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     

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     

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.     

Nitrate storage by phytoplankton in a coastal upwelling environment

The storage of nitrate by phytoplankton cells during the early phases of upwelling was studied in coastal stations off northern Spain (southern Bay of Biscay) between 1990 and 1994. In this region, a persistent upwelling during summer is characterised by intermittent pulses of variable intensity, and increased nutrient concentrations in the surface layer. The main effect of an upwelling pulse on phytoplankton distribution is the shifting of the chlorophyll a and primary production maxima to near the surface. When the upwelling relaxes, thermal stratification of the water column occurs, and a distinct subsurface chlorophyll maximum develops below the production maximum. An accumulation of intracellular nitrate characterized the early phases of upwelling (mean = 2.73 μmol N m⁻³), maximum concentrations being attained at depths where biomass and production values were moderate. In contrast, phytoplankton cells from non-upwelling situations contained significantly lower concentrations of intracellular nitrate (mean = 0.17 μmol N m⁻³). The variations in the intracellular pool of nitrate may result from the differential allocation of resources within the cell as a result of variations in the energy available, since the uptake and assimilation of nitrate is a relatively expensive process involving several enzymatic systems. We hypothesize that nitrate storage by phytoplankton cells is characteristic of early phases of upwelling and is linked to patterns of carbon fixation. Average nitrogen budgets for upwelling and non-upwelling situations indicate that intracellular nitrate reserves are not responsible for maintaining high phytoplankton growth rates, since they only account for <2% of daily primary production during upwelling events. Received: 28 August 1996 / Accepted 3 December 1996     

Interactions of inorganic nitrogen in the uptake and assimilation by marine phytoplankton

The effect of ambient ammonium concentration on the nitrate uptake rate of marine phytoplankton was investigated. These studies consisted of laboratory experiments using unialgal species and field experiments using natural phytoplankton communities. In laboratory experiments, ammonium suppressed the uptake rates of nitrate and nitrite. Approximately 30 min were required for ammonium to exhibit its fully inhibitory effect on nitrate uptake. At high ammonium concentration (>3 g-at/l), a residual nitrate uptake rate of approximately 0.006 h^-1 was observed. When the ambient ammonium concentration was reduced to a value less than 1 g-at/l, the suppressed nitrate uptake rate subsequently attained a value comparable to that observed before the addition of ammonium. A range of 25 to 60% reduction in the nitrate uptake rate of natural phytoplankton communities was observed at ambient ammonium concentrations of 1.0 g-at/l. A mechanism is proposed for the suppression of nitrate uptake rate by ammonium through feedback control of the nitrate permease system and/or the nitrate reductase enzyme system. The feedback control is postulated to be regulated by the level of total amino acids in the cell.Contribution No. 936 from the Department of Oceanography, University of Washington, Seattle, Washington 98195, USA. This paper represents a portion of a dissertation submitted to the Department of Oceanography, University of Washington, Seattle, in partial fulfillment of the requirements for the Ph.D. degree.     

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.     

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.     

Assessment of simultaneous uptake of nitrogenous nutrients (15N) and inorganic carbon (13C) by natural phytoplankton populations

The simultaneous uptake of nitrogenous nutrients and inorganic carbon was measured in shipboard incubations of natural phytoplankton populations, using tracer additions of ¹³C-bicarbonate and ¹⁵N-labelled nitrogenous substrates. From March 1991 through March 1992, three stations on the Scotian Shelf (eastern Canada) were sampled monthly at ten depths in the euphotic zone. Additions of labelled nitrogen compounds ranged between 0.5 and 98% of ambient concentrations. Most of the C/N (at/at) uptake ratios were lower than the Redfield ratio, suggesting that nitrogen was not limiting. The fixation of carbon with and without addition of nitrate, ammonium or urea was generally similar. Some samples presented significant differences in carbon uptake rate between the four treatments, but these differences were not related to nitrogen enrichment (percent or nitrogen species). Given these results, the double-labelling method appears to be a reliable tool for measuring the simultaneous uptake of carbon and nitrogen by natural phytoplankton.     

Synergistic effects of nitrate and ammonium ion on the growth and uptake kinetics of Monochrysis lutheri in continuous culture

Continuous-culture results for Monochrysis lutheri grown on 12 h light-12 h dark cycles with a spectrum of ratios of nitrate and ammonium serving as limiting nutrients are compared with continuous light, exclusively nitrate, and exclusively ammonium-limited data for this species. The diel effects of the light regime on the maximum specific uptake rate are examined for both nitrate and ammonium. Synergistic effects on uptake by various initial concentrations of these two nutrients are presented. Preconditioning with light-dark cycles did not affect maximum uptake rate, but preconditioning on a combination of nitrate and ammonium gave much lower uptake rates than those observed for populations preconditioned on either nutrient exclusively. The implications of high maximum specific-uptake rates compared to maximum specific-growth rates in terms of the range of nitrate and ammonium ion concentrations associated with nutrient limitation are reviewed.Hawaii Institute of Marine Biology Contribution No. 477.     

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.     

Regulation of nitrate and ammonium uptake in the Greenland Sea

The uptake of nitrate and ammonium was investigated experimentally during early spring 1989 in the Greenland Sea, with particular attention placed on the roles of irradiance, nitrogen concentrations and nitrateammonium interactions. The phytoplankton assemblage was dominated by the colonial prymnesiophyte Phaeocystis pouchetii. Nitrate concentrations ranged from undetectable at the end of the cruise to greater than 10 M, and ammonium levels ranged from less than 0.1 to 1.9M. The uptake of both nitrate and ammonium as a function of irradiance was found to be a saturation response. Photoinhibition occurred and was found to be greater for ammonium uptake. Ammonium uptake also saturated at irradiance levels five times lower than those needed to saturate nitrate uptake. Nitrate and ammonium uptake as a function of nitrogen concentration also was characterized by a saturation response, with the estimated half-saturation constant (K _s) value for nitrate uptake being 0.29 M. Elevated ammonium concentrations inhibited nitrate uptake, and the response appeared to be one of exponential decrease with increasing concentrations of ammonium. The most important factor in the Greenland Sea influencing ammonium uptake during the spring was irradiace, while both irradiance and ammonium concentrations played major roles in regulating nitrate uptake and new production.     

Seasonal variations in the utilization of ammonium and nitrate by photoplankton in Vineyard Sound,Massachusetts, USA

The nitrogenous nutrition of the phytoplankton in Vineyeard Sound, Massachusetts, USA was investigated over a 15-month period. Highest rates of ammonium uptake were observed immediately prior to, or during, the diatom bloom periods, and with one exception were found in the <10 m size class. The saturating rate of ammonium uptake correlated well with temperature and gave Q₁₀ values of 2.6–3.2; correlations with ambient solar irradiation were not nearly so clear. Uptake rates of ammonium exceeded those of nitrate except during the winter bloom of the diatom Rhizosolenia delicatula; yet calculation of the f ratio revealed that nitrate was relatively important in the nitrogenous nutrition of the phytoplankton throughout the year.Contribution no. 5096 from the Woods Hole Oceanographic Institution     

Persistent blooms of surf diatoms along the Pacific coast,USA

Productivity was studied in two diatom species, Chaetoceros armatum T. West and Asterionella socialis Lewin and Norris, which form persistent dense blooms in the surf zone along the Pacific coast of Washington and Oregon, USA. Past observations have shown that surf-diatom standing stock usually declines in summer along with concentrations of nitrate and ammonium. Using the ¹⁴C method, photosynthetic rates in natural surf samples were measured monthly for one year (October 1981 through September 1982) at a study site on the Washington coast. Also measured were temperature, salinity, dissolved nutrients, particulate carbon and nitrogen (used as estimates of phytoplankton C and N), and chlorophyll a. Assimilation numbers (P _max) were higher in summer (5 to 8 g C g^-1 chl a h^-1) than in winter (3 to 4gC). Specific carbon incorporation rates (µ_max) showed no obvious seasonality, mostly falling within the range of 0.09 to 0.13 g C g^-1 C(POC) h^-1. The discrepancy between the seasonal trends for chlorophyll-specific and carbon-specific rates reflects a change in the carbon-to-chlorophyll ratio. Because of seasonal differences in daylength and light intensity, daily specific growth rates () are thought to be higher in summer than in winter. Neither ammonium enrichment assays nor particulate carbon-to-nitrogen ratios provided convincing evidence for nitrogen limitation during summer, and the observed changes in diatom abundance cannot be explained on this basis. Both the high diatom concentrations and their seasonal variations probably are due mainly to factors other than growth rates; two factors considered important are diatom flotation and seasonal changes in wind-driven water transport. C. armatum usually dominates the phytoplankton biomass in the surf zone, and evidence suggests that this species is strongly dominant in terms of primary production.Contribution No. 1391 of the School of Oceanography, University of Washington, Seattle, Washington, USA     

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.     

Nitrate- and silicate-competition among antarctic phytoplankton

Natural phytoplankton from antarctic waters in the Drake Passage were used for competition experiments in semicontinuous cultures. The outcome of interspecific competition for silicate and nitrate was studied at a range of Si:N ratios (from 2.6:1 to 425:1) and at three different dilution rates. For five species Monod kinetics of silicate-and nitrate-limited growth has been established. Comparison between theoretical predictions derived from Monod kinetics and the outcome of competition experiments showed only minor deviations. Contrary to literature data, considerable depletion of nitrate was found in antarctic seawater. Both the concentrations of soluble silicate and of nitrate were too low to support maximum growth rates of some of the diatom species under investigation.     

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.     

Stimulation and inhibition of phytoplankton growth by low molecular weight hydrocarbons

Experiments on 4 phylogenetically different phytoplankton exposed in culture to a range of concentrations of benzene, toluene and xylene showed a variety of growth responses for marine microalgae. The degree of influence of these aromatic hydrocarbons, all components of fuel oils and crude oils, varied with concentration, compound and species. Stimulation of growth in Dunaliella tertiolecta resulted from low μg/l concentrations of all three compounds, Skeletonema costatum showed no growth enhancement, while Cricosphaera carterae and Amphidinium carterae were intermediate in their reactions. Closed culture vessels were found to be necessary to retain these volatile hydrocarbons. Many of the previous laboratory studies on oil using standard methods — cotton plugs, screw caps or beakers — have overlooked the important influence of the volatile fraction. The species-specific stimulation of low concentrations was further shown in experiments with mixtures of No. 2 fuel oil. The volatile fraction was most biologically reactive, being the source of growth enhancement at low levels and a major growth inhibitor at high concentrations. Thus, a significant environmental effect of oil on marine primary production could be the growth stimulation of particular species by low molecular weight aromatic compounds resulting in an alteration of the natural phytoplankton community structure and its trophic relationships.     

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.     

Simultaneous nitrogen and silicate deficiency of a phytoplankton community in a coastal jet-front

The nutritional status of a phytoplankton community was investigated in a coastal jet-front located in the Gulf of St. Lawrence, Canada, in 1987. During the sampling period, the frontal community was mainly composed of the diatomsChaetoceros debilis, Skeletonema costatum, Thalassiosira gravida andC. pelagicus. As previously reported for the St. Lawrence, some frontal stations were depleted both in nitrate and silicate. At stations impoverished in nitrate, internal nitrate pool concentrations were low or undetectable, suggesting that cells had not, recently, been exposed to a nitrate flux which exceeded the nitrate assimilation rate. At these impoverished stations, however, ambient and intracellular concentrations of ammonium and urea were high, suggesting that the community was not nitrogen-deficient. The comparison between the ambient silicate concentrations and the silicate requirement (K _s ) of the dominant diatoms suggests thatC. debilis andS. costatum were Si-deficient. This is further supported by the low silicate uptake rates and intracellular concentrations measured at the silicate impoverished stations. The silicate deficiency also resulted in a decrease in the seston and phytoplankton N:C ratios.Please address all correspondence and requests for reprints to Dr Levasseur at his new address: Maurice Lamontagne Institute, 850 Route de la Mer, Mont-Joli, Québec G5H 3Z4, Canada     

不同氮源对4种海洋微藻生长的影响

采用实验室一次性培养的方法,研究了硝氮、氨氮、尿素和混合氨基酸等4种不同氮源对典型赤潮藻赤潮异弯藻Heterosigma akashiwo、凯伦藻Karenia sp.、球形棕囊藻Phaeocystis globosa和常见浮游植物优势种类角毛藻Chaetoceros sp.生长的影响。结果表明,这4种海洋微藻不仅能利用无机氮硝氮和氨氮,而且也均能利用有机氮尿素和混合氨基酸。赤潮异弯藻、凯伦藻和角毛藻均在以硝氮为唯一氮源时,比生长速率分别达到最大值0.45、0.52和0.70 d-1;而球形棕囊藻在以硝氮和尿素为唯一氮源时,比生长速率均达最大值0.65 d-1。可溶性有机氮库中的重要组成成分尿素和氨基酸均能显著促进4种海洋微藻的生长;相比较而言,赤潮异弯藻和凯伦藻更加喜好有机氮氨基酸,而球形棕囊藻和角毛藻更加喜好尿素。海洋微藻具备利用有机氮源的能力,无疑扩展了其氮营养来源,在无机氮缺乏而有机氮丰富的水体中,它们在浮游植物群落中更具有竞争优势。