COMPARISON OF HALF-SATURATION CONSTANTS FOR GROWTH AND NITRATE UPTAKE OF MARINE PHYTOPLANKTON 2

Growth rates and rates of nitrate uptake by N-depleted cells were measured for an oceanic diatom, Chaetoceros gracilis, and a neritic diatom, Asterionella japonica, as functions of nitrate concentration of the medium. Both growth and N-uptake rates appeared to be hyperbolic with nitrate concentration and could be fit to an equation of Michaelis-Menten form: where v is rate, V^m. is the maximum rate, S is nitrate concentration, and K_sis the half-saturation constant. K_svalues for uptake and growth were similar if not identical for each species. Uptake experiments can provide a presumptive measure of K_sfor growth, thought to be an ecologically significant characteristic of a species.     

Optical characteristics of the harmful dinoflagellate Alexandrium tamarense in response to different nitrogen sources

The chlorophyll a specific absorption coefficient a_ph^* and absorption ratio (a_ph ratio) of Alexandrium tamarense at five concentrations of nitrate, ammonium and urea over a range from 6 to 100 μM were examined. The experimental results were compared to two coastal diatom species and a prymnesiophyte to identify differences in absorption ratios. Cells exposed to increasing nitrate concentrations were characterized by an increase in a_ph^* at 443, 490, 510, 555 and 675 nm. However, ammonium and urea induced low a_ph^* values at their lowest and highest concentrations. The a_ph relative to 510 or 555 nm was constant regardless of the concentration of the N source, but dependent on the N source. Oxidized N induced a lower a_ph ratio than the reduced form. Comparisons of the a_ph ratio among taxonomic groups revealed significant differences. The a_ph ratio of A. tamarense was 20–30 and >50% lower than those of two diatoms and a prymnesiophyte, respectively. The a_ph ratio of the present study could assist in increasing the capability for detecting harmful species such as A. tamarense.     

INTRASPECIFIC COMPARISONS OF NITRATE UPTAKE IN THREE MARINE DIATOMS1

Short-term (within 5 min) and long-term (≤2 h) rates of nitrate uptake were determined for the marine diatoms, Nitschiella longissima (Cleve), Skeletonema costatum (Greville) Cleve and Asterionella japonica (Cleve). Pigment levels, cell carbon, nitrogen and cell volume were also determined for cells in the logarithmic and stationary phases of growth. For each species, one clone isolated from oligotrophic coastal water and one clone isolated from eutrophic coastal water were compared. Long-term NO₃^? uptake typically followed saturation kinetics describable by the Michaelis-Menten expression. Under experimental conditions, half-saturation constants ranged from 0.6 to 2.2 μM NO₃^?. Generally, the oligotrophic clones had lower Ks and V_max (on a per cell basis) than their eutrophic counterparts, though this was only statistically significant in one pair of clones. Eutrophic and oligotrophic clones also differed in their short-term response to nutrient addition; oligotrophic clones showed greatest rate of uptake at the lowest nitrate addition while uptake by eutrophic clones increased with increasing nitrate concentration. However, all clones had very similar V_max values expressed on a dry weight basis. Under N-starvation, cellular C and pigment levels (and N to a lesser extent) generally declined more in eutrophic than in oligotrophic clones. While the differences between inshore and offshore clones were not great, the results are consistent with the hypothesis that eutrophic waters support algae which grow faster and are less conservative biochemically than cells in oligotrophic waters.     

Fate of nitrate during initial nitrate utilization by nitrogen-depleted dwarf bean

The fate of nitrate and nitrogen-15 was followed during the apparent induction phase (6h) for nitrate uptake by N-depleted dwarf bean (Phaseolus vulgaris L. ev. Witte Krombek). Experiments were done with intact plants and with detached root systems. Qualitatively and quantitatively, xylem exudation from detached roots was a bad estimate of the export of NO^?₃ or NO^?₃-¹⁵N from roots of intact plants. In vivo nitrate reductase activity (NRA) agreed well with in situ reduction, calculated as the difference between uptake and accumulation in whole plants, provided NRA was assayed with merely endogenous nitrate as substrate (‘actual’ NRA). The majority (75%) of the entering nitrate remained unmetabolized. Both nitrate reduction and nitrate accumulation occurred predominantly in the root system. Some (< 25%) of the root-reduced nitrate-N was translocated to the shoot. Nitrate uptake occurred against the concentration gradient between medium and root cells, and probably against the gradient of the electro-chemical potential of nitrate. Part of the energy expended for NO^?₃ absorption came from the tops, since decapitation and ringing at the stem base restricted nitrate uptake.     

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.     

NEUTRAL LIPID AND CARBOHYDRATE PRODUCTIVITIES AS A RESPONSE TO NITROGEN STATUS IN ISOCHRYSIS SP. (T‐ISO; HAPTOPHYCEAE): STARVATION VERSUS LIMITATION1

Partitioning of the carbon (C) fixed during photosynthesis between neutral lipids (NL) and carbohydrates was investigated in Isochrysis sp. (Haptophyceae) in relation to its nitrogen (N) status. Using batch and nitrate‐limited continuous cultures, we studied the response of these energy reserve pools to both conditions of N starvation and limitation. During N starvation, NL and carbohydrate quotas increased but their specific growth rates (specific rates of variation, μ_CAR and μ_NL) decreased. When cells were successively deprived and then resupplied with NO₃, both carbohydrates and neutral lipids were inversely related to the N quota (N:C). These negative relationships were not identical during N impoverishment and replenishment, indicating a hysteresis phenomenon between N and C reserve mobilizations. Cells acclimated to increasing degrees of N limitation in steady‐state chemostat cultures showed decreasing NL quota and increasing carbohydrate quota. N starvation led to a visible but only transient increase of NL productivity. In continuous cultures, the highest NL productivity was obtained for the highest experimented dilution rate (D = 1.0 d^?1; i.e., for non N‐limited growth conditions), whereas the highest carbohydrate productivity was obtained at D = 0.67 d^?1. We used these results to discuss the nitrogen conditions that optimize NL productivities in the context of biofuel production.     

Potential contributions of vertically migrating Rhizosolenia to nutrient cycling and new production in the open ocean

We present a numerical model of nutrient uptake and photosynthesisduring migrations of the marine diatom Rhizosolenia that wasdeveloped to estimate fluxes of carbon and nitrogen due to thesemigrations in the open ocean. The predicted specific rate ofincrease of Rhizosolenia was 0.11–0.15 day¹, whereas thetotal time for one migration cycle ranged between 3 and 5 days.Using published estimates of Rhizosolenia abundance, we estimatethat new primary production due to Rhizosolenia migrations rangesbetween 0.018 and 0.033 mmol N m^–2 day^–1. Thesevalues represent up to 17% of new production due to turbulentdiffusive fluxes of nitrate into the euphotic zone and are ofthe same order of magnitude as new production due to nitrogenfixation in tropical oceans. Large-scale contributions of Rhizosoleniato oceanic new production are limited by their relatively lowstanding crop. Variations in the formulation of losses withdepth greatly affected gross and net fluxes of carbon and nitrogen.Better characterization of losses of Rhizosolenia and improvedestimates of its abundance will help determine more accuratelythe contributions of Rhizosolenia to global biogeochemical cycles. ¹Present address: Department of Agricultural and EnvironmentalScience, The Queen's University of Belfast New forge Lane, BelfastBT95PX, UK     

CHEMICAL COMPOSITION AND PHOTOSYNTHETIC CHARACTERISTICS OF ETHMODISCUS REX (BACILLARIOPHYCEAE): EVIDENCE FOR VERTICAL MIGRATION1

Ethmodiscus rex (Rattray) Wiseman and Hendey cells from near surface net tows in the Southwest Atlantic Ocean and Caribbean Sea were examined for chemical composition, internal nutrient pool concentrations, and oxygen evolution characteristics. Elemental ratios indicated nitrogen limitation with C:N:P ratios of 125:9:1 (atoms), and carbon: chlorophyll (chl) ratios of 129:1 (weight). However, internal nitrate pools (1.4–27.1 mM) suggested that cells were not N-limited. Intracellular NO₃^? accounted for up to 54% (range = 3–54%) of the total N quota in some samples. Photosynthetic parameters were consistent with a high-light-adapted population and suggested an instantaneous maximum chl-specific photosynthetic rate (P^B_max) of 4.8–12.4 nmol O₂·μg chl^?1· h^?1. Respiration rates varied ten-fold and were inversely related to P^B_max Ethmodiscus chemical composition and buoyancy characteristics are similar to vertically migrating Rhizosolenia mats and the non-motile dinoflagellate Pyrocystis noctiluca Murray (Schuett). The presence of internal NO₃^? pools in Ethmodiscus suggests that this genus is also vertically migrating to exploit sub-surface nitrogen pools. Such behavior may be widespread in large, non-motile oceanic phytoplankton. Based on ascent rate data, chemical composition, and photosynthetic rates, we estimate that the entire division–migration cycle for Ethmodiscus requires at least 7–12 days.     

COMPARATIVE KINETIC STUDIES OF NITRATE-LIMITED GROWTH AND NITRATE UPTAKE IN PHYTOPLANKTON IN CONTINUOUS CULTURE1

A comparative study of nitrate-limited growth and nitrate uptake was carried out in chemostat cultures of Ankistrodesmus falcatus (Corda) Ralfs., Asterionella formosa Hass., and Fragilaria crotonensis Kit. In each species growth rate (μ) was related to total cell nitrogen or cell quota (q) by the empirical Droop growth function. Nitrate uptake was a function of both external N concentration and q. The apparent maximum uptake rate (V_m') at a given μ was inversely related to q – q₀, where q₀ is the minimum quota. The apparent half-saturation constant for uptake, (K_m') appears to show a slight inverse trend with μ, although statistical analysis shows that this trend is inconclusive. When q approaches q₀, V_m' is several orders of magnitude greater than μq, the calculated steady-state uptake rate. As q increases, however, the difference between these two variables decreases sharply until q approaches q_m, the cell quota for nitrogen-rich cells. At this point the difference between μq and V_m' disappears. This behavior is explained by the feedback regulation of N uptake. The inverse relationship between V_m' and q – q₀ can be described by an empirical three-parameter equation.     

BIOLOGICAL AND CHEMICAL CHARACTERISTICS OF THE GIANT DIATOM ETHMODISCUS (BACILLARIOPHYCEAE) IN THE CENTRAL NORTH PACIFIC GYRE

Cells of the giant diatom Ethmodiscus Castr. gathered from the upper 15 m were examined for O₂ evolution, nitrate reductase activity (NRA), C and N composition, internal NO concentrations, , and ¹⁵NO, ¹⁵NH , and ³²Si uptake in a series of cruises in the central N. Pacific gyre. The δ¹⁵N (2.56–5.09 ‰), internal NO concentrations (0.0– 11.5 mM NO⁻), and NRA (6.7 ± 4.7 × 10⁻⁴μM NO cell ⁻¹·h⁻¹) were consistent with recent exposure to elevated nitrate concentrations and utilization of deep NO as a primary N source. These results are similar to other diatoms that migrate vertically to the nutricline as part of their life cycle. Rate measures (Si[OH]₄ uptake, NRA, and O₂ evolution) indicated surface doubling times from 45 h to 75 h. Both NO and NH uptake in surface waters were low and inadequate to supply N needs at surface NO and NH concentrations. Our results suggest a partitioning in nutrient acquisition, with N acquired at depth and C and Si acquired at the surface. Doubling rates were two to three times higher than predicted from cell volume and C content models. These data are consistent with the observed elemental content being lower than expected because of the dominance of cell volume by the vacuole. Our calculations suggest that Ethmodiscus contributes little to the biogeochemistry of the upper water column via upward nutrient transport. Although reported as a paleo-upwelling indicator, thisevidence suggests that Ethmodiscus has adapted to the nutrient-poor open ocean by a vertical migration strategy and has biological characteristics inconsistent with a upwelling indicator.     

NUTRITIONAL STUDIES OF TWO RED ALGAE. II. KINETICS OF AMMONIUM AND NITRATE UPTAKE1, 2

Similar NH₄₊ and NO₃^?.uptake kinetic patterns were observed in Neoagardhiella baileyi (Harvey ex Kiitzing) Wyinne & Taylor and Gracilaria foliifera (Forssk?l) Borgesen. NO₃^? was taken up in a rate-sturating fashion described by the Michaelis-Menten equation. NH₄₊ uptake was multicomponent: a saturable component was accompanied by a diffusive or a high K component showing no evidence of saturation (at ≤50 μM [NH₄₊]). Nitrogen starved plantsi(C/N atom ratios > ca. 10) showed higher transient rates of NH₄₊ uptake at a given concentration than plants not N-Iimited. Only plants with high N content exhibited diel changes inNH₄₊ uptake rates, and showed transient rates of NH₄₊ accumulation which did not greatly exceed the capacity to incorporate N in steady-state growth. NH₄₊ was preferred over NO_3?even in plants preconditioned on NO_3?as the sole N. source, NO_3? uptake was suppressed at 5μM [NH₄₊], but simultaneous uptake occurred at unsurpressed rates at lower concentrations. Potential for N accumulation was greater via NH₄₊uptake than via NO_3?uptake. Changing capacity for NH₄₊ uptake with N content appears to be a mechanism whereby excessive accumulation of N was avoided by N-.satiated plants but a large accumulation was possible for N-depleted plants.     

Nitrate (15NO3) limitation affects nitrogen partitioning between metabolic and storage sinks and nitrogen reserve accumulation in chicory (Cichorium intybus L.)

In chicory, we examined how NO₃ ⁻ supply affected NO₃ ⁻ uptake, N partitioning between shoot and root and N accumulation in the tuberized root throughout the vegetative period. Plants were grown at two NO₃ ⁻ concentrations: 0.6 and 3 mM. We used ¹⁵N-labelling/chase experiments for the quantification of N fluxes between shoot and root and for determining whether N stored in the tuberized root originates from N remobilized from the shoot or from recently absorbed NO₃ ⁻. The rate of ¹⁵NO₃ ⁻ uptake was decreased by low NO₃ ⁻ availability at all stages of growth. In young plants (10–55 days after sowing; DAS), in both NO₃ ⁻ treatments the leaves were the strongest sink for ¹⁵N. In mature (tuberizing) plants, (55–115 DAS), the rate of ¹⁵NO₃ ⁻ uptake increased as well as the amount of exogenous N allocated to the root. In N-limited plants, N allocation to the tuberized root relied essentially on recent N absorption, while in N-replete plants, N remobilized from the shoot contributed more to N-reserve accumulation in the root. In senescing plants (115–170 DAS) the rate of ¹⁵NO₃ ⁻ uptake decreased mainly in N-replete plants whereas it remained almost unchanged in N-limited plants. In both NO₃ ⁻ treatments the tuberized root was the strongest sink for recently absorbed N. Remobilization of previously absorbed N from shoot to tuberized root increased greatly in N-limited plants, whereas it increased slightly in N-replete plants. As a consequence, accumulation of the N-storage compounds vegetative storage protein (VSP) and arginine was delayed until later in the vegetative period in N-limited plants. Our results show that although the dynamics of N storage was affected by NO₃ ⁻ supply, the final content of total N, VSP and arginine in roots was almost the same in N-limited and N-replete plants. This indicates that chicory is able to build up a store of available N-reserves, even when plants are grown on low N. We also suggest that in tuberized roots there is a maximal capacity for N accumulation, which was reached earlier (soon after 100 DAS) in N-replete plants. This hypothesis is supported by the fact that in N-replete plants despite NO₃ ⁻ availability, N accumulation ceased and significant amounts of N were lost due to N efflux. Received: 14 October 1996 / Accepted: 4 February 1997     

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.     

EFFECTS OF LOW NITROGEN LEVELS AND VARIOUS NITROGEN SOURCES ON GROWTH AND WHORL DEVELOPMENT IN ACETABULARIA (CHLOROPHYTA)1

Nitrate, ammonia, urea, and glycine were compared as nitrogen sources for Acetabularia mediterranea. Cells grew normally in media containing nitrate or urea, while cells did not grow at all when the same amount of N was supplied as ammonium ion. The utilization of glycine remains questionable. Cells in medium without added N (NDM) increased in length and some formed reproductive caps. The whorls of vegetative cells showed considerable hypertrophy in NDM and in glycine. This hypertrophy was due to the elongation of only the first-(a₁) and second- (a₂) order articles. When cut, the basal portion of cells without added N regenerated new apices with whorls. The development of these whorls was inversely proportional to the NO₂ concentration. Analyses showed that the intracellular nitrogen pool in young cells and regenerating bases was very small, about 1/10 of that of fully grown cells. Therefore we suggest that trace amounts of N contaminants in the medium supported growth and development, the uptake of which was facilitated by the hypertrophied whorls, under N-limited conditions.     

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

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

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.     

RAPID AMMONIUM‐ AND NITRATE‐INDUCED PERTURBATIONS TO CHL a FLUORESCENCE IN NITROGEN‐STRESSED DUNALIELLA TERTIOLECTA (CHLOROPHYTA)1

When NH₄ ⁺ or NO₃ ^? was supplied to NO₃ ^? ‐stressed cells of the microalga Dunaliella tertiolecta Butcher, immediate transient changes in chl a fluorescence were observed over several minutes that were not seen in N‐replete cells. These changes were predominantly due to nonphotochemical fluorescence quenching. Fluorescence changes were accompanied by changes in photosynthetic oxygen evolution, indicating interactions between photosynthesis and N assimilation. The magnitude of the fluorescence change showed a Michaelis‐Menten relationship with half‐saturation concentration of 0.5 μM for NO₃ ^? and 10 μM for NH₄ ⁺ . Changes in fluorescence responses were characterized in D. tertiolecta both over 5 days of N starvation and in cells cultured at a range of NO₃ ^? ‐limited growth rates. Variation in responses was more marked in starved than in limited cells. During N starvation, the timing and onset of the fluorescence responses were different for NO₃ ^? versus NH₄ ⁺ and were correlated with changes in maximum N uptake rate during N starvation. In severely N‐starved cells, the major fluorescence response to NO₃ ^? disappeared, whereas the response to NH₄ ⁺ persisted. N‐starved cells previously grown with NH₄ ⁺ alone showed fluorescence responses with NH₄ ⁺ but not NO₃ ^? additions. The distinct responses to NO₃ ^? and NH₄ ⁺ may be due to the differences between regulation of the uptake mechanisms for the two N sources during N starvation. This method offers potential for assessing the importance of NO₃ ^? or NH₄ ⁺ as an N source to phytoplankton populations and as a diagnostic tool for N limitation.     

EFFECTS OF NITROGEN SOURCES ON P-LIMITED GROWTH OF ANABAENA FLOS-AQUAE1

The effects of N₂, nitrate and ammonia as N sources were investigated in P-limited and nutrient-sufficient cultures of Anabaena flos-aquae (Lyngb.) Bréb. The maximum growth rate (μ_m) was highest at 1.34 d^?1 with ammonia, compared to 1.18 with nitrate and 0.95 d^?1 with N₂. There was no difference in P requirement between N₂ and nitrate cultures. Under P-limited conditions, the increase in cell P with growth rate (μ) was identical. With N₂ as the N source, cell-N concentrations in P-limited cells increased with μ as did cell P, and the cellular N:P ratio remained the same (14) within the range of μ examined. With nitrate, however, cell N concentrations were high and independent of n, except at a low μ. It appears that this organism fixes atmospheric N₂ only at the minimum concentration required to maintain a μ. The acetylene reduction rate increased with μ in both N_2- and nitrate-grown cells, but the rate was lower in nitrate. Under P-limitation, there was no difference in net C-fixation rate per cell between N₂ and nitrate cultures at a given μ. However, the rate per unit of chlorophyll a (chl a) was higher in N₂ than in nitrate cultures, and the rate was independent of μ with N₂ but was a linear function of nitrate supplied. The maximum C-fixation rate in nutrient sufficient cells was highest with ammonia, followed by nitrate and N₂. The cellular chl a concentration was correlated with the total cell-N concentrations regardless of H and the source of N.     

Nutrient uptake and alkaline phosphatase (ec 3:1:3:1) activity of emiliania huxleyi (PRYMNESIOPHYCEAE) during growth under n and p limitation in continuous cultures

Emiliania huxleyi (strain L) expressed an exceptional P assimilation capability. Under P limitation, the minimum cell P content was 2.6 fmol P·cell^?1, and cell N remained constant at all growth rates at 100 fmol N·cell^?1. Both, calcification of cells and the induction of the phosphate uptake system were inversely correlated with growth rate. The highest (cellular P based) maximum phosphate uptake rate (V_max^P) was 1400 times (i.e. 8.9 h^?1) higher than the actual uptake rate. The affinity of the P‐uptake system (dV/dS) was 19.8 L·μmol^?1·h^?1 at μ = 0.14 d^?1. This is the highest value ever reported for a phytoplankton species. V_max and dV/dS for phosphate uptake were 48% and 15% lower in the dark than in the light at the lowest growth rates. The half‐saturation constant for growth was 1.1 nM. The coefficient for luxury phosphate uptake (Q_maxt/Q_min) was 31. Under P limitation, E. huxleyi expressed two different types of alkaline phosphatase (APase) enzyme kinetics. One type was synthesized constitutively and possessed a V_max and half‐saturation constant of 43 fmol MFP·cell^?1·h^?1 and 1.9 μM, respectively. The other, inducible type of APase expressed its highest activity at the lowest growth rates, with a V_max and half‐saturation constant of 190 fmol MFP·cell^?1·h^?1 and 12.2 μM, respectively. Both APase systems were located in a lipid membrane close to the cell wall. Under N‐limiting growth conditions, the minimum N quotum was 43 fmol N·cell^?1. The highest value for the cell N‐specific maximum nitrate uptake rate (V_max^N) was 0.075 h^?1; for the affinity of nitrate uptake, 0.37 L·μmol^?1·h^?1. The uptake rate of nitrate in the dark was 70% lower than in the light. N‐limited cells were smaller than P‐limited cells and contained 50% less organic and inorganic carbon. In comparison with other algae, E. huxleyi is a poor competitor for nitrate under N limitation. As a consequence of its high affinity for inorganic phosphate, and the presence of two different types of APase in terms of kinetics, E. huxleyi is expected to perform well in P‐controlled ecosystems.