ACCLIMATION OF NITRATE UPTAKE BY PHYTOPLANKTON TO HIGH SUBSTRATE LEVELS1

A literature review of data on nitrate uptake by phytoplankton suggests that nitrate levels above 20 μmol N·L^?1 generally stimulated uptake rates in cultured unicellular algae and natural phytoplankton communities. This phenomenon indicates that phytoplankton cells acclimate to elevated nitrate levels by increasing their uptake capacity in a range of concentrations previously considered to be saturating. Cyanobacteria and flagellates were found to present a considerable capacity for acclimation, with low (0.1–2 μmol N·L^?1) half‐saturation values (K_s) at low (5–20 μmol N·L^?1) substrate levels and high (1–80 μmol N·L^?1) K_s values at high (30–100 μmol N·L^?1) substrate levels. However, some diatom genera (Rhizosolenia, Skeletonema, Thalassiosira) also appeared to possess a low affinity nitrate uptake system (K_s between 18 and 120 μmol N·L^?1), which can help resolve the paradox of their presence in enriched seas. It follows that present models of nitrate uptake can severely underestimate the effects of high nitrate concentrations on phytoplankton dynamics and development. A more adequate approach would be to consider the possibility of multiphasic uptake involving several phase transitions as nitrate concentrations increased. Because it is a nonlinear phenomenon featuring strong thresholds, this effect appears to override that of other variables, such as irradiance, temperature, and cell size. Within the present context of eutrophication and for a range of concentrations that is becoming more and more ecologically relevant, equations are tentatively presented as a first approach to estimate K_s from ambient nitrate concentrations.     

Growth and nitrate uptake properties of plants grown at different relative rates of nitrogen supply. II. Activity and affinity of the nitrate uptake system in Pisum and Lemna in relation to nitrogen availability and nitrogen demand

Abstract Net nitrate uptake rates were measured and the kinetics calculated in non-nodulated Pisum sativum L. cv. Marma and Lemna gibba L. adapted to constant relative rates of nitrate-N additions (R_A), ranging from 0.03 to 0.27 d^?1 for Pisum and from 0.05 to 0.40 d^?1 for Lemna, V_max of net nitrate uptake (measured in the range 10 to 100 mmol m^?3 nitrate, i.e. ‘system I’) increased with R_A in the growth limiting range but decreased when R_A exceeded the relative growth rate (RGR), K_m was not significantly related to changes in R_A. On the basis of previous ¹³N-flux experiments, it is concluded that the differences in V_max at growth limiting R_A are attributable to differences in influx rates. Linear relationships between V_max and tissue nitrogen concentrations were obtained in the growth limiting range for both species, and extrapolated intercepts relate well with the previously defined minimal nitrogen concentrations for plant growth (Oscarson, Ingemarsson & Larsson, 1989). Analysis of V_max for net nitrate uptake on intact plant basis in relation to nitrogen demand during stable, nitrogen limited, growth shows an increased overcapacity at lower R_A values in both species, which is largely explained by the increased relative root size at low R_A. A balancing nitrate concentration, defined as the steady state concentration needed to sustain the relative rate of increase in plant nitrogen (R_N), predicted by R_A, was calculated for both species. In the growth limiting range, this value ranges from 3.5 mmol m^?3 (R_A 0.03 d^?1) to 44 mmol m^?3 (R_A 0.21 d^?1) for Pisum and from 0.2 mmol m^?3 (R_A 0.05 d^?1) to 5.4 mmol m^?3 (R_A 0.03 d^?1) for Lemna. It is suggested that this value can be used as a unifying measure of the affinity for nitrate, integrating the performance of the nitrate uptake system with nitrate flux and long term growth and demand for nitrogen.     

INHIBITION OF CARBON UPTAKE AND STIMULATION OF NITRATE UPTAKE AT LOW SALINITIES IN FUCUS DISTICHUS (PHAEOPHYCEAE)1

Inorganic carbon and nitrate uptake were examined in whole plants of Fucus distichus L. (Powell) incubated in dilutions of synthetic ocean water and media with different concentrations of Na⁺, K⁺ and Cl^?. Reduction in salinity from normal seawater (33 ppt) decreased carbon uptake rate but increased nitrate uptake rate by 50% each. Substitution of K⁺ for Na⁺ at constant ionic strength decreased nitrate uptake. Substitution of K⁺ or mannitol for Na⁺ decreased carbon uptake. Neither the uptake of nitrate or carbon was changed by substituting SO₄^2- for Cl^?. Ionophores, valinomycin and monensin, inhibited both nitrate uptake and carbon fixation from 20 to 70% of control rates. The stimulation of nitrate uptake at low salinity may be beneficial to plants in estuarine tidal environments in which nitrate is supplied by the fresh water source.     

EVALUATION OF COMPETITIVE ABILITY OF STAURASTRUM LUETKEMUELLERII (CHLOROPHYCEAE) AND MICROCYSTIS AERUGINOSA (CYANOPHYCEAE) UNDER P LIMITATION1

The desmid Staurastrum luetkemuellerii Donat et Ruttner and the cyanobacterium Microcystis aeruginosa Kütz. showed pronounced differences in chemical composition and ability to maintain P fluxes. The cellular P:C ratio (Q_p) and the surplus P:C ratio (Q_sp) were higher in M. aeruginosa, indicating a lower yield of biomass C per unit of P. The subsistence quota (Q_p) was 1.85 μg P·mg C^?1in S. luetkemuellerii and 6.09 μg P·mg C^?1in M. aeruginosa, whereas the respective Q_p of P saturnted organisms (Q_s) were 43 and 63 μg P·mg C^?1. These stores could support four divisions in S. luetkemuellerii and three divisions in M. aeruginosa, which suggests that the former exhibited highest storage capacity (Q_s/Q₀). M. aeruginosa showed a tenfold higher activity of alkaline phosphatase than S. luetkemuellerii when P starved. The optimum N:P ratio (by weight) was 5 in S. luetkemuellerii and 7 in M. aeruginosa. The initial uptake of P_i pulses in the organisms was not inhibited by rapid (<1 h) internal feedback mechanisms and the short term uptake rote could be expressed solely as a function of ambient P_i. The maximum cellular C-based uptake rate (V_m) in P starved M. aeruginosa was up to 50 times higher than that of S. luetkemuellerii. It decreased with increasing growth rate (P status) in the former species and remained fairly constant in the latter. The corresponding cellular P-based value (U_m= V_m/Q_p) decreased with growth rate in both species and was about 10 times higher in P started M. aeruginosa than in S. luetkemuellerii. The average half saturation constant for uptake (K_m) was equal for both species (22 μg P·L^?1) and varied with the P status. S. luetkemuellerii exhibited shifts in the uptake rate of P_i that were characterized by increased affinity (U^m/K^m) at low P_i, concentrations (<4 μg P·L^?1) compared to that at higher concentrations. The species thus was well adapted to uptake at low ambient P_i, but M. aeruginosa was superior in P_i uptake under steady state and transient conditions when the growth rate was lower than 0.75 d^?1. Moreover, M. aeruginosa was favored by pulsed addition of P_i. M. aeruginosa relpased P_i at a higher rate than S. luetkemuellerii. Leakage of P_i from the cells caused C-shaped μ vs. P_i curves. Therefore, no unique K_s for growth could be estimated. The maximum growth rate (μ_m) (23° C) was 0.94 d^?1for S. luetkemuellerii and 0.81 d^?1for M. aeruginosa. The steady state concentration of P_i (P*) was lower in M. aeruginosa than in S. luetkemuellerii at medium growth rates. The concentration of P_i at which the uptake and release of P_i was equal (P_c was, however, lower in S. luetkemuellerii.     

EFFECT OF VITAMIN CONCENTRATIONS ON GROWTH AND DEVELOPMENT OF VITAMIN-REQUIRING ALGAE1

Vitamin-requiring marine algae, Cyclotella nana, Monochrysis lutheri, and Amphidinium carterae, were grown in batch culture with limiting concentrations of vitamin B₁₂, thiamine, and biotin, respectively. Cell numbers, average cell volumes, biomasses, ¹¹CO₂ uptake rates, and chlorophyll a contents were determined daily. Maximum ¹⁴CO₂ uptake rates in most vitamin concentrations were obtained at 2 days with C. nana and M. lutheri and at 4 days with A. carterae after starved cultures were exposed to the vitamin. Radiocarbon uptake rates approximately reflect biomass increases. Cell numbers were proportional to vitamin concentrations when cells were incubated for 2 to 3 more days. Cell sizes varied depending on time of incubation. Chlorophyll a content did not always reflect vitamin concentrations. Maximum carbon assimilation rates (K_m) and saturation constants (K_s) determined from ¹⁴CO₂, uptake rates in different vitamin concentrations during early incubation were higher than when determined from cell number in log phase growth. Dissolved vitamin B₁₂, thiamine, and biotin in many samples of seawaters were in the ranges which influence the growth rate, cell size, and chlorophyll a content of C. nana, M. lutheri, and A. carterae, respectively, in laboratory studies. The effects of vitamins on these algae in situ may be similar.     

VARIABILITY IN NITRATE UPTAKE KINETICS IN THALASSIOSIRA PSEUDONANA (BACILLARIOPHYCEAE)1

Short-term (1–9 min) nitrate uptake kinetics were measured in Thalassiosira pseudonana (Hust.) Hasle & Heimdal grown in nitrate-limited, ammonium-limited, and nitrate-sufficient continuous cultures. For all cultures, maximal nitrate uptake rates did not develop until approximately 3 min after nitrate addition; thereafter, nitrate uptake rates remained constant or declined slightly. The K_s and V_max for the nitrate-limited cultures were higher at any growth rate than those for the ammonium-limited or nitrate-sufficient cultures. Thus, much higher nitrate concentrations would be required to saturate nitrate uptake in nitrate-limited Thalassiosira pseudonana than is usually considered necessary. The lack of data for other species grown under a range of environmental conditions makes it difficult to generalize about the effect of preconditioning on nitrate uptake kinetics.     

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

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

TEMPERATURE DEPENDENCE OF GROWTH AND PHOSPHORUS UPTAKE IN TWO SPECIES OF VOLVOX (VOLVOCALES,CHLOROPHYTA)1

The growth of Volvox globator L. and Volvox aureus Ehr. was measured at five temperatures and nine phosphorus concentrations. Growth rates were hyperbolically related to phosphorus concentrations for all temperatures using a Monod growth model. Optimal growth rates of 1.17 and 1.00 doublings d^?1 were obtained at 20°C for V. globator and V. aureus, respectively. Neither species grew at 5°C. The half-saturation constants for growth, K_s, were lower for V. aureus. Phosphorus uptake by both species was also dependent upon external phosphorus concentrations and temperature. At all temperatures, maximum phosphorus uptake (μmol P colony^?1 min^?1) was similar for both species; however, the half-saturation constants for uptake showed significant differences between the species. Comparisons of the kinetic constants for growth and phosphorus uptake suggest that V. aureus will outcompete V. globator under phosphorus limited, conditions.     

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.     

VARIABILITY OF NITRATE UPTAKE CAPACITY IN MACROCYSTIS PYRIFERA (LAMINARIALES,PHAEOPHYTA) WITH NITRATE AND LIGHT AVAILABILITY1

The nitrate uptake capacity of mature blade tissue of the giant kelp, Macrocystis pyrifera (L.) C. Ag., was examined as a function of the availability of light and nitrate. Time course measurements indicated that nitrate uptake rate, as measured by the incorporation of ¹⁵N, was significantly increased by N starvation. The response was linear over the first hour of exposure regardless of the N status of the tissue indicating that surge uptake was not responsible for the increase. The Michaelis-Menten parameters V_max and K_s, however, were not significantly changed by either growth nitrate concentration or growth irradiance as a result of high variability among blades. Similarly, the initial slope (α) of the nitrate uptake kinetics curves was unaffected. Concentration of photosynthetic pigments increased in response to increased nitrate availability but not to increased growth irradiance. Time course and pigment data demonstrated that mature blade tissue responds to increased N availability by decreasing its capacity to take up nitrate and by increasing its investment in photosynthetic pigments, perhaps for N storage or enhanced light-harvesting capabilities and the increase in reducing power available for N assimilation. This study provides evidence for a dynamic regulatory system that responds to changes in nitrate availability in an integrated manner.     

Effect of NH4 +:NO3 - ratios on growth and nitrogen uptake by onions

The modelling of ion uptake by plants requires the measurement of kinetic and growth parameters under specific conditions. The objective of this study was to evaluate the effect of nine NH _inf4 ^sup+ :NO _inf3 ^sup− ratios on onions (Allium cepa L.). Twenty-eight to 84 day-old onion plants were treated with NH _inf4 ^sup+ :NO_f3/sup− ratios ranging from 0 to 100% of each ionic species in one mM solutions in a growth chamber. Maximum N influx (I_max) was assessed using the N depletion method. Except at an early stage, ionic species did not influence significantly I_max, the Michaelis constant (K_m) and the minimum concentration for net uptake (C_min). I_max for ammonium decreased from 101 to 59 pmole cm^-2 s^-1 while I_max for nitrate increased from 26 to 54 pmole cm^-2 s^-1 as the plant matured. On average, K_m and C_min values were 14.29 μM, and 5.06 μM for ammonium, and 11.90 μM and 4.54 μM for nitrate, respectively. In general, the effect of NH₄ ⁺:NO₃ ^- ratios on root weight, shoot weight and total weight depended on plant age. At an early stage, maximum plant growth and N uptake were obtained with ammonium as the sole source of N. At later stages, maximum plant growth and N uptake were obtained as the proportion of nitrate increased in the nutrient solution. The was no apparent nutrient deficiency whatever NH₄ ⁺:NO₃ ^- ratio was applied, although ammonium reduced the uptake of cations and increased the uptake of phosphorus. The research was supported by the Natural Sciences and Engineering Research Council of Canada.     

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     

Growth of dominant zooplankton species feeding on plankton microflora in Lake Shira

Batch cultures and continuous flow cultures were used to study the growth rates of zooplankton species from Shira lake, the rotifer Brachionus plicatilis Muller and calanoid copepod Arctodiaptomus salinus Daday, which were fed on phytoplankton and bacterioplankton from the lake. Analyses of the birth and survival rates were used to demonstrate that the lake phytoplankton, consisting mostly of cyanobacteria and diatomaceous algae, is inadequotes for optimal realisation of the reproductive potential of B. plicatilis when compared with the bacterial diet. The study revealed that the kinetic growth characteristics of the two zooplankters were similar: B. plicatilis r _max, 0.120 d^–1; S ₀, 0.253; and K _s, 0.114 mg dry mass l^–1; and for A. salinus r _max, 0.129 d^–1; S ₀, 0.240; and K _s, 0.171 mg dry mass l^–1. Fluctuations in natural food concentration reduced the growth rate of both species. Even though the threshold concentration of food for B. plicatilis and A. salinus were quite similar, the copepods were less sensitive to food limitation.     

Effect of four heavy metals on the biology of Nostoc muscorum

Summary This study presents the effects of Cr, Pb, Ni and Ag on growth, pigments, protein, DNA, RNA, heterocyst frequency, uptake of NH₄ ⁺ and N0₃ ^–, loss of electrolytes (Na⁺ and K⁺), nitrate reductase and glutamine synthetase activities ofNostoc muscorum. The statistical tests revealed a direct positive correlation between the metal concentration and inhibition of different processes. Ni was found to be more toxic against growth, pigments and heterocyst differentiation compared to the other metals. Inhibition of pigment showed the following trend: chlorophyll > phycocyanin > carotenoid. No generalized trend for inhibition of macromolecules was observed. The loss of K⁺ and Na⁺ as affected by Cr, Ni and Pb was similar but more pronounced for K⁺ than Na⁺. The inhibition of physiological variables depicted the following trend: Na⁺ loss > K⁺ loss > glutamine synthetase > NH₄ uptake > growth > N0₃ ^– uptake > nitrate reductase > heterocyst frequency. This study therefore suggests that loss of electrolytes can be used as a first signal of metal toxicity in cyanobacteria. However, further study is needed to confirm whether the abnormality induced by nickel (branch formation) is a physiological or genetic phenomenon.     

EFFECTS OF FLOWING WATER ON NITROGEN- AND PHOSPHORUS-LIMITED PHOTOSYNTHESIS AND OPTIMUM N:P RATIOS BY SPIROGYRA FLUVIATILIS (CHAROPHYCEAE)1,2

The effects of flowing water on net photosynthesis, dark respiration, specific growth rate, and optimum N:P ratios by Spirogyra fluviatilis Hilse were assessed. The alga was cultivated under nitrogen or phosphorus limitation in laboratory streams at three flow velocities: 3, 12, and 30 cm·s^?1. The Droop equation adequately described respiration and photosynthesis (PS_net) as a function of N or P cell quota (Q^N or Q^p). The data show that for N- or P-limited Spirogyra fluviatilis, flowing water is physiologically costly. Generally, flowing water had little effect on respiration rates; however, the proportion of gross photosynthesis devoted to dark respiration did increase with flow velocity. For photosynthesis, the minimum N and P cell quotas increased with velocity, and the theoretical PS_net maxima for N and P both appeared greatest at 12 cm·s^?1. The Droop models showed that for any given Q^N or Q^p, PS_net, was reduced by the 30-cm·s^?1 treatment. Consistent with this finding, independent estimates of specific growth rates for P-limited S. fluviatilis in the laboratory streams were inversely related to flow velocity when ambient PO₄^?3 was undetectable. However, growth was not diminished at the fastest velocity when PO₄^?3 was available for uptake. Thus, the increase in cellular phosphorus demand can be offset by flow-enhanced P uptake when conditions permit; otherwise, growth will be impaired. The optimum N:P ratios for S. fluviatilis at 3, 12, and 30 cm·s^?1 were 50, 58, and 52 by atoms, respectively, when calculated for PS_net= 0. The optimum ratios were inversely related to PS_net and decreased to approximately 20 when PS_net was near maximum. The potential for flowing water to mediate nutrient partitioning among lotic algae by altering growth rates and optimum nutrient ratios is discussed.     

STUDIES OF CARBON AND NITROGEN METABOLISM BY THREE MARINE PHYTOPLANKTON SPECIES IN NITRATE-LIMITED CONTINUOUS CULTURE1,2

Characteristics of carbon production, excretion and dark respiration, and nitrate uptake kinetics were studied using continuous culture techniques for Thalassiosira allenii Takano, Monorhrysis lutheri Droop and Dunaliella tertiolccta Butcher. Fur T. allenii. the ratio of dark C loss to daytime net C production varied between 0.1 and 0.2 over a growth rate range from ca. 0.005 to 0.06 h^-1. For M. lutheri and D. tertiolecta. this same ratio varied belween 0.2 and 0.3 between growth rates of ca. 0.005 and 0.025 h^-1, but declined at higher growth rates when the dark nitrate uptake capacity of the cells was exceeded by the pumping rate. Carbon excretion rates averaged less than 1.5% of daytime net C production rates. Productivity indices showed little correlation with growth rate, due to the significant poisitive correlation between chl a:C ratios and growth rate. Chlorophyll a:C ratios for T. allenii were less than 0.01 al growth rates less than 0.03 h^-1, and appoached zero at zero growth rate. Dark nitrate maximum uptake rates for M. lutheri, D. tertiolecta and T. allenii averaged 23, 64 and 120%, respectively, of light nitrate maximum uptake rates. Excretion of nitrite was observed during most nitrate uptake experiments. This excretion reduced net uptake of nitrate spikes in the dark for M. lutheri and D. tertiolecta by 79 and 23%, respectively.     

Intracellular nitrite accumulation: The cause of growth inhibition of Microcystis aeruginosa exposure to high nitrite level

The aim of the present study is to test the role of intracellular nitrite in external nitrite suppressing algal growth. We examined the growth of Microcystis aeruginosa at different nitrite levels under high nitrate conditions and without nitrate conditions. There were higher intracellular nitrite and lower P_m^chla, R_d ^chla, α^chl, maximum cell density and specific growth rate in high nitrate group than nitrate absence group at 5 mg NO₂^?‐N L^?1. At 10 and 15 mg NO₂^?‐N L^?1, P_m^chla, R_d ^chla, α^chl, maximum cell densities and specific growth rates in the high nitrate group became higher than those of the nitrate absence group, while a lower intracellular nitrite in the high nitrate group than nitrate absence group was observed. In addition, the intracellular nitrite and the growth of M. aeruginosa in the high nitrate group did not change from 5 to 10 mg NO₂^?‐N L^?1. In the nitrite uptake experiment, with nitrite concentration increasing from 5 to 15 mg NO₂^?‐N L^?1, maximum nitrite uptake rate of alga increased, and half‐saturation constant of alga decreased. These results indicate that external nitrite inhibited algal growth through stimulating intracellular nitrite rise, which resulted from overexpression of nitrite transporter.     

The effect of endogenous and externally supplied nitrate on nitrate uptake and reduction in sugarbeet seedlings

The pericarp of the dormant sugarbeet fruit acts as a storage reservoir for nitrate, ammonium and -amino-N. These N-reserves enable an autonomous development of the seedling for 8–10 d after imbibition. The nitrate content of the seed (1% of the whole fruit) probably induces nitrate-reductase activity in the embryo enclosed in the pericarp. Nitrate that leaks out of the pericarp is reabsorbed by the emerging radicle. Seedlings germinated from seeds (pericarp was removed) without external N-supply are able to take up nitrate immediately upon exposure via a low-capacity uptake system (v_max = 0.8 mol NO ₃ ^- ·(g root FW)^–1·h^–1; K_s = 0.12 mM). We assume that this uptake system is induced by the seed nitrate (10 nmol/seed) during germination. Induction of a high-capacity nitrate-uptake system (v_max = 3.4 mol NO ₃ ^- ·(g root FW)^–1·h^–1; K_s = 0.08 mM) by externally supplied nitrate occurs after a 20-min lag and requires protein synthesis. Seedlings germinated from whole fruits absorb nitrate via a highcapacity uptake mechanism induced by the pericarp nitrate (748 nmol/pericarp) during germination. The uptake rates of the high-capacity system depend only on the actual nitrate concentration of the uptake medium and not on prior nitrate pretreatments. Nitrate deprivation results in a decline of the nitrate-uptake capacity (t_1/2 of v_max = 5 d) probably caused by the decay of carrier molecules. Small differences in K_s but significant differences in v_max indicate that the low- and high-capacity nitrate-uptake systems differ only in the number of identical carrier molecules.Abbreviations NR nitrate reductase - pFPA para-fluorophenylalanine This work was supported by a grant from Bundesministerium für Forschung und Technologie and by Kleinwanzlebener Saatzucht AG, Einbeck.     

Inhibitory effect of dihydroxyacetone onGluconobacter oxydans: Kinetic aspects and expression by mathematical equations

Summary Microbial conversion of glycerol into dihydroxyacetone (DHA) byGluconobacter oxydans was subjected to inhibition by excess substrate. Comparison of cultures containing increasing initial DHA contents (0 to 100 g l^–1) demonstrated that DHA also inhibited this fermentation process. The first effect was on bacterial growth (cellular development stopped when DHA concentration reached 67 gl^–1), and then on oxidation of glycerol (DHA synthesis only occurred when the DHA concentration in the culture medium was lower than 85 g l^–1). Productivity, specific rates and, to a lesser extent, conversion yields decreased as initial concentrations of DHA increased. The changes in the specific parameters according to increasing initial DHA contents were described by general equations. These formulae satisfactorily express the concave aspect of the curves and the reduction in biological activity when the cells were in contact with DHA concentrations of up to 96 g l^–1.Abbreviations X, S, P biomass, substrate, product concentrations - r _x,r _s,r _p rates of growth, consumption and production - ,q _s,q _p specific rates of growth, glycerol consumption and DHA production - Y _x/s, Y_p/s conversion yields of substrate into biomass and product - K _s constant of affinity of cells to the substrate - K _ip product inhibition constant - P _m threshold concentration of DHA in substrate     

FACTORS REGULATING THE SPATIAL DISTRIBUTION OF THE FILAMENTOUS ALGA PITHOPHORA OEDOGONIA (CHLOROPHYCEAE) IN AN INDIANA LAKE1

Pithophora oedogonia (Mont.) Wittr. biomass in Surrey Lake, Indiana was greater in the littoral than in the pelagial region. Although mean soluble reactive phosphorus concentrations did not differ between the two areas, nitrate concentrations were almost six times higher in the cove than in the open water. Using laboratory cultures of Pithophora, the half saturation constant (K_s at 20° C relating filament growth to external concentrations of nitrate-nitrogen was determined to be 1.23 mg L^?1 (=88 μM)and for phosphate-phosphorus, 0.1 mg L^?1 (=3.22 μM). These values were used to calculate a NO₃-N/PO₄-P atomic ratio of 27.6. Comparison of this value with NO₃-N/PO₄-P ratios in Surrey Lake showed that nitrogen limiting conditions were prevalent in the open water section of the lake. Alkaline phosphatase and dark ammonia uptake analyses on field collected filaments from the shallow and deep water sections confirmed the hypothesis that nitrate is the major factor limiting growth of Pithophora in Surrey Lake.