NUTRITIONAL STUDIES OF TWO RED ALGAE. I. GROWT RATE AS A FUNCTION OF NITROGEN SOURCE AND CONCENTRATION1

Gracilaria foliifera (Forsskal) Borgesen and Neoagardhiella bailiyi (Harvey ex Kiitzing) Wynne & Taylor were grown in continuous-flow culture under controlled environmental conditions in 15 liter experimental chambers. Growth rate was related to the source and concentration of nitrogen enrichment supplied to the plants, Growth rate appeared to follow saturation-type nutrient uptake kinetics for plants receiving ammonium, nitrate, urea of sewage effluent enrichments. Ammonium enrichment produced higher growth rates than nitrate of sewage enrichment. The lowest growth rates occurred in the chambers receiving unenriched seawater or urea. The low estimated constants (K) for growth were in the range of 0.2–0.4 (M N for all N enrichments examined. The low estimated values of K compare closely with those found for microalgae and indicate that both species possess the ability to utilize very low concentrations of N.     

KINETICS OF NITRATE,AMMONIUM, AND UREA UPTAKE BY FOUR INTERTIDAL SEAWEEDS FROM NEW ZEALAND1

The competitive ability for N uptake by four intertidal seaweeds, Stictosiphonia arbuscula (Harvey) King et Puttock, Apophlaea lyallii Hook. f. et Harvey, Scytothamnus australis Hook. f. et Harvey, and Xiphophora gladiata (Labillardière) Montagne ex Harvey, from New Zealand is described by the uptake kinetics for NO₃^?, NH₄⁺, and urea. This is the first study to report uptake kinetics for N uptake by a range of southern hemisphere intertidal seaweeds in relation to season and zonation. Species growing at the highest shore positions had higher NO₃^? and urea uptake at both high and low concentrations and had unsaturable NH₄⁺ uptake in both summer and winter. Although there was evidence of some feedback inhibition of V_max for NO₃^? uptake by Stictosiphonia arbuscula growing at the lower vertical limits of its range, rates were high compared with species growing lower on the shore. Our results highlight the superior competitive ability for N uptake of certain high intertidal seaweeds, and consistent with our previous findings we can conclude that intertidal seaweeds in southeast New Zealand are adapted to maximizing N acquisition in a potentially N‐limiting environment.     

METABOLIC AND ECOLOGICAL CONSTRAINTS IMPOSED BY SIMILAR RATES OF AMMONIUM AND NITRATE UPTAKE PER UNIT SURFACE AREA AT LOW SUBSTRATE CONCENTRATIONS IN MARINE PHYTOPLANKTON AND MACROALGAE1

Marine phytoplankton and macroalgae acquire important resources, such as inorganic nitrogen, from the surrounding seawater by uptake across their entire surface area. Rates of ammonium and nitrate uptake per unit surface area were remarkably similar for both marine phytoplankton and macroalgae at low external concentrations. At an external concentration of 1 μM, the mean rate of nitrogen uptake was 10±2 nmol·cm^?2·h^?1 (n=36). There was a strong negative relationship between log surface area:volume (SA:V) quotient and log nitrogen content per cm² of surface (slope=?0.77), but a positive relationship between log SA:V and log maximum specific growth rate (μ_max; slope=0.46). There was a strong negative relationship between log SA:V and log measured rate of ammonium assimilation per cm² of surface, but the slope (?0.49) was steeper than that required to sustain μ_max (?0.31). Calculated rates of ammonium assimilation required to sustain growth rates measured in natural populations were similar for both marine phytoplankton and macroalgae with an overall mean of 6.2±1.4 nmol·cm^?2·h^?1 (n=15). These values were similar to maximum rates of ammonium assimilation in phytoplankton with high SA:V, but the values for algae with low SA:V were substantially less than the maximum rate of ammonium assimilation. This suggests that the growth rates of both marine phytoplankton and macroalgae in nature are often constrained by rates of uptake and assimilation of nutrients per cm² surface area.     

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.     

IN SITU UPTAKE KINETICS OF AMMONIUM AND PHOSPHATE AND CHEMICAL COMPOSITION OF THE RED SEAWEED GRACILARIA TIKVAHIAE1

The uptake kinetics of ammonium and phosphate by Gracilaria tikvahiae McLachlan were studied under field conditions. Seaweeds, pulse fed once a week for 6 h over a 4-week period, had maximum uptake rates of 19 μmol·g fwt^?1·h^?1 for ammonium and 0.28 μmol·g fwt^?1·h^?1 for phosphate. For both nutrients there was a positive linear correlation between uptake rate (v) and concentration (S) over the entire range of concentration tested. In a nutrient depletion experiment, the phosphate uptake curve determined over a wide range of concentrations consisted of two stages of saturation at low concentrations, and a linear phase at high concentrations. Ash free dry weight, chlorophyll a, phycoerythrin, and protein content were higher in pulse fed plants than in control plants receiving no nutrient additions, while the reverse held true for carbohydrate contents and the C/N ratios. The C/N ratio inversely correlated with ammonium and phosphate uptake rate as well as protein and phycoerythrin content, and positively with carbohydrate content.     

UPTAKE OF NITRATE,AMMONIUM, AND UREA BY NITROGEN-STARVED CULTURES OF MICROMONAS PUSILLA (PRASINOPHYCEAE): TRANSIENT RESPONSES1

Nitrogen uptake rates were measured as a function of time following saturating additions (15 μMg-at N·^?1) of ¹⁵N-labelid ammonium, urea, and nitrate to N-starved cultures of the picoflagellate Micromonas pusilla Butcher. Uptake rates were estimated from both the accumulation of ¹⁵N into the cells and the disappearance of nitrogen from the medium. Transient elevated (surge) uptake rates of NH₄⁺ and urea were observed after enrichment. During the first 5 min the initial urea and NH₄⁺ uptake rates were 2- and 4-fold greater than the maximum growth rate (μM_max)observed prior to No₃^? depletion in the cultures. The elevated urea uptake rates declined quickly to a relatively constant value, whereas the initial rates of NH₄⁺ uptake declined rapidly but were followed by a subsequent increase prior to remaining roughly constant. Nitrate was not taken up as readily by N-starved M. pusilla as the reduced N forms. Although NO₃⁺ uptake commenced immediately after enrichment (i.e. no lag period) the N-Specific rate over the next 6 h averaged half the μM_max observed during NO₃^? replete conditions.     

15N MEASUREMENTS OF AMMONIUM AND NITRATE UPTAKE BY ULVA FENESTRATA (CHLOROPHYTA) AND GRACILARIA PACIFICA (RHODOPHYTA): COMPARISON OF NET NUTRIENT DISAPPEARANCE,RELEASE OF AMMONIUM AND NITRATE,AND 15N ACCUMULATION IN ALGAL TISSUE 1

Ammonium and nitrate uptake rates in the macroalgae Ulva fenestrata (Postels and Ruprecht) (Chlorophyta) and Gracilaria pacifica (Abbott) (Rhodophyta) were determined by ¹⁵N accumulation in algal tissue and by disappearance of nutrient from the medium in long‐term (4–13 days) incubations. Nitrogen‐rich algae (total nitrogen> 4% dry weight [dw]) were used to detect isotope dilution by release of inorganic unlabeled N from algal thalli. Uptake of NH₄ ⁺ was similar for the two macroalgae, and the highest rates were observed on the first day of incubation (45 μmol N·g dw ^{? 1}·h ^{? 1} in U. fenestrata and 32 μmol N·g dw ^{? 1}·h ^{? 1} in G. pacifica). A significant isotope dilution (from 10 to 7.9 atom % enrichment) occurred in U. fenestrata cultures during the first day, corresponding to a NH₄ ⁺ release rate of 11 μmol N·g dw ^{? 1}·h ^{? 1}. Little isotope dilution occurred in the other algal cultures. Concurrently to net NH₄ ⁺ uptake, we observed a transient free amino acid (FAA) release on the first day in both macroalgal cultures. The uptake rates estimated by NH₄ ⁺ disappearance and ¹⁵N incorporation in algal tissue compare well (82% agreement, defined as the percentage ratio of the lower to the higher rate) at high NH₄ ⁺ concentrations, provided that isotope dilution is taken into account. On average, 96% of added ¹⁵NH₄ ⁺ was recovered from the medium and algal tissue at the end of the incubation. Negligible uptake of NO₃ ^? was observed during the first 2–3 days in both macroalgae. The lag of uptake may have resulted from the need for either some N deprivation (use of NO₃ ^? pools) or physiological/metabolic changes required before the uptake of NO₃ ^? . During the subsequent days, NO₃ ^? uptake rates were similar for the two macroalgae but much lower than NH₄ ⁺ uptake rates (1.97–3.19 μmol N·g dw ^{? 1}·h ^{? 1}). Very little isotope dilution and FAA release were observed. The agreement between rates calculated with the two different methods averaged 91% in U. fenestrata and 95% in G. pacifica. Recovery of added ¹⁵NO₃ ^? was virtually complete (99%). These tracer incubations show that isotope dilution can be significant in NH₄ ⁺ uptake experiments conducted with N‐rich macroalgae and that determination of ¹⁵N atom % enrichment of the dissolved NH₄ ⁺ is recommended to avoid poor isotope recovery and underestimation of uptake rates.     

EFFECTS OF PHOTOCYCLES AND PERIODIC AMMONIUM SUPPLY ON THREE MARINE PHYTOPLANKTON SPECIES. II. AMMONIUM UPTAKE AND ASSIMILATION1

The relative influence of the photoperiod and of periodic ammonium pulses in entraining the cell division cycle in nitrogen-limited cyclostat cultures differs dramatically in Hymenomonas carterae Braarud and Fagerl, Amphidinium carteri Hulburt and Thalassiosira weissflogii Grun. We examined how each species processes an NH₄⁺ pulse at various times during the cell cycle and the L/D cycle. Rates of NH₄⁺ uptake and changes in cellular concentrations of NH₄⁺, free amino acids, and protein were examined after the addition of an NH₄⁺ pulse. Depletion of NH₄⁺ from the medium occurred earlier when the pulse was given at the beginning of the light period than at the beginning of the dark period in H. carterae and A. carteri. Depletion took longer in the T. weissflogii cultures and the kinetics were similar during both stages of the photocycle in this species. Similarly, the temporal phasing and maximum pool sizes varied with timing of the NH₄⁺ pulse in H. carterae and A. carteri but complete assimilation was relatively rapid. More persistent pools of NH₄⁺ and free amino acids accumulated in T. weissflogii, and the patterns of assimilation varied little as a function of the timing of the pulse with respect to the photocycle. Although nitrogen metabolism occurred rapidly in nitrogen-limited H. carterae and A. carteri, the entrainment of the cell division cycle by the photoperiod resulted in a large degree of uncoupling between completion of nitrogen assimilation and cell division. It is hypothesized that the strong entrainment of the cell division cycle of T. weissflogii by NH₄⁺ pulses results from a relatively slow rate of nitrogen metabolism.     

MEASURING RATES OF AMMONIUM ASSIMILATION IN MARINE ALGAE: USE OF THE PROTONOPHORE CARBONYL CYANIDE m-CHLOROPHENYLHYDRAZONE TO DISTINGUISH BETWEEN UPTAKE AND ASSIMILATION

A method for determining rates of ammonium assimilation in marine algae is described. Ammonium assimilation is defined as the decrease in total (medium + cellular) ammonium. The protonophore carbonyl cyanide m- chlorophenylhydrazone (CCCP) was used to distinguish between uptake and assimilation of ammonium. Ammonium uptake by nitrogen-replete and nitrogen-starved cells of the diatom Phaeodactylum tricornutum Bohlin and the green macroalga Enteromorpha sp. was completely (98%–99%) inhibited in the presence of 100 μM CCCP. In addition to inhibiting further uptake of ammonium, CCCP promoted the release of unassimilated ammonium by nitrogen-replete and nitrogen-starved P. tricornutum and Enteromorpha that had been allowed to take up ammonium for a period. Most (97.5%) of preaccumulated ¹⁴C-methylammonium was released by nitrogen-starved P. tricornutum in the presence of CCCP. Specific rates of ammonium assimilation in nitrogen-replete cultures of P. tricornutum were identical to the maximum growth rate, but specific rates in nitrogen-starved cultures were fourfold greater. Rates of ammonium assimilation in Enteromorpha during both the surge and the internally controlled uptake phases were the same as the internally controlled rate of uptake, suggesting that the latter is a reliable measure of the maximum rate of assimilation.     

NITRATE UPTAKE KINETICS AND CLONAL VARIABILITY IN THE NERITIC DIATOM BIDDULPHIA AURITA1

Nitrate uptake experiments were conducted at 18 and 25 C on subtropical and temperate isolates of the neritic diatom Biddulphia aurita (Lyngb.) Breb. & Godey. Two clones (STX-88 and B1) were grown in a 12:12 LD cycle in NO₃-limited continuous cultures used for uptake experiments. The half-saturation constant (K_s) and the maximum NO₃- uptake rate (V_m) were measured under conditions in which external NO₃- concentration controlled uptake rate. The kinetic constants for both clones were higher at the higher temperature. The subtropical clone (STX-88) had lower kinetic values than temperate clone (B1) at both temperatures. The differences appear to be of adaptive significance since STX-88 has a NO₃- uptake system, described by its lower K_s value, which minimizes the double stress effects of high temperature-low NO₃- concentration conditions characteristic of its habitat. Cytoplasmic enzyme electrophoretic analyses were conducted on the clones. Differences in banding patterns and mobilities showed evidence of allelic substitution. The results suggest genetic variation at the intraspecific level.     

AMMONIUN AND NITRATE UPTAKE BY THE MARINE MACROPHYTES HYPNEA MUSVUFORMIS (RHODOPHYTA) AND MACROCYSTIS PYRIFERA (PHAEOPHYTA)1, 2

NH₄⁺ and NO₃^? uptake were measured by continuous sampling with an autoanalyzer. For Hypnea musciformis (Wulfen) Lamouroux, NO₃^?up take followed saturable kinetics (K₂=4.9 μg-at N t^?1, V_max= 2.85 μg- at N, g(wet)^?1. h^?1. The ammonium uptake data fit a trucatd hyperbola, i.e., saturation was not reach at the concentrations used. NO₃^? uptake was reduced one-half in the presence of NH₄⁺, but presence of NO₃^? had no effect on NH₄⁺ uptake. Darkness reduced both NO₃^? and NH₄⁺ uptake by one-third to one-half. For Macrocystis pyrufera (L) C. Agardh, NO₃^? uptake followed saturable kinetices: K₂=13.1 μg-at N. l^?1. V_max=3.05 μg-at N. g(wet)^?1. h^?1.NH₄⁺ uptake showed saturable kinetics at concentration below 22 μg-at N l -1 (K₂=5.3 μg-at N.1–1, V_max= 2.38 μg-at N G (wet)^?1.h^?1: at higher concentration uptake increased lincarly with concentrations. NO₃^?and NH₄⁺ were taken up simulataneously: presence of one form did not affect uptake of the other.     

METABOLIC REGULATION OF AMMONIUM UPTAKE BY ULVA RIGIDA (CHLOROPHYTA): A COMPARTMENTAL ANALYSIS OF THE RATE-LIMITING STEP FOR UPTAKE1

Non-linear time courses of ammonium (NH₄⁺) depletion from the medium and internal accumulation of soluble nitrogen (N) in macroalgae imply that the rate-limiting step for ammonium uptake changes over time. We tested this hypothesis by measuring the time course of N accumulation in N-limited Ulva rigida C. Agardh. Total uptake was measured as removal of NH₄⁺ from medium. Rates for the component processes (transport of NH₄⁺ across the membrane = R_v assimilation of tissure NH₄⁺ into soluble N compounds = R_a, assimilation of tissue NH₄⁺ into soluble N compounds = R_a and incorporation of soluble N compounds into macromolecules = R₁) were determined by measuring the rate of labelling of the major tissue N pools after the addition of ¹⁵N-ammonium. The results indicate that nitrogen-specific rates (mass N taken up / mass N present / unit time) are ranked in the order of R_t < R_a < R₁ Absolute uptake rates (μmol N. mg dry wt^?1. h^?1) showed a different relationship. Membrane transport appears to be inhibited when NH₄⁺ accumulates in the tissue. Maximum uptake rates occur when assimilation of NH₄⁺ into soluble N compounds begins. Assimilation of NH₄⁺ into soluble N compounds was initially faster than incorporation of soluble N compounds into macromolecules. Implications of rate limitations caused by differences in maximal rates and maximal pool sizes are discussed.     

粉绿狐尾藻和凤眼莲对不同形态氮吸收动力学研究

采用改进常规耗竭法,研究了粉绿狐尾藻(Myriophyllum aquaticum)、凤眼莲(Eichhornia crassipes)在抑菌和非抑菌两种处理中对NO3-、NH4+吸收的动力学特征。结果表明:两种植物对NO3-和NH4+的最大吸收速率(Imax)和亲和力(1/Km)有显著差异,凤眼莲对NO3-、N H4+的吸收速率显著高于狐尾藻,说明凤眼莲更适宜用于污染水体养分的去除;不同植物对NO3-、NH4+表现出吸收偏好性,凤眼莲对NO3-有较高的Imax值和亲和力,而狐尾藻对NH4+有较高的Imax值和亲和力;抑菌处理能显著增加凤眼莲对NH4+的吸收速率,NO3-浓度高于1.00 mmol/L时,抑菌处理能显著减小凤眼莲对NO3-的吸收速率,狐尾藻对NO3-、N H4+的吸收未受到抑菌处理的显著影响。       

MOLYBDENUM DEPENDENCE,NITRATE UPTAKE AND PHOTOSYNTHESIS OF FRESHWATER PLANKTON ALGAE1,2

The effects of molybdenum on photosynthesis and nitrate uptake by two species of freshwater algae, Navicula pelliculosa (Bréd.) Hilse and Chlamydomonas reinhardtii Dang. Were examined. Photosynthetic rates in cells of N. Pelliculosa deprived of Mo for 48 h were significantly lower than in nondeprived cells; the rates were 2.6 and 4.5 μg C/10⁶ cells/h, respectively. There was no significant reduction in photosynthetic rates of C. reinhardtii under the 2 treatments, the rates were both ca. 6.0 μg C/10⁶ cells/h. These observed rates in the 2 species were not altered by added Mo concentrations as high as 1.0 ppm. The chlorophyll a values in 48 h Mo-deprived cells were 1.15 μg/10⁶ cells compared to 1.57 μg/10⁶ cells provided with this metal. Molybdenum deficiency did not affect the chlorophyll a concentration in Chlamydomonas; the chlorophyll levels were 2.44μg/10⁶ cells. Nitrate uptake by Mo-deprived cells of N. Pelliculosa was significantly lower than in cells cultured in Mo; the rates were 0.087 and 0.238 μM NO_a⁻/10⁶ cells/h, respectively. Uptake rates by C. reinhardtii were similar with or deprived of Mo. The K_m values for No₃⁻ uptake were 14.9 and 148.0 μM for N. Pelliculosa and C. reinhardtii, respectively.     

KINETICS OF AMMONIUM ASSIMILATION IN TWO SEAWEEDS, ENTEROMORPHA SP. (CHLOROPHYCEAE) AND OSMUNDARIA COLENSOI (RHODOPHYCEAE)

The kinetics of ammonium assimilation were investigated in two seaweeds from northeastern New Zealand, Enteromorpha sp. (Chlorophyceae, Ulvales) and Osmundaria colensoi (Hook. f. et Harvey) R.E. Norris (Rhodophyceae, Ceramiales), with the use of a recently developed method for measuring assimilation. In contrast to ammonium uptake, which was nonsaturable, ammonium assimilation exhibited Michaelis–Menten kinetics in both species. Maximum rates of assimilation (V_max) were 27 and 12 μmol·(g DW)⁻¹·h⁻¹ for Enteromorpha sp. and O. colensoi, respectively, with half-saturation (K_m) constants for assimilation of 18 and 41 μM. At environmentally relevant concentrations, assimilation accounted for all of the ammonium taken up by both species. The maximum rate of assimilation in Enteromorpha sp. resembled very closely that of the ammonium assimilatory enzyme, glutamine synthetase, when activities of the latter were measured in the presence of subsaturating substrate (glutamate and ATP) concentrations. Moreover, the initial rate of glutamine production (measured with HPLC) following ammonium enrichment was almost identical to the rates determined above. The rate of ammonium assimilation was therefore determined by three independent methods, two of which involve in vivo measurements, and it is suggested that the use of assimilation kinetics may be useful when examining the nutrient relations of seaweeds.     

AMMONIUM,NITRATE, PHOSPHATE,AND INORGANIC CARBON UPTAKE IN AN OLIGOTROPHIC LAKE: SEASONAL VARIATIONS AMONG LIGHT RESPONSE VARIABLES1

The dependence of substrate saturated uptake of ¹⁵NH₄⁺, ¹⁵NO₃^?, ³²PO₄^3?, and ¹⁴CO₂ on photosynthetic photon flux density (PPFD or photsynthetically active radiation, 400–700 nm) was characterized seasonally in oligotrophic Flathead Lake, Montana. PO₄^3? uptake was not dependent upon PPFD at any time of the year, whereas NH₄⁺, NO₃^?, and CO₂ uptake were consistently dependent on PPFD over all seasons. Maximal rates of NH₄⁺, NO₃^? and CO₂ uptake usually occurred near 40% of surface PPFD, which corresponded to about 5 m in the lake; inhibition was evident at PPFD levels greater than 40%. NH₄⁺, NO₃^? and PO₄^3? were incorporated in the dark at measurable rates most of the year, whereas dark CO₂ uptake was always near 0 relative to light uptake. CO₂ and NO₃^? uptake were more strongly influenced by PPFD than was NH₄^3? uptake. The PPFD dependence of PO₄^3?, NH₄⁺, NO₃^? and CO₂ uptake may affect algal growth and nutrient status by influencing the balance in diel and seasonal C:N:P uptake ratios.     

RAPID AMMONIUM UPTAKE BY FRESHWATER PHYTOPLANKTON1

A natural assemblage of freshwater phytoplankton was removed from an oligotrophic lake and grown at N:P supply ratios of 5:1, 15:1 and 45:1 (by atoms) in semicontinuous culture. After a minimum of 31 days at an average daily growth rate of 0.5 d^?1, experiments were conducted examining the time-course of saturated ammonium uptake rates. Cultures grown under the two lowest N:P supply ratios demonstrated greatly elevated initial, specific uptake rates for ammonium and were able to sequester between 7 and 21% of their daily N requirement in less than 5 min. The initial rates declined rapidly but were followed by a subsequent increase and decrease over a 120 min period.     

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.     

SIMULTANEOUS ASSIMILATION OF AMMONIUM AND NITRATE BY GELIDIUM NUDIFRONS (GELIDIALES: RHODOPHYTA)1

Simultaneous assimilation of NH₄ and NO₃ by Gelidium nudifrons Gardner was observed in culture experiments of 4 possible combinations of NH₄ and NO₃. The combinations tested were those in which the concentration of both N sources were in the range of 3.0–4.0 μg-atN · l^?1; both in the range of 0.5–1.0 μg-atN · l^?1; one in the 3.0–4.0 μg-atN · l^?1 range and the other in the 0.5–1.0 μg-atN · l^?1 range; and, visa versa. The data suggest that the pools of both NH₄ and NO₃ are simultaneously available for algal assimilation.