Nitrogen and phosphorus uptake by the Brazilian kelp Laminaria abyssalis (Phaeophyta) in culture

The uptake of ammonium, nitrate and phosphate by laboratory-grown young sporophytes of Laminaria abyssalis was measured in a perturbed system (batch mode) at 18 °C and 35 ± 5 µE m^–2 s^–1 photon flux density. Uptake of all appeared to follow saturation-type nutrient uptake kinetics. The NO _inf3 ^sup– (K _s = 14.0 µM, V _max = 5.0 µmol h^–1 g^–1 dry wt) and NH _inf4 ^sup+ (K _s = 4.6 µM, V _max= 2.0 µmol h^–1 g^–1 dry wt) were taken up simultaneously, although NH _inf4 ^sup+ was taken up more rapidly. Values of K ₃ and V _max for phosphate were, respectively, 2.21 µM and 0.83 µmol h^–1 g^–1 dry wt. Nitrate and phosphate were both consumed in similar rates (V _max /Ks 0.37) at low concentrations. NH _inf4 ^sup+ , thus, might be a more efficient form of N fertilizer if artificial enrichment of seawater is used.     

Effect of light intensity and ammonium-N on carotenogenesis ofTrentepohlia odorata andDunaliella bardawil

AxenicTrentepohlia odorata was cultured at three different NH₄Cl levels (3.5 × 10^–2, 3.5 × 10^–3, 3.5 × 10^–4 M) and three different light intensities (48, 76, 122 µmol m^–2 s^–1). Chloride had no effect on growth over this range of concentration. High light intensity and high NH₄Cl concentration enhanced the specific growth rate. The carotenoid content increased under a combination of high light intensity and low N concentration. WhenD. bardawil was exposed to the same combination of growth conditions, there was an increase in its carotenoid content. The light saturation and the light inhibition constants (K _s andK _i, respectively) for growth, and the saturation constant (K _m) for NH₄Cl were determined. TheK _s andK _i values were higher inT. odorata (66.7 and> 122 mol m^–2 s^–1, respectively) than inD. bardawil (5.1 and 14.7 µmol m^–2 s^–1, respectively). TheK _m value determined at 122 µmol m^–2 s^–1, however, was lower inT. odorata (0.048 µM) than inD. bardawil (0.062 µM).Author for correspondence     

Uptake of sulphate,taurine, cysteine and methionine by symbiotic and free-living dinoflagellates

Sulphate uptake by Amphidinium carterae, Amphidinium klebsii and Gymnodinium microadriaticum grown on artificial seawater medium with sulphate, cysteine, methionine or taurine as sulphur source occurred via an active transport system which conformed to Michaelis-Menten type saturation kinetics. Values for K _m ranged from 0.18–2.13 mM and V _max ranged from 0.2–24.2 nmol · 10⁵ cells^–1 · h^–1. K _m for symbiotic G. microadriaticum was 0.48 mM and V _max was 0.2 nmol · 10⁵ cells^–1 · h^–1. Sulphate uptake was slightly inhibited by chromate and selenate, but not by tungstate, molybdate, sulphite or thiosulphate. Cysteine and methionine (0.1 mM), but not taurine, inhibited sulphate uptake by symbiotic G. microadriaticum, but not by the two species of Amphidinium. Uptake was inhibited 45–97% under both light and dark conditions by carbonylcyanide 3-chlorophenylhydrazone (CCCP); under dark conditions sulphate uptake was 40–60% of that observed under light conditions and was little affected by 3-(3,4-dichlorophenyl) 1,1-dimethylurea (DCMU).The uptake of taurine, cysteine and methionine by A. carterae, A. klebsii, cultured and symbiotic G. microadriaticum conformed to Michaelis-Menten type saturation kinetics. K _m values of taurine uptake ranged from 1.9–10 mM; for cysteine uptake from 0.6–3.2 mM and methionine from 0.001–0.021 mM. Cysteine induced a taurine uptake system with a K _m of 0.3–0.7 mM. Cysteine and methionine uptake by all organisms was largely unaffected by darkness or by DCMU in light or darkness. CCCP significantly inhibited uptake of these amino acids. Thus energy for cysteine and methionine uptake was supplied mainly by respiration. Taurine uptake by A. carterae was independent of light but was inhibited by CCCP, whereas uptake by A. klebsii and symbiotic G. microadriaticum was partially dependent on photosynthetic energy. Taurine uptake by cultured G. microadriaticum was more dependent on photosynthetic energy and was more sensitive to CCCP. Cysteine inhibited uptake of methionine and taurine by cultured and symbiotic G. microadriaticum to a greater extent than in the Amphidinium species. Methionine did not greatly affect taurine uptake, but did inhibit cysteine uptake. Taurine did not affect the uptake of cysteine or methionine.     

Therlt11 andraec1 mutants ofArabidopsis thaliana lack the activity of a basic-amino-acid transporter

The concentration dependence of the influx ofl-lysine in excised roots ofArabidopsis thaliana seedlings was analyzed for the wild-type (WT) and two mutants,rlt11 andraec1, which had been selected as resistant to lysine plus threonine, and to S-2-aminoethyl-l-cysteine, respectively. In the WT three components were resolved: (i) a high-affinity, low-capacity component [K _m = 2.2 M;V _max = 23 nmol·(g FW)^–1·h^–1]; (ii) a low-affinity, high-capacity component [K _m = 159 M;V _max = 742 nmol·(g FW)^–1·h^–1]; (iii) a component which is proportional to the external concentration, with a constant of proportionalityk = 104 nmol·(g FW)^–1 h^–1];·mM^–1. The influx ofl-lysine in the mutants was lower than in the WT, notably in the concentration range 0.1–0.4 mM, where it was only 7% of that in the WT. In both mutants the reduced influx could be fully attributed to the absence of the low-affinity (high-K _m ) component. This component most likely represents the activity of a specific basic-amino-acid transporter, since it was inhibited by several other basic amino acids (arginine, ornithine, hydroxylysine, aminoethylcysteine) but not byl-valine. The high-affinity uptake ofl-lysine may be due to the activity of at least two general amino acid transporters, as it was inhibitable byl-valine, and could be further dissected into two components with a high affinity (K _i = 1–5 M; and a low affinity (K _i = 0.5–1mM) forl-valine, respectively. Therlt11 andraecl mutant have the same phenotype and the corresponding loci were mapped on chromosome 1, but it is not yet clear whether they are allelic.Abbreviations AEC S-2-aminoethyl-l-cysteine - K _i equilibrium constant - WT wild-type     

l-serine and GABA uptake by synaptosomes during postnatal development of rat

Postnatal development changes in mechanisms of synaptosomal amino acid transport have been studied in rat cerebral cortex. Specific uptake of radiolabeled l-serine was examined and compared with that of radiolabeled GABA using synaptosomes-enriched fractions freshly prepared from cerebral cortex at different postnatal days from the birth to young adulthood. The preparations were incubated with 10 nM of [³H]l-serine and 10 nM of [³H]-GABA in either the presence or absence of NaCl, KCl or choline chloride, at 2 and 30 °C, for different periods up to 30 min. The uptake of [³H]l-serine was temperature dependent in synaptosomal fractions prepared from cerebral cortex of rats in postnatal days 5, 7, 13 and 21, but stronger dependence was observed in adult brain, irrespective of the presence of Na⁺, K⁺ or choline ions. At all postnatal ages studied, [³H]-GABA uptake showed a high activity in the presence of Na⁺ ions and at 30 °C. The values of K_m were 90–489 μM in l-serine uptake. However, in the uptake of GABA the values of K_m were 80–150 μM. The highest values of V_max were obtained at 5 and 21 postnatal days for both transport systems. These results indicate that the uptake of l-serine and GABA are regulated differentially during postnatal development.     

Kinetics of urea uptake by Melosira italica (Ehr.) Kütz at different luminosity conditions

The kinetic parameters K_m and V_max for urea uptake by Melosira italica were determined at 160 μeinsteins m⁻² s⁻¹ and in the dark. The transport systems showed an affinity for the substrate and a storing capacity in the dark (K_m = 65.07 μM; V_max = 2.18 nmoles 10⁵ cells ⁻¹ h⁻¹) greater than under 160 μE m⁻² s ⁻¹ (K_m = 111.2 μM; V_max = 1.11 nmoles 105 cells⁻¹ h⁻¹). Similarly, a reduction in consumption rate of urea under increasing photon flux densities was observed. The use of an inhibitor (potassium cyanide) indicated that the uptake process requires metabolic energy. That urea transport is more important in darkness, may constitute a survival strategy in which this compound is utilized by cells mainly during heterotrophic growth.     

Phosphate uptake capacity of summer phytoplankton of the Loosdrecht Lakes in relation to phosphorus loading and irradiance

Summer populations of the phytoplankton of the Loosdrecht Lakes were enclosed in laboratory scale enclosures (LSE), supplied with 7.5 g P.l^–1.d^–1 and 105 g P.l^–1.d^–1, respectively. The maximum initial phosphate uptake rate (V_m) was related to irradiance and primary production. At phosphate uptake saturating light-irradiance V_m values up to 4 times the V_m values in the dark were measured.The phosphate uptake capacity per unit dry weight remained more or less constant throughout the experiments in the LSE receiving the lower amount of phosphorus, and declined in the LSE receiving the higher amount of phosphorus. Within the range of V_m values measured (<10 g P.mg DW^–1.h^–1 or 1,3 g P. g chla ^–1.h^–1), the growth rate of the phytoplankton was not influenced by alterations in phosphorus availability.     

Nature and kinetic characteristics of L-DOPA uptake in rat renal proximal tubules

Summary The present study was performed with the aim to determine the kinetics and the caracteristics of cellular uptake of L-3,4-dihydroxyphenylalanine (L-DOPA) in rat renal proximal tubules. Incubation of renal tubules at 4°C in the presence of increasing concentrations of L-DOPA results in a linear and concentration-dependent accumulation of the substrate. In experiments carried out at 37°C, the accumulation of L-DOPA in renal tubules was found to be greater than that occurring at 4°C and showed a trend for saturation. The saturable component of L-DOPA uptake was derived from the total amount of L-DOPA accumulated in renal tubules at 37°C subtracted with the values obtained in experiments conducted at 4°C. The V_max and K_m values for the saturable component of L-DOPA uptake in renal tubules were, respectively, 241 ± 32 fmol µg protein^–1min^–1 and 567 ± 63 µM. Cyanine 863 (5 and 10 µM) was found to decrease the tubular uptake of L-DOPA, whereas probenecid (50 µM) did not change the rate of uptake of L-DOPA into renal tubules. The V_max and K_m values for the saturable component of L-DOPA uptake in renal tubules incubated in the presence of 10 µM cyanine 863 were, respectively, 97 ± 11 fmol µg protein^–1min^–1 and 160 ± 22 µM. It is suggested that the anionic L-DOPA may behave as an amphoteric substance, both hydroxyl groups in the aromatic ring determining the binding of the molecule to the organic cation transporter.     

Redistribution of hepatocyte chloride during l-alanine uptake

We used ion-sensitive, double-barrel microelectrodes to measure changes in hepatocyte transmembrane potential (V _m), intracellular K⁺, Cl^-, and Na⁺ activities (a ⁱ _k, a _Cl ⁱ and a _Na ⁱ ), and water volume during l-alanine uptake. Mouse liver slices were superfused with control and experimental Krebs physiological salt solutions. The experimental solution contained 20 m l-alanine, and the control solution was adjusted to the same osmolality (305 mOsm) with added sucrose. Hepatocytes also were loaded with 50 mm tetramethylammonium ion (TMA⁺) for 10 min. Changes in cell water volume during l-alanine uptake were determined by changes in intracellular, steady-state TMA⁺ activity measured with the K⁺ electrode. Hepatocyte control V _m was -33±1 mV. l-alanine uptake first depolarized V _m by 2±0.2 mV and then hyperpolarized V _m by 5 mV to-38±1 mV (n = 16) over 6 to 13 min. During this hyperpolarization, a _Na ⁱ increased by 30% from 19±2 to 25±3 mm (P < 0.01), and a _K ⁱ did not change significantly from 83±3 mm. However, with added ouabain (1 mm) l-alanine caused only a 2-mV increase in V _m, but now a _K ⁱ decreased from 61±3 to 54±5 mm (P < 0.05). Hyperpolarization of V _m by l-alanine uptake also resulted in a 38% decrease of a _Cl ⁱ from 20±2 to 12±3 mm (P < 0.001). Changes in V _m and V _Cl — V _m voltage traces were parallel during the time of l-alanine hyperpolarization, which is consistent with passive distribution of intracellular Cl^– with the V _m in hepatocytes. Added Ba²⁺ abolished the l-alanineinduced hyperpolarization, and a _Cl ⁱ remained unchanged. Hepatocyte water volume during l-alanine uptake increased by 12±3%. This swelling did not account for any changes in ion activities following l-alanine uptake. We conclude that hepatocyte a _K ⁱ is regulated by increased Na⁺-K⁺ pump activity during l-alanine uptake in spite of cell swelling and increased V _m due to increased K⁺ conductance. The hyperpolarization of V _m during l-alanine uptake provides electromotive force to decrease a _Cl ⁱ . The latter may contribute to hepatocyte volume regulation during organic solute transport.This work was supported by grant AA-08867 from the Alcohol, Drug Abuse, and Mental Health Association.     

Direct measurements of nanomolar nitrate uptake by the marine diatom Phaeodactylum tricornutum (Bohlin). Implications for studies of oligotrophic ecosystems

A new automated procedure for nanomolar nitrate analysis was applied to the study of uptake of low nitrate concentrations (< 100 ngat l^–1) by phytoplankton. The precision of this analytical method (± 3 ngat l^–1) made it possible to monitor the absorption of very low quantities of nitrate over short term periods by a low cell-density culture of the marine diatom Phaeodactylum tricornutum, where the levels of particulate nitrogen and chlorophyll were equivalent to those found in oligotrophic areas (0.5 µgat N l^–1 and 0.4 µg l^–1 respectively). By continuous monitoring of nitrate disappearance from the culture medium, we are able to describe accurately the transient uptake responses of the diatom after a spike addition of trace quantities of nitrate and thus to provide new information on the still largely unknown small-scale phenomenon of pulsed nitrate supply in the upper layer of stratified oceans and rapid uptake of these nitrate patches by phytoplankton.The results show that a N-limited culture of Phaeodactylum tricornutum is immediately capable of taking up trace quantities of nitrate (< 100 ngat l^–1) at high rates (0.10–0.14 h^–1) . These initial rates are one order of magnitude higher than the theoretical rates calculated from the Michaelis-Menten equation and are close to the level of V _max (0.15 h^–1) obtained when cells are exposed to saturating nitrate concentrations. This rapid initial uptake would be a considerable advantage in oligotrophic areas where nanomolar nitrate supply is thought to be episodic. The present results suggest that phytoplankton evolve adaptations to utilize the available nitrate at the spatial and temporal scales at which it occurs. On the other hand, we can consider this physiological adaptation as evidence of nitrate pulses in the field which would invalidate the steady-state approach to the oligotrophic ecosystems.     

Purification and characterization of a high molecular mass serine carboxypeptidase from <Emphasis Type="Italic">Monascus pilosus</Emphasis>

Two serine carboxypeptidases, MpiCP-1 and MpiCP-2, were purified to homogeneity from Monascus pilosus IFO 4480. MpiCP-1 is a homodimer with a native molecular mass of 125 kDa composed of two identical subunits of 61 kDa, while MpiCP-2 is a high mass homooligomer with a native molecular mass of 2,263 kDa composed of about 38 identical subunits of 59 kDa. This is unique among carboxypeptidases and distinguishes MpiCP-2 as the largest known carboxypeptidase. The two purified enzymes were both acidic glycoproteins. MpiCP-1 has an isoelectric point of 3.7 and a carbohydrate content of 11%, while for MpiCP-2 these values were 4.0 and 33%, respectively. The optimum pH and temperature were around 4.0 and 50°C for MpiCP-1, and 3.5 and 50°C for MpiCP-2. MpiCP-1 was stable over a broad range of pH between 2.0 and 8.0 at 37°C for 1 h, and up to 55°C for 15 min at pH 6.0, but MpiCP-2 was stable in a narrow range of pH between 5.5 and 6.5, and up to 50°C for 15 min at pH 6.0. Phenylmethylsulfonylfluoride strongly inhibited MpiCP-1 and completely inhibited MpiCP-2, suggesting that they are both serine carboxypeptidases. Of the substrates tested, benzyloxycarbonyl-l-tyrosyl-l-glutamic acid (Z-Tyr-Glu) was the best for both enzymes. The K_m, V_max, K_cat and K_cat/K_m values of MpiCP-1 for Z-Tyr-Glu at pH 4.0 and 37°C were 1.33 mM, 1.49 mM min^–1, 723 s^–1 and 545 mM^–1 s^–1, and those of MpiCP-2 at pH 3.5 and 37°C were 1.55 mM, 1.54 mM min^–1, 2,039 s^–1 and 1,318 mM^–1 s^–1, respectively.     

Purification and characterization of a new type of serine carboxypeptidase from<Emphasis Type="Italic"> Monascus purpureus</Emphasis>

Carboxypeptidase produced by Monascus purpureus IFO 4478 was purified to homogeneity. The purified enzyme is a heterodimer with a molecular mass of 132 kDa and consists of two subunits of 64 and 67 kDa. It is an acidic glycoprotein with an isoelectric point of 3.67 and 17.0% carbohydrate content. The optimum pH and temperature were 4.0 and 40 °C, respectively. The enzyme was stable between pH 2.0 and 8.0 at 37 °C for 1 h, and up to 50 °C at pH 5.0 for 15 min. The enzyme was strongly inhibited by piperastatin A, diisopropylfluoride phosphate (DFP), phenylmethylsulfonylfluoride (PMSF), and chymostatin, suggesting that it is a chymotrypsin-like serine carboxypeptidase. Monascus purpureus carboxypeptidase was also strongly inhibited by p-chloromercuribenzoic acid (PCMB) but not by ethylenediaminetetraacetic acid (EDTA) and 1,10-phenanthroline, indicating that it requires cysteine residue but not metal ions for activity. Benzyloxycarbonyl-l-tyrosyl-l-glutamic acid (Z-Tyr-Glu), among the substrates tested, was the best substrate of the enzyme. The K_m, V_max, K_cat, and K_cat/K_m values of the enzyme for Z-Tyr-Glu at pH 4.0 and 37 °C were 0.86 mM, 0.917 mM min^–1, 291 s^–1, and 339 mM^–1 s^–1, respectively.     

Uptake of ethanolamine in neuronal and glial cell cultures

The uptake of radioactive ethanolamine has been studied in exclusively neuronal and glial cell cultures from dissociated cerebral hemispheres of chick embryos. Both cell types show saturable kinetics; neurons have an apparentK _m of 6.7 M,V _max 41.4 pmol mg prot.^–1 min^–1 and glial cells aK _m of 119.6 M,V _max 3,917 pmol mg prot^–1 min^–1. The lower affinity of the transport and the 100 fold increase inV _max observed in glial cells correlated with a more important accumulation of free ethanolamine found in glial cells and with a higher degree of phosphorylation of ethanolamine. The uptake appeared to be temperature and Na⁺ ions dependent but was not affected by CN^– or ouabain. Monomethyl-, dimethylethanolamine and choline were effective in inhibiting the uptake. Little or no effect was observed with serine, methionine, carnitine, alanine or glutamate.     

EFFECT OF THE SINKING RATE OF TWO DIATOMS (THALASSIOSIRA SPP.) ON UPTAKE FROM LOW CONCENTRATIONS OF PHOSPHATE1

The effect of the sinking rate, or rate of medium flow (φ) on the rate of phosphate incorporation (V) by the planktonic diatoms Thalassiosira fluviatilis Hust. and T. pseudonana Hasle & Heimdal in batch and chemostat cultures was determined by passing medium at defined flow rates (0.5–25.0 mm·min^?1) over algae on membrane filters. At concentrations from 1 to 100 μg phosphorus·l^?1 V, increases with increasing velocity of flow, approaching a maximum value (V_m) as described by the empirical relationship: where K_φ is the sinking rate value when V = 1/2 V_m+ V_o and V_o is the uptake at 0 rate of flow. By comparing uptake at controlled flow with uptake in a vigorously stirred medium, the phosphate concentration in the cell boundary layer can be determined. The sinking rate that reduces the phosphate concentration in the boundary layer to half of nominal concentration in the medium is much lower for the larger T. fluviatilis than for T. pseudonana. For both diatoms, it is inversely related to the nominal concentration.     

Nitrate Uptake by the Halotolerant Cyanobacterium <Emphasis Type="Italic">Aphanothece halophytica</Emphasis> Grown Under Non-Stress and Salt-Stress Conditions

We have compared the characteristics of nitrate uptake by Aphanothece halophytica grown under non-stress and salt-stress conditions. Both cell types showed essentially similar patterns of nitrate uptake toward ammonium, nitrite, and DL-glyceraldehyde. Although the affinities of nitrate to non-stress cells and salt-stress cells were not significantly different, i.e., K_s = 416 and 450 µM, respectively, the V_max value for non-stress cells was about twofold of that for salt-stress cells (9.1 vs 5.3 µmol min^–1 mg^–1 Chl). Nitrate uptake by A. halophytica was found to be dependent on Na⁺. Ammonium inhibited nitrate uptake, and the presence of methionine sulfoximine could not release the inhibition by ammonium. Nitrite appeared to competitively inhibit nitrate uptake with a K_i value of 84 µM. Both chloride and phosphate anions did not affect nitrate uptake. DL-Glyceraldehyde, an inhibitor of CO₂ fixation, caused a reduction in the uptake of nitrate.Received: 22 October 2002 / Accepted: 6 December 2002     

Presence of an X AG − carrier in frog (Rana esculenta) red blood cells

Evidence is presented that the high levels of internal l-glutamic and l-aspartic acid in frog Rana esculenta red blood cells are due to the existence of a specific carrier for acidic amino acids of high affinity K _m = 3 m and low capacity (V_max) 0.4 mol l-Glu · Kg^–1 dry cell mass · 10 min^–1. It is Na+ dependent and the incorporation of l-glutamic acid can be inhibited by l and d-aspartate and l-cysteic acid, while d-glutamic does not inhibit. Moreover, this glutamic uptake shows a bell-shaped dependence on the external pH. All these properties show that this carrier belongs to the system X _AG ^– family. Besides the incorporation through this system, l-glutamic acid is also taken up through the ASC system, although, under physiological conditions, this transport is far less important, since it has relatively low affinity K _m 39 m but high capacity (V _max) 1.8 mol l-Glu · Kg^–1 dry cell mass · 10 min^–1.     

Model for the production of L-tryptophan from L-serine and indole by immobilized cells in a three-phase liquid-impelled loop reactor

Production of L-tryptophan from L-serine and indole catalyzed by Escherichia coli, immobilized in k-carrageenan gel beads, is technically feasible in the liquidimpelled loop reactor (LLR), using an organic solvent, e.g. n-dodecane.With L-serine in large excess intrinsic reaction kinetics is approximately first order with respect to indole, with a reaction constant of 8.5×10^–5 m³ kg _dw ^–1 s^–1.The overall process kinetics is jointly controlled by intrinsic kinetics and by intraparticle mass transfer resistance, which can be quantified using an effectiveness factor.Mass transfer of indole from the organic to the aqueous phase and from the aqueous to the gel phase are relatively fast and thus have negligible influence in the overall process kinetics, under the operational conditions tested. However, they may become important if the process is intensified by increasing the cell concentration in the gel and/or the gel hold-up in the reactor.A simple model which includes indole mass balances over the aqueous and organic phases, mass transfer and reaction kinetics, with parameters experimentally determined in independent experiments, was successful in simulating L-tryptophan production in the LLR.List of Symbols a, b, c coefficients of the equilibrium curve for indole between organic and aqueous phases - A, B, C, D, E, F auxiliary variables used in liquid-liquid mass transfer studies - a _x specific interfacial area referred to the volume of the aqueous phase (m^–1) - A _x interfacial area (m²) - a _Y specific interfacial area referred to the volume of the organic phase (m^–1) - A _Y interfacial area (m²) - C _b substrate concentration in the bulk of the aqueous phase (kg m^–3) - C _e substrate concentration in exit stream (kg m^–3) - C _E biocatalyst concentration referred to the aqueous phase (kg m^–3) - C _{E s} biocatalyst concentration referred to the volume of gel (kg m^–3) - C _s substrate concentration at the gel surface (kgm^–3) - d, e, f coefficients of the equilibrium curve for indole between aqueous and organic phases - dp particle diameter (m) - K ₂ kinetic constant (s^–1) - K ₁ kinetic constant K₂/K_M (kg^–1 m³ s^–1) - K _M Michaälis-Menten constant (kgm^–3) - K _X mass transfer coefficient referred to the aqueous phase (ms^–1) - K _Xa_X volumetric mass transfer coefficient based on the volume of the aqueous phase (s^–1) - k _Y mass transfer coefficient referred to the organic phase (ms^–1) - K _Ya_Y volumetric mass transfer coefficient based on the volume of the organic phase (s^–1) - N _X mass flux of indole from organic to aqueous Phase (kg m^–2s^–1) - N _Y mass flux of indole from aqueous to organic phase (kg m^–2s^–1) - Q _e volumetric flow rate in exit stream (m³s^–1) - Q _f volumetric flow rate in feed stream (m³s^–1) - _obs observed reaction rate (kg s^–1 m^–3) - intrinsic reaction rate (kg s^–1 m^–3) - Re Reynolds number - Sc Schmidt number - Sh Sherwood number - t time (s) - u superficial velocity (m s^–1) - V _max maximum reaction rate (kg s^–1m^–3) - V _S volume of the support (m³) - V _X volume of aqueous phase (m³) - V _Y volume of the organic phase (m³) - X indole concentration in the aqueous phase (kgm^–3) - Y indole concentration in the organic phase (kg m^–3 Greek Letters overall effectiveness factor - _e external effectiveness factor - _i internal effectiveness factor - Thiele module A fellowship awarded to one of us (D.M.R.)by INICT is gratefuly acknowledged.     

Heterotrophic glucose uptake and respiration in Lake Kinneret

The turnover times of glucose, averaged for 0–10 m in the upper waters of Lake Kinneret and measured by the addition of single or multiple concentrations of substrate, ranged from 23 to 188 hours and 1 to 87 hours respectively. Potential uptake rates (estimated as V_max) ranged from 0.095 to 1.94 µg glucose l^–1h^–1, while measured uptake rates varied from 0.09 to 1.1 µg glucose l^–1h^–1. Concentrations of dissolved carbohydrates and glucose averaged 0.71 mg glucose equivalents l^–1 and 39 µg glucose l^–1 respectively. No evident relationships between glucose cycling and any fractions of dissolved organic matter, phytoplankton biomass or primary productivity were found. Turnover times were generally most rapid immediately after the decline of the spring Peridinium bloom. The respiration percentage of incorporated glucose ranged from 25% to 61% with highest values during the summer months. Respiration may be influenced by the nature of the indigenous bacterial population as well as by temperature. Daily heterotrophic glucose carbon uptake was about 9% of the photosynthetic incorporation and could provide a bacterial yield of about 7 × 10⁴ ml^–1d^–1.     

Uptake of iron byGeotrichum candidum,a non-siderophore producer

Summary Geotrichum candidum (isolate 1–9) pathogenic on citrus fruits, appears to lack siderophore production. Iron uptake byG. candidum is mediated by two distinct iron-regulated, energy-and temperature-dependent transport systems that require sulfhydryl groups. One system exhibits specificity for either ferric or ferrous iron, whereas the other exhibits specificity for ferrioxamine-B-mediated iron uptake and presumably other hydroxamate siderophores. Radioactive iron uptake from⁵⁹FeCl₃ showed an optimum at pH 6 and 35° C, and Michaelis-Menten kinetics (apparentK _m = 3 m,V _max = 0.054 nmol · mg^–1 · min^–1). The maximal rate of Fe²⁺ uptake was higher than Fe³⁺ (V _max = 0.25 nmol · mg^–1 · min^–1) but theK _m was identical. Reduction of ferric to ferrous iron prior to transport could not be detected. The ferrioxamine B system exhibits an optimum at pH 6 and 40° C and saturation kinetics (K _m = 2 M,V _max = 0.22 nmol · mg^–1 · min^–1). The two systems were distinguished as two separate entities by negative reciprocal competition, and on the basis of differential response to temperature and phenazine methosulfate. Mössbauer studies revealed that cells fed with either⁵⁷FeCl₃ or⁵⁷FeCl₂ accumulated unknown ferric and ferrous binding metabolites.     

Ammonium and nitrate uptake in gap,generalist and understory species of the genus Piper

Summary We studied root net uptake of ammonium (NH ₄ ⁺ ) and nitrate (NO ₃ ^– ) in species of the genus Piper (Piperaceae) under high, intermediate and low photosynthetically active photon flux densities (PFD). Plants were grown hydroponically, and then transferred to temperature controlled (25° C) root cuvettes for nutrient uptake determinations. Uptake solutions provided NH ₄ ⁺ and NO ₃ ^– simultaneously (both) or separately (single). In the first experiment, seven species of Piper, from a broad range of rainforest light habitats ranging from gap to understory, were screened for mineral nitrogen preference (100 M NH ₄ ⁺ and/or 100 M NO ₃ ^– ) at intermediate PFD (100 mol m^–2 s^–1). Preference for NH ₄ ⁺ relative to NO ₃ ^– , defined as the ratio of NH ₄ ⁺ (both):NO ₃ ^– (both) net uptake, was higher in understory species than in gap species. Ammonium repression of NO ₃ ^– uptake, defined as the ratio of NO ₃ ^– (single): NO ₃ ^– (both) net uptake, was also higher in understory species as compared to gap species. In a second set of experiments, we examined the effect of nitrogen concentration (equimolar, 10 to 1000 M) on NH ₄ ⁺ preference and NH ₄ ⁺ repression of NO ₃ ^– net uptake at high (500 mol m^–2 s^–1) and low (50 mol m^–2 s^–1) PFD in a gap (P. auritum), generalist (P. hispidum) and understory species (P. aequale). All species exhibited negligible NH ₄ ⁺ repression of NO ₃ ^– net uptake at high PFD. At low PFD, NH ₄ ⁺ preference and repression of NO ₃ ^– net uptake occurred in all species (understory > generalist > gap), but only at intermediate nitrogen concentrations, i.e. between 10 and 200 M. Ammonium repression of net NO ₃ ^– uptake decreased or increased rapidly (in < 48 h) after transitions from low to high or from high to low PFD respectively. No significant diurnal patterns in NO ₃ ^– or NH ₄ ⁺ net uptake were observed.CIWDPB publication # 1130