. It was competitively inhibited by ammonium (K _i =7 M). Uptake exhibited a narrow pH maximum around pH 7.0.Up to 200-fold gradients across the membrane were formed within 40–60 min. Gradient formation was inhibited by carbon starvation, azide or cyanide. Pre-accumulated methylamine was released by ammonium pulses to more than 80%, indicating only minor metabolization.The synthesis of the transport system was repressed by ammonium in high concentrations.     

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.     

REVERSIBLE KINETIC MODEL FOR THE SHORT-TERM REGULATION OF METHYLAMMONIUM UPTAKE IN TWO PHYTOPLANKTON SPECIES,DUNALIELLA TERTIOLECTA (CHLOROPHYCEAE) AND PHAEODACTYLUM TRICORNUTUM (BACILLARIOPHYCEAE)1

Methylammonium, an ammonium analog, was used to study the short-term kinetics of ammonium uptake in a diatom, Phaeodactylum tricornutum Bohlin, and a green alga, Dunaliella tertiolecta Butcher. Time courses of methylammonium disappearance were measured over a wide range of initial substrate concentrations for the two species. It was shown that feedback inhibition, described mathematically by a reversible enzyme kinetic model, can be used to explain the data. For the two species, there was good agreement between the kinetic parameters obtained from the analysis of the uptake versus substrate curve and those from the fit of the reversible kinetic model to the time-course data. All time courses of CH₃NH₃⁺ disappearance could be described by constants V_m and k_s. Ammonium time-course data show some similarities to its analog, methylammonium. Our study suggests that the apparent change in V_m and k_s with time measured after the addition of saturating ammonium concentrations reflects an uncoupling between transport and assimilation of the substrate rather than a real change in the kinetic parameters of the transport mechanism.     

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.     

The uptake of nitrate and ammonium nitrogen in Chara hispida L.: the contribution of the rhizoid

Abstract The uptake of ammonium and nitrate nitrogen by cultured plants of the green freshwater alga Chara hispida L. has been compared quantitatively with the contribution of its rhizoidal tissue. In the short-term, the rhizoid takes up 7–20% of the ammonium nitrogen, and about 15% of the nitrate that is taken up by whole plants under similar conditions. The uptake was studied over a range of both temperatures and external concentrations. The apparent activation energy for the uptake of NH₄⁺ and NO₃^? by the whole plant was found to be 50 kJ mol^?1 and 30 kJ mol^?1, respectively. For the rhizoid, the values were similar for both nitrogenous ions, 106 kJ mol^?1 and 70–100 kJ mol^?1. The rhizoidal uptake mechanism for ammonium nitrogen operates more efficiently compared to that in the whole plant. Nitrate is taken up by the rhizoid by a mechanism with a substrate affinity higher than in the plant taken as a whole. The possible ecological significance of the results is discussed.     

THE RELATIVE IMPORTANCE OF WATER MOTION ON NITROGEN UPTAKE BY THE SUBTIDAL MACROALGA ADAMSIELLA CHAUVINII (RHODOPHYTA) IN WINTER AND SUMMER1

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 (V_max, K_s) 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.     

Amine transport at the plasmalemma of Riccia fluitans

Green thallus cells of the aquatic liverwort, Riccia fluitans, are rapidly depolarized in the presence of 1–20 μM NH₄Cl and 5–100 μM CH₃NH₃Cl, respectively. Simultaneously, the membrane conductance is increased from 0.41 to 1.2 S · m^?2. Uptake of [¹⁴C]methylamine is stimulated by increasing [K⁺]_o and inhibited by increasing [Na⁺]_o or [H⁺]_o, is highly voltage sensitive, and saturates at low amine concentrations.Double-reciprocal plots of (a) maximal membrane depolarization and (b) methylamine uptake vs. external amine concentration give apparent K_m values of 2 ± 1 μM ammonia and 25–50 μM methylamine; K_m values for changes in conductance and membrane current are greater and voltage dependent. Whereas the amine transport into the cell is strongly inhibited by CN^?, the amine efflux is stimulated.The current-voltage characteristics of the ammonia transport are represented by a sigmoid curve with an equilibrium potential of ?60 mV, and this is understood as a typical carrier curve with a saturation current of about 70 mA · m^?2. It is further concluded that the evidently carrier-mediated transport is competitive for the two amines tested, and that ammonia and methylamine are transported in the protonated form as NH₄⁺ and CH₃NH₃⁺ into the cytoplasm.     

Acclimatization of K+ uptake to changes in root temperature: experiments with oilseed rape and barley in flowing solution culture

Abstract Changes in the net uptake rate of K⁺ and in the average tissue concentration of K⁺ were measured over 14 d in response to changes in root temperature with oilseed rape (Brassica napus L. cv. Bien venu) and barley (Hordeum vulgare L. cv. Atem). Plants were grown in flowing nutrient solutions containing 2.5 mmol m^?3 K⁺ and were acclimatized over 49 d (rape) or 28 d (barley) to low root temperature (5°C) prior to steady–state treatments at root temperatures between 3 °C and 25 °C, with common air temperature. Uptake of K⁺ was monitored continuously over 14 d and nitrogen was supplied as NH₄⁺+ NO^?₃ or NH⁺₄ or NO^?₃. Unit absorption rates of K⁺ increased with time and with root temperature up to Day 4 or 5 following the change in root temperature. Thereafter they usually approached steady-state, with Q₁₀? 2.0 between 7 °C and 17°C, although rates became similar between 7 °C and 13°C. Uptake of K⁺ by rape plants was invariably greater under NO^?₃ nutrition compared with NH⁺₄. The percentage K⁺ in the plant dry matter increased with temperature from 2% at 3 °C to 4% at 25 °C in rape, but there was less effect of temperature on the average concentrations of K⁺ in the plant fresh weight or plant water content. Concentrations of K⁺ in the leaf water fraction of rape plants decreased with increasing root temperature, but in barley they increased with increasing root temperature. Concentrations of K⁺ in the root water fraction were relatively stable with respect to root temperature. The results are discussed in terms of compensatory changes in K⁺ uptake following a change in root temperature and the relationships between growth, shoot: root ratio and K⁺ composition of the plant.     

NH4 + transport system of a psychrophilic marine bacterium, Vibrio sp. strain ABE-1

NH₄ ⁺ transport system of a psychrophilic marine bacterium Vibrio sp. strain ABE-1 (Vibrio ABE-1) was examined by measuring the uptake of [¹⁴C]methylammonium ion (¹⁴CH₃NH^{3 +}) into the intact cells. ¹⁴CH₃NH₃ ⁺ uptake was detected in cells grown in medium containing glutamate as the sole nitrogen source, but not in those grown in medium containing NH₄Cl instead of glutamate. Vibrio ABE-1 did not utilize CH₃NH₃ ⁺ as a carbon or nitrogen source. NH₄Cl and nonradiolabeled CH₃NH₃ ⁺ completely inhibited ¹⁴CH₃NH₃ ⁺ uptake. These results indicate that ¹⁴CH₃NH₃ ⁺ uptake in this bacterium is mediated via an NH₄ ⁺ transport system and not by a specific carrier for CH₃NH₃ ⁺. The respiratory substrate succinate was required to drive ¹⁴CH₃NH₃ ⁺ uptake and the uptake was completely inhibited by KCN, indicating that the uptake was energy dependent. The electrochemical potentials of H⁺ and/or Na⁺ across membranes were suggested to be the driving forces for the transport system because the ionophores carbonylcyanide m-chlorophenylhydrazone and monensin strongly inhibited uptake activities at pH 6.5 and 8.5, respectively. Furthermore, KCl activated ¹⁴CH₃NH₃ ⁺ uptake. The ¹⁴CH₃NH₃ ⁺ uptake activity of Vibrio ABE-1 was markedly high at temperatures between 0° and 15°C, and the apparent K _m value for CH₃NH₃ ⁺ of the uptake did not change significantly over the temperature range from 0° to 25°C. Thus, the NH₄ ⁺ transport system of this bacterium was highly active at low temperatures. Received: August 1, 1998 / Accepted: October 8, 1998     

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.     

The role of N-remobilisation and the uptake of NH4+ and NO3- by Lolium perenne L. in laminae growth following defoliation under field conditions

Several studies have previously shown that shoot removal of forage species, either by cutting or herbivore grazing, results in a large decline in N uptake (60%) and/or N₂ fixation (80%). The source of N used for initial shoot growth following defoliation relies mainly on mobilisation of N reserves from tissues remaining after defoliation. To date, most studies investigating N-mobilisation have been conducted, with isolated plants grown in controlled conditions. The objectives of this study were for Lolium perenne L., grown in a dense canopy in field conditions, to determine: 1) the contribution of N-mobilisation, NH₄ ⁺ uptake and NO₃ ^- uptake to growing shoots after defoliation, and 2) the contribution of the high (HATS) and low (LATS) affinity transport systems to the total plant uptake of NH₄ ⁺ and NO₃ ^-. During the first seven days following defoliation, decreases in biomass and N-content of roots (34% and 47%, respectively) and to a lesser extent stubble (18% and 43%, respectively) were observed, concomitant with mobilisation of N to shoots. The proportion and origin of N used by shoots (derived from reserves or uptake) was similar to data reported for isolated plants. Both HATS and LATS contributed to the total root uptake of NH₄ ⁺ and NO₃ ^-. The V_max of both the NH₄ ⁺ and NO₃ ^- HATS increased as a function of time after defoliation, and both HATS systems were saturated by substrate concentrations in the soil at all times. The capacity of the LATS was reduced as soil NO₃ ^- and NH₄ ⁺ concentrations decreased following defoliation. Data from ¹⁵N uptake by field-grown plants, and uptake rates of NH₄ ⁺ and NO₃ ^- estimated by excised root bioassays, were significantly correlated, though uptake was over-estimated by the later method. The results are discussed in terms of putative mechanisms for regulating N uptake following severe defoliation. This revised version was published online in June 2006 with corrections to the Cover Date.     

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.     

Permeability of ammonia,methylamine and ethylamine in the cyanobacterium,Synechococcus R-2 (Anacystis nidulans) PCC 7942

Summary Permeabilities of ammonia (NH₃), methylamine (CH₃NH₂) and ethylamine (CH₃CH₂NH₂) in the cyanobacterium (cyanophyte)Synechococcus R-2 (Anacystis nidulans) have been measured. Based on net uptake rates of DCMU (dichlorophenyldimethylurea) treated cells, the permeability of ammonia was 6.44±1.22 m sec^–1 (n=13). The permeabilities of methylamine and ethylamine, based on steady-state¹⁴C labeling were more than ten times that of ammonia (P _methylamine=84.6±9.47 m sec^–1 (76),P _ethylamine=109±11 m sec^–1 (55)). The apparent permeabilities based on net uptake rates of methylamine and ethylamine uptake were significantly lower, but this effect was partially reversible by ammonia, suggesting that net amine fluxes are rate limited by proton fluxes to an upper limit of about 700 nmol m^–2 sec^–1. Increasing concentrations of amines in alkaline conditions partially dissipated the pH gradient across the cell membrane, and this property could be used to calculate the relative permeabilities of different amines. The ratio of ethylamine to methylamine permeabilities was not significantly different from that calculated from the direct measurements of permeabilities; ammonia was much less effective in dissipating the pH gradient across the cell membrane than methylamine or ethylamine. An apparent permeability of ammonia of 5.7±0.9 m sec^–1 could be calculated from the permeability ratio of ammonia to methylamine and the experimentally measured permeability of methylamine. The permeability properties of ammonia and methylamine are very different; this poses problems in the interpretation of experiments where¹⁴C-methylamine is used as an ammonia analogue.     

Ammonium uptake kinetics in root and leaf cells of Zostera marina L.

Ammonium uptake rates and the mechanism for ammonium transport into the cells have been analysed in Zostera marina L. In the cells of this species, a proton pump is present in the plasmalemma, which maintains the membrane potential. However, this seagrass shows a high-affinity transport mechanism both for nitrate and phosphate which is dependent on sodium and is unique among angiosperms. We have then analysed if the transport of another N form, ammonium, is also dependent of sodium. First, we have studied ammonium transport at the cellular level by measurements of membrane potentials, both in epidermal root cells and mesophyll cells. And second, we have monitored uptake rates in whole leaves and roots by depletion experiments. The results showed that ammonium is taken up by a high-affinity transport system both in root and leaf cells, although two different of kinetics could be discerned in mesophyll cells (with affinity constants of 2.2 ± 1.1 μM NH₄⁺, in the range 0.01-10 μM NH₄⁺, and 23.2 ± 7.1 μM NH₄⁺, at concentrations between 10 and 500 μM NH₄⁺). However, only one kinetic could be observed in epidermal root cells, which showed a K_m = 11.2 ± 1.0 μM NH₄⁺, considering the whole ammonium concentration range assayed (0.01-500 μM NH₄⁺). The higher affinity of leaf cells for ammonium was consistent with the higher uptake rates observed in leaves, with respect to roots, in depletion experiments at 10 μM NH₄⁺ initial concentration. However, when an initial concentration of 100 μM was assayed, the difference between uptake rates was reduced, but still being higher in leaves. Variations in proton or sodium-electrochemical gradient did not affect ammonium uptake, suggesting that the transport of this nutrient is not driven by these ions and that the ammonium transport mechanism could be different to the transport of nitrate and phosphate in this species.     

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.