Partitioning of nitrogen and carbon in cultures of the marine diatom Thalassiosira fluviatilis supplied with nitrate,ammonium, or urea

Small or negligible differences in growth rates, average cell size, yields in cell numbers and total cell volumes were found in cultures of Thalassiosira fluviatilis inriched with nitrate, ammonium, or urea. Intracellular pools of unassimilated nitrate, nitrate, and ammonium were found in nutrient-rich conditions, but urea was not accumlated internally. Nitrogen assimilation into organic combination rather than nitrogen nutrient uptake was a critical rate-limiting step in nitrogen utilization. The free amino acid pool, protein, lipid-associated nitrogen, pigments, and total cell nitrogen were all highest in young or mature phase cells and decreased with age in senescent cells, whereas chitan, lipid, carbohydrate, and total cellular carbon all continued to increase during senescence. Dissolved organic nitrogen compounds accumulated in the medium only during senescence. C:N and lipid:protein were sensitive indicators of nitrogen depletion and age in T. fluviatilis.     

Interactions between nitrate and ammonium uptake: variation with growth rate,nitrogen source and species

The interaction between nitrate and ammonium uptake was examined as a function of preconditioning growth rate and nitrogen source by adding nitrate, ammonium, or both to nitrogen-sufficient,-deficient, and-starvedSkeletonema costatum (Grev.) Cleve and nitrogen-deficientChaetoceros debilis Cleve. By simultaneously measuring the internal accumulation of intermediates of nitrogen assimilation and the rates of nitrogen assimilation, the metabolic control of nitrogen uptake could be assessed. After the simultaneous addition of nitrate and ammonium to culture, both nitrate and ammonium uptake rates were decreased in comparison with the rates observed when each was added alone, although nitrate uptake was usually decreased more than ammonium uptake. Since both nitrate and ammonium uptake rates vary with time, preconditioning growth conditions, nitrogen sources present, and species, it was necessary to use several different indices to quantify inhibition. In general, ammonium inhibition of nitrate uptake inS. costatum was greatest in cultures preconditioned to ammonium and those at low growth rates, whereas ammonium uptake was inhibited most in cultures preconditioned to nitrate. In nitrogen-deficientC. debilis, nitrate uptake was more inhibited by ammonium, but uptake returned to normal rates more quickly than inS. costatum, whereas inhibition of ammonium uptake was similar. These results explain why the interaction between nitrate and ammonium uptake in the field can be so variable. Inhibition of uptake is not controlled by internal ammonium or total amino acids, nor is it related to the inability to reduce nitrate. Instead, inhibition must be determined in part by the external concentration of nitrogen compounds and in part by some intermediate(s) of nitrogen assimilation present inside the cell.Bigelow Laboratory Contribution No. 82022     

CO2 enrichment inhibits shoot nitrate assimilation in C3 but not C4 plants and slows growth under nitrate in C3 plants

The CO2 concentration in Earth's atmosphere may double during this century. Plant responses to such an increase depend strongly on their nitrogen status, but the reasons have been uncertain. Here, we assessed shoot nitrate assimilation into amino acids via the shift in shoot CO2 and O2 fluxes when plants received nitrate instead of ammonium as a nitrogen source (deltaAQ). Shoot nitrate assimilation became negligible with increasing CO2 in a taxonomically diverse group of eight C3 plant species, was relatively insensitive to CO2 in three C4 species, and showed an intermediate sensitivity in two C3-C4 intermediate species. We then examined the influence of CO2 level and ammonium vs. nitrate nutrition on growth, assessed in terms of changes in fresh mass, of several C3 species and a Crassulacean acid metabolism (CAM) species. Elevated CO2 (720 micromol CO2/mol of all gases present) stimulated growth or had no effect in the five C3 species tested when they received ammonium as a nitrogen source but inhibited growth or had no effect if they received nitrate. Under nitrate, two C3 species grew faster at sub-ambient (approximately 310 micromol/mol) than elevated CO2. A CAM species grew faster at ambient than elevated or sub-ambient CO2 under either ammonium or nitrate nutrition. This study establishes that CO2 enrichment inhibits shoot nitrate assimilation in a wide variety of C3 plants and that this phenomenon can have a profound effect on their growth. This indicates that shoot nitrate assimilation provides an important contribution to the nitrate assimilation of an entire C3 plant. Thus, rising CO2 and its effects on shoot nitrate assimilation may influence the distribution of C3 plant species.     

Short-term interaction between nitrate and ammonium uptake inThalassiosira pseudonana: Effect of preconditioning nitrogen source and growth rate

The effect of preconditioning nitrogen source and growth rate on the interaction between nitrate and ammonium uptake was determined inThalassiosira pseudonana (Clone 3H). A new method, using cells on a filter (Parslow et al. 1985), allowed continuous measurement of uptake from 0.5 to 9 min after the addition of nitrate, ammonium, or both, with no variation in concentration during the course of the experiment. For each preconditioning N source and growth rate, a series of uptake experiments was conducted, including controls with only nitrate or only ammonium, and others with different combinations of concentrations of nitrate and ammonium. For the first time, preference for ammonium was separated from inhibition of nitrate uptake by ammonium. Ammonium was the preferred N source, i.e. if nitrate and ammonium were presented separately, ammonium uptake rates exceeded nitrate uptake rates. Preference for ammonium varied with both preconditioning N source and growth rate. Inhibition of nitrate uptake by ammonium, determined by comparing nitrate uptake in the presence and absence of ammonium, was observed at ammonium concentrations > 1µM, but was rarely complete. Inhibition of nitrate uptake by ammonium was less in the ammonium-limited culture than in the cultures growing on nitrate, but invariant with growth rate in the nitrate-grown cultures. Below 1µM ammonium, nitrate uptake was often stimulated and rates exceeded those in the controls without ammonium. Ammonium uptake was not inhibited by the presence of nitrate.T. pseudonana fits the classical view of the interaction between nitrate and ammonium uptake in some respects, such as preference for ammonium, and inhibition of nitrate uptake by ammonium concentrations > 1µM. However, at ammonium concentrations typical of most marine environments, nitrate uptake occurs at rapid rates. In other respects, N uptake inT. pseudonana deviates from the classical view in the following ways: (1) stimulation of nitrate uptake by low concentrations of ammonium; (2) lack of inhibition of nitrate uptake by ammonium at low nitrate concentrations; and (3) variation in preference and inhibition with preconditioning, which is markedly different for other species. Because of the apparent enormous species variation in the interaction between nitrate and ammonium uptake and the lack of detailed information for a variety of species, it is difficult to generalize about the effect of ammonium on nitrate uptake, especially in the field, where prior N availability and species composition are not usually addressed.     

Effect of nitrogen supply on nitrogen uptake,accumulation and assimilation in Porphyra perforata (Rhodophyta)

Porphyra perforata J. Ag. was collected from a rocky land-fill site near Kitsilano Beach, Vancouver, British Columbia, Canada and was grown for 4 d in media with one of the following forms of inorganic nitrogen: NO ₃ ^- , NH ₄ ⁺ and NO ₃ ^- plus NH ₄ ⁺ and for 10 d in nitrogen-free media. Internal nitrogen accumulation (nitrate, ammonium, amino acids and soluble protein), nitrate and ammonium uptake rates, and nitrate reductase activity were measured daily. Short initial periods (10 to 20 min) of rapid ammonium uptake were common in nitrogen-deficient plants. In the case of nitrate uptake, initial uptake rates were low, increasing after 10 to 20 min. Ammonium inhibited nitrate uptake for only the first 10 to 20 min and then nitrate uptake rates were independent of ammonium concentration. Nitrogen starvation for 8 d overcame this initial suppression of nitrate uptake by ammonium. Nitrogen starvation also resulted in a decrease in soluble internal nitrate content and a transient increase in nitrate reductase activity. Little or no decrease was observed in internal ammonium, total amino acids and soluble protein. The cultures grown on nitrate only, maintained high ammonium uptake rates also. The rate of nitrate reduction may have limited the supply of nitrogen available for further assimilation. Internal nitrate concentrations were inversely correlated with nitrate uptake rates. Except for ammonium-grown cultures, internal total amino acids and soluble protein showed no correlation with uptake rates. Both internal pool concentrations and enzyme activities are required to interpret changes in uptake rate during growth.     

Saturated uptake kinetics: transient response of the marine diatom Thalassiosira pseudonana to ammonium,nitrate, silicate or phosphate starvation

Changes in the saturated uptake kinetics of the limiting nutrient were followed as Thalassiosira pseudonana (Clone 3 H) batch cultures entered ammonium, nitrate, silicate and phosphate starvation. Cultures starved of ammonium or phosphate developed very high specific uptake capacities over a 24 to 48 h starvation period, due to both decreases in cell quota and increases in uptake rates per cell. In particular, the cell phosphorus quota decreased ca. 8-fold during phosphate starvation and specific uptake rates exceeded 100 d^-1. In contrast, cultures entering nitrate or silicate starvation underwent little or no further cell division, and the uptake capacity declined during starvation. After 24 to 48 h starvation, an induction requirement for uptake of nitrate or silicate was apparent. These responses are consistent with adaptation to the pattern of supply of these nutrients in the field.     

Inorganic nitrogen metabolism in marine bacteria: Nitrate uptake and reduction in a marine pseudomonad

Growth experiments in batch cultures indicated that the uptake of nitrate by the marine pseudomonad PL1 was inhibited in the presence of ammonia provided that the ammonia concentration was higher than 1 mM. At ammonia concentrations of less than about 1 mM, however, both nitrate and ammonia were utilised simultaneously. The saturation constants for nitrate and ammonia uptake were both 2.6x10^-4 M, and similar to the Michaelis constants of nitrate reductase for nitrate (2.9x10^-4 M) and glutamine synthetase for ammonia (2x10^-4 M). Nitrate reductase activity linked to NADH was detected in chemostat-grown cultures with nitrate as nitrogen source, and in cultures containing limiting concentrations of nitrate and ammonia, ammonia or glutamate. Enzyme synthesis appeared to be repressed in cultures containing an excess of ammonia or glutamate. Chemostat cultures utilised ammonia or glutamate in preference to nitrate, while there was no marked preference between ammonia and glutamate.     

Seasonal variation in total and soluble tissue nitrogen of Pleurophycus gardneri (Phaeophyceae: Laminariales) in relation to environmental nitrate

Seasonal variations in tissue nitrogen (ethanol soluble nitrate and ninhydrin positive substances, as well as total nitrogen) of different thallus parts of Pleurophycus gardneri Setchell and Saunders were monitored simultaneously with ambient seawater nitrate from 1982 until 1984 in Bamfield, Vancouver Island, British Columbia, Canada. A trend of low, nearly zero levels in ambient nitrate typical for the area in late spring and early summer normally contrasts with average nitrate concentrations of 10 mol NO₃ ^- l^-1 in late fall and winter. Total nitrogen content was greater in the perennial thallus parts, stipe and holdfast than in the annual blade and peaked in fall and early winter. The longitudinal thallus distribution of nitrate revealed a distinct and significant concentration of nitrate in the haptera reaching at maximum 8% nitrate-N of the internal total nitrogen. Internal nitrate concentration ranged from 20 to 5 000 times the ambient nitrate concentration in the midrib, and from 40 to 3 100 times in the wing, while the range was greatest with 400 to 14 000 times in the haptera. P. gardneri contained at most about 7 mol NO₃ ^- g fresh wt^-1 in the blade, which corresponds to about 6% of total tissue nitrogen. Ninhydrin positive substances comprised the major portion of the soluble N pool in P. gardneri and showed a pronounced seasonality. Concentrations of ninhydrin positive substances ranged from 20 to 800 g N g fresh wt^-1 in the midrib and in the wing. In the stipe, ninhydrin positive substances varied from 180 to 2 200 g N g fresh wt^-1, and from 250 to 1 200 g N g fresh wt^-1 in the haptera. Evidence is given that (1) the perennial parts, stipe and haptera of P. gardneri contain the majority of nitrogen products independent of season and ontogenetic stage; (2) ninhydrin positive substances are the most abundant internal nitrogen constituents; (3) the low N values in the blade in summer suggest a nitrogen limited growth; and (4) nitrate may not be the predominant external nitrogen source.     

The rapid response of the marine diatom Skeletonema costatum to changes in external and internal nutrient concentration

A nitrogen-deficient batch culture of the marine diatom Skeletonema costatum, when resupplied with a mixture of nitrate and ammonium, showed an initial enhanced nitrate uptake rate leading to a large internal concentration (pool) of nitrate. Following this initial nitrate uptake event, nitrate uptake ceased, and nitrate assimilation was inhibited until the ammonium present was used. At this point, nitrate uptake resumed and nitrate assimilation began. No internal ammonium pool was observed during nitrate utilization, but a large nitrate pool remained throughout the utilization of external nitrate. The internal nitrate pool decreased rapidly after exhaustion of nitrate from the culture medium, but growth of cellular particulate nitrogen continued for about 24 h. A mathematical simulation model was developed from these data. The model cell consisted of a nitrate pool, ammonium pool, dissolved organic nitrogen pool, and particulate nitrogen. It was found that simple Michaelis-Menten functions for uptake and assimilation gave inadequate fit to the data. Michaelis-Menten functions were modified by inclusion of inhibitory and stimulatory feedback from the internal pools to more accurately represent the observed nutrient utilization.     

Dynamique du phytoplancton et matière organique particulaire dans la zone euphotique de l'Atlantique Equatorial

At two fixed stations in the Equatorial Atlantic Ocean (0°–4° W), the physical, chemical and biological properties of the euphotic layer were determined for 14 d (Station A: 5–18 February, 1979) and 13 d (Station B: 20 October–7 November, 1979), respectively. The stability of the water column allowed comparison of 3 different “systems”: (i) a well-illuminated and nitrate-depleted mixed layer; (ii) a chlorophyll maximum layer (chl a _max) in the thermocline which is poorly illuminated (6.3% of surface irradiance); (iii) a well-illuminated but nitrate-rich (>0.9 μg-at l^-1) mixed layer. In each layer the particulate organic carbon (COP), nitrogen (NOP) and phosphorus (POP) contents were measured and compared with the phytoplankton biomass. In the chlorophyll maximum layer, the phytoplankton biomass contributed significantly to the total particulate organic matter (between 55 and 75%). In the nitrate-depleted mixed layer, the results varied according to whether the regression technique [COP=f(chl a)] was used, or the chl a synthesis during the incubation of the samples. With the former technique, the phytoplankton carbon (C _p) content appeared minimal, because the y intercept, computed using all the data of the water column, was probably overestimated for this layer. POP would be more associated with living protoplasm than with carbon and nitrogen in the three layers. In the chlorophyll a maximum layer it constitutes a valuable detritus-free biomass measurement, since 80% of the POP consist of phytoplankton phosphorus. The assimilation numbers (NA=μg C μg chl a ^-1 h^-1) were high in all three layers, but the highest values were recorded in the nitrate-depleted mixed layer (NA=15 μg C μg chl a ^-1 h^-1). In the chlorophyll maximum layer, light would be a limiting factor during incubation: between 10²⁵ and 8.10²⁴ quanta m^-2 d^-1 NA and light are positively correlated independant of nitrate concentration. The growth rates of phytoplankton (μ) were estimated and compared to the maximum expected growth rate. Our main conclusion was that despite very low biomass and nutrient content, the mixed layer was in a highly dynamic state, as evidenced by high rates of phytoplankton growth and short nutrient turnover times (1 d or less for PO-P₄ in the mixed layer versus 3 d in the thermocline). The presence of nitrate in the water column allows the development of a higher phytoplankton biomass but does not increase growth rate.     

Studies on catalytic reduction of nitrate in groundwater

Catalytic reduction of nitrate in groundwater by sodium formate over the catalyst was investigated. Pd-Cu/γ-Al₂O₃ catalyst was prepared by impregnation and characterized by brunauer-emmett-teller (BET), inductive coupled plasma (ICP), X-ray diffraction (XRD), transmission electron microscopy (TEM) and energy dispersive X-ray (EDX). It was found that total nitrogen was effectively removed from the nitrate solution (100 mg/L) and the removal efficiency was 87%. The catalytic activity was affected by pH, catalyst amount used, concentration of sodium formate, and initial concentration of nitrate. As sodium formate was used as reductant, precise control in the initial pH was needed. Excessively high or low initial pH (7.0 or 3.0) reduced catalytic activity. At initial pH of 4.5, catalytic activity was enhanced by reducing the amount of catalyst, while concentrations of sodium formate increased with a considerable decrease in N₂ selectivity. In which case, catalytic reduction followed the first order kinetics.     

Significance of cellular nitrate content in natural populations of marine phytoplankton growing in shipboard cultures

Phytoplankton intracellular nitrate concentrations have been monitored in a 56-h experiment on a shipboard culture of surface sea water from an upwelling region. These measurements were related to parameters of biomass (particulate nitrogen) and nitrate assimilation using the ¹⁵N isotope technique and the nitrate reducase (NR) assay. The procedure for measuring cellular nitrate concentrations is described. This parameter exhibited diurnal variations, ranging from 3.1 to 20.6 ng-at nitrate per g-at particulate nitrogen, and could be correlated positively with NR activity. Nitrogen budgets show that NR activity represents only 12% of nitrate incorporation in organic phytoplankton material when nitrate is available in the sea water. However, upon depletion of the environmental nitrate (zero uptake), NR activity can fully account for the decrease of internal nitrate. From the results, it seems that internal nitrate content is a better index of nitrate consumption by marine phytoplankton than the external concentration of nitrate-nitrogen.     

Regulation of growth in Laminaria digitata: use of in-vivo nitrate reductase activities as an indicator of nitrogen limitation in field populations of Laminaria spp.

The seasonal variation in growth rate of a population of Laminaria digitata (Huds.) Lamour growing at Arbroath, Scotland was studied between August 1981 and September 1982, and was found to follow the biphasic annual cycle typical of this genus. Growth rates were maximum (0.3 cm cm^-1 mo^-1) in early June and minimum (0.05 cm cm^-1 mo^-1) between September and January. An analysis of the relationship between the seasonal changes in environmental factors (inorganic nitrogen concentrations, irradiance and temperature) with those of growth rate and the accumulation or mobilisation of cellular reserves of carbohydrates and nitrate, indicated that growth was nitrogen-limited between June and October and light-limited (with a possible co-involvement of temperature) for the remainder of the year. These conclusions were supported by the seasonal changes in the ratio of actual: potential in-vivo nitrate reductase activities in L. digitata, thus confirming the suitability of this technique for monitoring the occurrence of nitrogen limitation in Laminaria spp. The seasonal changes in blade nitrate reductase activities closely followed those of growth rate, with maximum activities [0.3 mol NO ₃ ^- reduced g^-1 (wet wt) h^-1] being present in late May and minimum levels [0.01 mol NO ₃ ^- reduced g^-1 (wet wt) h^-1] occurring between November and March. The correlation observed between nitrate reductase activities and growth rate is consistent with the ability of Laminaria spp. to store excess inorganic nitrogen, available during winter and early spring, as NO ₃ ^- , and with the requirement to conserve enzyme protein during the summer period of nitrogen limitation.     

Nitrogenous nutrition in the euphotic zone of the Central North Pacific Gyre

The uptake rates of the three nitrogen compounds ammonium, nitrate, and urea were measured in the oligotrophic North Central Pacific Gyre in August–September 1985. The measurements were performed by using ¹⁵N-labelled substrates and incubating for short-time periods (3 to 4 h) under simulated in situ conditions. Ambient concentrations of the nitrogenous nutrients were generally below 0.10 mol l^-1. The average total daily nitrogen uptake rate, integrated over the euphotic zone, was 12.5 mmol N m^-2 d^-1. Diel studies in the upper water mass resulted in a calculated phytoplankton growth rate of 1.3 d^-1. Ammonium was the dominating nutrient, accounting for on the average 54% of the total nitrogen uptake, while urea uptake represented 32% and nitrate 14%. Ammonium uptake rates at a coastal station off the Hawaiian Islands were very close to the rates found at the oceanic station. Organisms <3 m dominated the nitrogen assimilation, being responsible for about 75% of the ammonium uptake. The nitrogen uptake rates in this study seem to be higher than those found by earlier investigations in the area, but correlated well with other productivity measurements performed during the same cruise.     

Regulation of photochemical activity in cultured symbiotic dinoflagellates under nitrate limitation and deprivation

The effects of nitrate limitation and nitrate starvation on the photochemical efficiency of photosystem II (Fv/F m) were examined in batch cultures of two species of symbiotic dinoflagellates, Symbiodinium kawagutii and S. pilosum. F v/F m values were determined along growth curves and show that the F v/F m values are negatively correlated with external nitrogen concentrations in cultures of both species. Changes in growth irradiances in the batch cultures due to increments of the cell densities were estimated S. kawagutii cultures showed a negative correlation between F v/F m and growth irradiance. These results indicate that F v/F m is dependent on the light history of the cultures and on the individual sensitivity of each species, and independent of their nutrient status. Nitrate starvation was analyzed by measuring changes in the quantum yield of fluorescence (Fv/F m), electron transport rate (ETR) and non-photochemical quenching (NPQ) at five time points along the growth curves under three conditions: control (C), without nitrogen (N–), and with ammonia (N+) as a nitrogen source sufficient to meet daily nitrogen requirements. Cells collected during the exponential growth phase and exposed to N– and N+ showed significant reductions in their maximum ETR relative to controls (20% in S. pilosum and 40% in S. kawagutii). The loss of electron transport capacity is consistent with a sink limitation rather than the result of nitrogen starvation. Under nitrate-starvation, the induction of NPQ resulted in effective protection against photosystem II damage in S. pilosum. In contrast, S. kawagutii cells failed to induced NPQ resulting in a concomitant increase in the excitation pressure over photosystem II leading to damage. Collectively the data indicate that F v/F m is not a robust indicator of nitrogen limitation in symbiotic dinoflagellates and that protection against photosystem II damage under sink limitations, is largely dependent on the differential capacities of each species to induce NPQ.Communicated by P.W. Sammarco, Chauvin     

Nitrate metabolism in abyssal waters

Activities of nitrate assimilation and nitrate reduction were measured 50 cm above the ocean floor (5,845 m and 5,207 m) by an in situ ¹⁵N tracer technique at stations in the subtropical (28°29.8′N; 144°58′E) and subarctic (44°10.2′N; 154°03′E) western North Pacific Ocean. Nitrate assimilation ranged from 0.009 to 0.11 μg-at N/1/day, and nitrate reduction from 0 to 0.42 μg-at N/1/day in the presence of added peptone and yeast extract. Nitrate assimilation was higher than nitrite formation at the southern station, but the reverse was the case at the subarctic station. No correlation was observed between bacterial growth and nitrate metabolizing activities. Data are also presented on the effect of hydrostatic pressures upon nitrate metabolism by microbial populations in the surface waters.     

Development of rapid ammonium uptake during starvation of batch and chemostat cultures of the marine diatom Thalassiosira pseudonana

Cell nitrogen quotas and uptake rates following ammonium additions were measured during ammonium-limited growth transients obtained by starving batch and chemostat cultures of Thalassiosira pseudonana (Clone 3 H). During starvation, cell quotas decreased by more than 50% in batch cultures. In chemostat cultures, the drop in cell quota during starvation decreased with dilution rate, from more than 50% at 1.45 d^-1, to less than 10% at 0.22 d^-1. Minimal levels of 3 to 4×10^-2 pg-at. N cell^-1 were reached after 24 h starvation in both batch and chemostat cultures. Uptake rates over the first minute of perturbation experiments were 3 times the long-term (10 to 30 min) rates. In batch cultures, specific uptake rates increased from 4 d^-1 to 20 d^-1 after 24 h starvation. Uptake rates per cell were independent of starvation time and dilution rate in chemostat cultures, but lower in non-starved batch cultures. The implications of these data for models of phytoplankton growth are discussed: the data support models which predict a depression in average growth rates when diatoms encounter microscale patches in oligotrophic environments.     

Ecotypic differentiation of Laminaria longicruris in relation to seawater nitrate concentration

The ultraplankton (cell diameters >3 μm), which compromises about 70% of the biomass of phytoplankton in subtropical surface waters near Oahu, Hawaii, was isolated for growth rate studies. The specific growth rate (μ) was estimated from the rate of increase of the chlorophyll biomass during incubations in the absence of grazers. This growth rate of the ultraplankton ranged from 0.037 to 0.071 h^?1 (=1.3 to 2.5 doublings d^?1) during a period when P:B ratios of 5 to 14.5 μg C μg^?1 chl a h^?1 prevailed. The co-occurrence of atypically high P:B ratios and nonlimiting ambient nutrient concentrations suggests that the calculated values are higher than those characteristic of such subtropical ecosystems in general. Rates of ammonium uptake and photosynthesis by the >3 μm fraction were also compared to those of larger fractions. Organisms in the >3 μm fraction assimilated NH ₄ ⁺ at a rate which was about 75% greater than that of the 3 to 20 μm size fraction. Comparison of μ and P:B data collected over a 2 mo period (November–December, 1980) shows that the correlation between these two rate indices is nonlinear. The predominance of small-celled phytoplankton in oligotrophic waters is explained, in part, by its higher μ, its higher nutrient assimilation rates, and the absence of its loss through sedimentation.     

Response of marine phytoplankton to nitrogen deficiency: Decreased nitrate uptake vs enhanced ammonium uptake

The uptake of nitrate and ammonium was measured separately in uni-algal, nitrogen-deficient cultures of four species of marine phytoplankton. Nitrogen-deficient phytoplankton took up ammonium at initial rates which greatly exceeded those measured for nitrogen-sufficient phytoplankton. However, nitrate uptake by nitrogendeficient cultures was generally much slower than either nitrate or ammonium uptake by nitrogen-sufficient cultures or ammonium uptake by nitrogen-deficient cultures. Considerable species differences were observed in the degree to which nitrogen deficiency increased ammonium uptake or decreased nitrate uptake. Loss of ability to take up nitrate, but enhanced ability to take up ammonium, as a result of nitrogen deficiency may be an adaptation to the different mechanisms by which nitrate and ammonium are supplied to the euphotic zone. In areas with an intermittent supply of nitrogen, changes in the ability of some species to take up nitrogen as a result of nitrogen starvation will influence species composition and complicate interpretations of measurements of nitrogen uptake.Contribution no. 1249 from the Department of Oceanography, University of Washington, and contribution no. 82006 from the Bigelow Laboratory for Ocean Sciences