Dissolved Inorganic Nitrogen Pools and Surface Flux under Different Brackish Marsh Vegetation Types,Common Reed (<Emphasis Type="Italic">Phragmites australis</Emphasis>) and Salt Hay (<Emphasis Type="Italic">Spartina patens</Emphasis>)

The current expansion of Phragmites australis into the high marsh shortgrass (Spartina patens, Distichlis spicata) communities of eastern U.S. salt marshes provided an opportunity to identify the influence of vegetation types on pools and fluxes of dissolved inorganic nitrogen (DIN). Two brackish tidal marshes of the National Estuarine Research Reserve system were examined, Piermont Marsh of the Hudson River NERR in New York and Hog Island in the Jacques Coustaeu NERR of New Jersey. Pools of DIN in porewater and rates of DIN surface flux were compared in replicated pairs of recently-expanded P. australis and neighboring S. patens-dominated patches on the high marsh surface. Both marshes generally imported nitrate (NO₃⁻) and exported ammonium (NH₄⁺), such that overall DIN was exported. No differences in surface exchange of NO₃⁻ or NH₄⁺ were observed between vegetation types. Depth-averaged porewater NH₄⁺ concentrations over the entire growing season were 56% lower under P. australis than under S. patens (average 1.4 vs. 3.2 mg NH₄⁺ L⁻¹) with the most profound differences in November. Porewater profiles showed an accumulation of NH₄⁺ at depth in S. patens and constant low concentrations in P. australis from the soil surface to 50 cm depth, with no significant differences in porewater salinity. Despite these profound differences in porewater, NH₄⁺ diffusion from soils of P. australis and S. patens were not measurably different, were similar to other published rates, and were well below estimated rates based on passive diffusion alone. Rapid adsorption and uptake by litter and microbes in surface soils of both communities may buffer NH₄⁺ loss to flooding tides in both communities, thereby reducing the impact of P. australis invasion on NH₄⁺ flux to flooding waters.     

Kinetics of nitrate uptake by freshwater algae

The kinetics of nitrate (NO₃ ^–) uptake, the maximum uptake velocity (V_m) and the half-saturation constant (K_s), were determined for 18 species of batch-cultured freshwater algae grown without nitrogen limitation. Values of K_s ranged from 0.25 to 6.94 µM l^–1 Chlorella pyrenoidosa Chick, and Navicula pelliculosa (Breb.) Hilse, respectively. Values of V_m ranged from 0.51 to 5.07 µM l^–1 h^–1 for Anabaena A7214 and Nitzschia W-32 O'Kelley, respectively. The mean positive values of K_s for Chlorophyta, Cyanophyta and Chrysophyta were 1.89, 3.67 and 4.07 µM l^–1, respectively. The mean values of V_m for the same phyla were 1.61, 1.02 and 2.97 µM l^–1 h^–1 10⁵ cells^–1, respectively. The ranges of these kinetic parameters encompass values of kinetic parameters for marine and freshwater species in batch culture, for freshwater algae grown in N-limited chemostats and for natural populations of freshwater phytoplankton. Thus, in spite of variability between species, uptake parameters for both marine and freshwater algae are identical.     

Effects of nutrients and light on periphyton biomass and nitrogen uptake in Mediterranean streams with contrasting land uses

1. Nutrient diffusing substrata (NDS) were used to determine the relative importance of nutrients and light as potential limiting factors of periphyton biomass and nitrogen (N) uptake in Mediterranean streams subjected to different human impacts. The nutrients examined were phosphorus (P) and N, and we also further differentiated between the response of periphyton communities to N species (i.e. NO₃‐N and NH₄‐N). To examine the effect of light and nutrients on periphyton biomass, chlorophyll a accrual rates on NDS located at open and closed canopy sites were compared. The effect of nutrient availability on periphyton uptake was measured by ¹⁵N changes on the NDS after NO₃‐¹⁵N short‐term nutrient additions. 2. Results show that light was the main factor affecting algal biomass in the study streams. Algal biomass was in general higher at open than at closed canopy sites. Nutrient availability, as simulated with the NDS experiments, did not enhance algal biomass accrual in either of the 2 light conditions. 3. In the control treatments (i.e. ambient concentrations), periphyton NO₃‐N uptake rates increased and C : N molar ratios decreased consistently with increases in N availability across streams. NO₃‐N uptake rates were altered when ambient N concentrations were increased artificially in the N amended NDS. Periphyton assemblages growing on N enriched substrata seemed to preferentially take up N diffusing from the substratum rather than N from the water column. This response differed among streams, and depended on ambient N availability. 4. Periphyton biomass was not significantly different between substrata exposed to the two forms of available N sources. Nonetheless, we found differences in the effects of both N sources on the uptake of N from the water column. NH₄‐N seemed to be the preferred source of N for periphyton growing on NDS. 5. Results suggest that the effect of riparian zones on light availability, although seldom considered by water managers, may be more important than nutrients in controlling eutrophication effects derived from human activities. Finally, our results confirm that not only increases in concentration, but also stoichiometric imbalances should be considered when examining N retention in human altered streams.     

AMMONIUM STIMULATION OF DARK CARBON FIXATION AS AN INDICATOR OF NITROGEN DEFICIENCY IN PHYTOPLANKTON: POTENTIAL ERRORS CAUSED BY AMMONIUM-OXIDIZING BACTERIA1

Stimulation of dark fixation of carbon by NH₄⁺ is often used as an indicator of phytoplankton N deficiency. This assay is based on the influence of available NH₄⁺ on anaplerotic CO₂ fixation by algae. However, carbon fixation by chemoautotrophic NH₄⁺-oxidizing bacteria may also be stimulated by NH₄⁺ enrichment, a process that can mask the algal response in natural communities. NH₄⁺ addition enhanced dark carbon fixation up to 300%, relative to unamended controls, in organisms collected on a 0.7-μm retention filter in oligotrophic Flathead Lake, Montana, but the effect was not detectable in the presence of nitrapyrin, an inhibitor of NH₄⁺-oxidizing bacteria. Dark carbon fixation was enhanced with addition of NH₄⁺ in organisms retained on 2-μm filters (which should allow passage of most bacteria). NH₄⁺ stimulated dark carbon fixation in N-deficient axenic cultures of Chlamydomonas reinhardtii Dang but not in N-replete cultures in both the presence and absence of nitrapyrin. Application of nitrapyrin or size fractionation treatments, to separate the processes of dark carbon fixation by nitrifiers and phytoplankton, may improve the efficacy of assays using NH₄⁺ stimulation of dark carbon fixation to specifically indicate N deficiency in natural algal communities.     

Phytoplankton nutrient deficiency in lakes of the McMurdo dry valleys, Antarctica

1. The influence of inorganic nitrogen and phosphorus enrichment on phytoplankton photosynthesis was investigated in Lakes Bonney (east and west lobes), Hoare, Fryxell and Vanda, which lie in the ablation valleys adjacent to McMurdo Sound, Antarctica. Bioassay experiments were conducted during the austral summer on phytoplankton populations just beneath the permanent ice cover in all lakes and on populations forming deep-chlorophyll maxima in the east and west lobes of Lake Bonney. 2. Phytoplankton photosynthesis in surface and mid-depth (13 m) samples from both lobes of Lake Bonney were stimulated significantly (P < 0.01) by phosphorus enrichment (2 μM) with further stimulation by simultaneous phosphorus plus NH₄⁺ (20 μM) enrichment. Similar trends were observed in deeper waters (18 m) from the east lobe of Lake Bonney, although they were not statistically significant at P < 0.05. Photosynthesis in this lake was never enhanced by the addition of 20 μM NH₄⁺ alone. Simultaneous addition of phosphorus plus nitrogen stimulated photosynthesis significantly (P < 0.01) in both Lake Hoare and Lake Fryxell. No nutrient response occurred in Lake Vanda, where activity in nutrient-enriched samples was below unamended controls; results from Lake Vanda are suspect owing to excessively long sample storage in the field resulting from logistic constraints. 3. Ambient dissolved inorganic nitrogen (DIN) (NH4⁺+ NO₂^?+ NO₃^?): soluble reactive phosphorus (SRP) ratios partially support results from bioassay experiments indicating strong phosphorus deficiency in Lake Bonney and nitrogen deficiency in Lakes Hoare and Fryxell. DIN : SRP ratios also imply phosphorus deficiency in Lake Vanda, although not as strong as in Lake Bonney. Particulate carbon (PC): particulate nitrogen (PN) ratios all exceed published ratios for balanced phytoplankton growth, indicative of nitrogen deficiency. 4. Vertical nutrient profiles in concert with low advective flux, indicate that new (sensu Dugdale & Goering, 1967) phytoplankton production in these lakes is supported by upward diffusion of nutrients from deep nutrient pools. This contention was tested by computing upward DIN : SRP flux ratios across horizontal planes located immediately beneath each chlorophyll maximum and about 2 m beneath the ice (to examine flux to the phytoplankton immediately below the ice cover). These flux ratios further corroborated nutrient bioassay results and bulk DIN : SRP ratios indicating phosphorus deficiency in Lakes Bonney and Vanda and potential nitrogen deficiency in Lakes Hoare and Fryxell. 5. Neither biochemical reactions nor physical processes appear to be responsible for differences in nutrient deficiency among the study lakes. The differences may instead be related to conditions which existed before or during the evolution of the lakes.     

Nutrient uptake and primary productivity in an urban estuary: using rate measurements to evaluate phytoplankton response to different hydrological and nutrient conditions

The northern San Francisco Estuary (nSFE) is an urban estuary supplied with anthropogenic nutrient inputs, yet spring blooms are uncommon and phytoplankton biomass is low. The low levels of chlorophyll (<5 µg L^?1) have likely contributed to declines in several native fishes, and there is a need to evaluate the conditions that could allow for increased phytoplankton. Increased ammonium (NH₄) loads have been hypothesized to modulate the magnitude of blooms in nSFE (the “NH₄ hypothesis”) as a result of inhibition of phytoplankton NO₃ uptake that limits access to the greater nitrogen (N) pool of nitrate (NO₃). This hypothesis, tested in enclosures, but not in the field until now, is that lack of access to NO₃ limits primary production and consequently the accumulation of chlorophyll. Here, we test this in the field with the following aims: (1) to observe the uptake response of phytoplankton in different flow and N loading conditions, (2) determine whether the sequence of uptake rates suggested by the “NH₄ hypothesis” occurs and (3) obtain depth-integrated nutrient uptake rates to better constrain published criteria for bloom formation. Weekly measurements of NH₄ and NO₃ uptake, and primary production rates were made during spring 2011–2012, along with nutrient and chlorophyll concentrations during two contrasting hydrological conditions of high vs low freshwater flow. In conditions with high freshwater flow (maximum of 2405 m³ s^?1), there were lower nutrient concentrations than with low/normal flows (e.g., NO₃ of 10 µmol L^?1 compared to 30 µmol L^?1), with low N uptake and primary production rates. With low flow (maximum of 1304 m³ s^?1), there was elevated chlorophyll and blooms occurred, especially in shallow well-lit shoals where chlorophyll reached 60 µg L^?1. The higher levels of chlorophyll and primary productivity resulted from uptake of ambient NO₃ by phytoplankton, and f-ratios >0.5. This was enabled by phytoplankton uptake of NH₄ to below inhibitory levels, as proposed by the “NH₄ hypothesis.” The depth-integrated uptake rate data were used to refine a model that yields flow and nutrient concentration criteria necessary for bloom formation and confirmed that washout flows were the most useful predictor of blooms. Understanding the interaction of phytoplankton biomass with nutrient variability requires evaluating changes in C and N uptake rates and river flow. These dynamic changes are central to understanding why some urban estuaries have lower productivity than expected, and would be difficult to evaluate using biomass data alone. This study points to the importance of treating inorganic N separately as NH₄ and NO₃ rather than lumping together as DIN and to use rate process data as a mechanistic way to understand, predict and minimize cultural eutrophication impacts.     

Ecology and dynamics of two intertidal algal communities on the littoral of the island of São Miguel (Azores)

The intertidal benthic algal communities of two sites located on opposite coasts of São Miguel Island (Azores), were studied over a 2-year period (September 1993–September 1995). At both sites (São Roque on the south coast and São Vicente on the north), the littoral region was surveyed from the upper intertidal down to the sublittoral fringe. The survey revealed five distinct zones, with a variable degree of overlapping. The two upper zones were characterized by animals (littorinids and barnacles, respectively). Lower down, algal communities formed three distinct zones: an upper Fucus spiralis/Gelidium microdon association, a more extensive turf zone, and a belt featuring erect or frondose algae. Upper in the eulittoral, the turf was mainly monospecific, and dominated by Caulacanthus ustulatus. The lower eulittoral turf was dominated by articulated coralline algae, the associated species differing between the two sites studied. Mainly erect algae (Pterocladiella, Asparagopsis, etc.) occurred furthest down the shore and extended into the nearby shallow sublittoral. Two intertidal communities were studied at each locality: the upper eulittoral (Caulacanthus turf in São Roque and the Fucus spiralis/Gelidium microdon association at São Vicente), and the lower eulittoral (the coralline turf). The lower littoral communities had a higher algal diversity. A general pattern was observed in the seasonal variation of biomass: the lower levels exhibited the higher values in late summer/early autumn, the period in which the upper levels had the lowest standing crops. Physical factors are proposed to account for this. No significant inter-annual variations could be detected, indicating relatively stable communities, at least on a short-term basis.     

Characterization of the affinity for nitrogen, uptake kinetics, and environmental relationships for Prorocentrum minimum in natural blooms and laboratory cultures

During the late spring and early summer of 1998, an extensive bloom of the dinoflagellate Prorocentrum minimum (>93% of phytoplankton cell density) developed in several tributaries of the Chesapeake Bay, USA. In January 1999, a bloom of mixed dinoflagellates (Heterocapsa rotundata, H. triquetra and P. minimum, with P. minimum forming 21% of total phytoplankton cells and 39% of the total biovolume) developed in the mesohaline Neuse Estuary, North Carolina, USA. During these blooms, experiments were carried out to characterize the nitrogen uptake kinetics of these assemblages with ¹⁵N isotopic techniques. Four nitrogenous substrates (NO₃⁻, NH₄⁺, urea, and a mixed amino acids substrate) were used to determine uptake rate and substrate preference. Rates of nitrogen uptake were also measured in P. minimum cultures grown on varying growth nitrogen substrates. The calculated kinetic parameters determined for the P. minimum-dominated field assemblages and the cultures indicated a preference for NH₄⁺. NH₄⁺ was also the primary nitrogen source supporting the blooms. In addition, a high affinity for urea was also found, and urea contributed significantly to the Neuse Estuary bloom. Furthermore, results showed that the regulation of uptake for each of the substrates was different: strong positive relationships between affinity and temperature were found for NH₄⁺ and amino acids, while a negative response was found for NO₃⁻, and very little response to temperature was noted for urea. These differences suggest that a diversity of nitrogen uptake mechanisms may aid the development and maintenance of P. minimum blooms.     

The effect of temperature on photosynthetic and respiratory electron transport system activity in the shallow and deep-living phytoplankton of a subalpine lake

SUMMARY. 1. The influence of temperature on in vivo photosynthetic and in vitro respiratory electron transport system (ETS) activity was determined over the season for the 3 m (warm-water) and a 20m (cold-water) phytoplankton communities in Castle Lake. The optimum temperature of photosynthesis at 3 m (X?=20.8°C) was significantly higher than the average optimum at 20 m (X?=14.8°C). 2. Seasonally, the photosynthetic temperature optimum increased when the blue-green alga Chroococcus limneticus Lemm. was present. The temperature characteristics of this organism were maintained even after it had settled into the cold water of the hypolimnion. 3. Temperature optima were not significantly different in experiments conducted under limiting or saturating photosynthetic photon flux densities (PPFD). 4. Short-term (1 h) preincubations with dissolved inorganic nitrogen (DIN) (?80 μg NH₄NO₃-N l^?1) had little effect on the temperature characteristics of photosynthesis while the longer (>24 h) incubations provided by a whole-lake epilimnetic DIN addition (?75 μg NH₄NO₃- N l^?1) significantly lowered the photosynthetic temperature optimum to 12.5°C. Once this epilimnetic DIN was depleted the optimum roseto25°C, a value higher than that present before the enrichment, which coincided with the growth of C limneticus. 5. Respiratory ETS activity usually began to inactivate between 19 and 20°C. However, when C. limneticus was abundant the inactivation temperature was often greater ihan 25°C. 6. The average energy of activation (E) and Q₁₀ value for the 3 m community (15.9 kcal mol^?1 and 2.6 respectively) were significantly higher than those at 20 m (14.2 kcal mol^?1 and 2.4 respectively). Seasonally, the highest E and Q₁₀ values of ETS activity occurred during the late-summer bloom of C. limneticus. 7. These results demonstrate that the epilimnetic and hypolimnetic phytoplankton communities in Castle Lake are physiologically distinct with regards to their temperature characteristics.     

NITROGEN UPTAKE BY FUCUS SPIRALIS (PHAEOPHYCEAE)1,2

Ammoniun, nitrate and nitrite update by Fucus spiralis L. from the Massachusetts coast was examined. Uptake of all appeared to follow saturation type nutrient uptake kinetics, with uptake often restricted at ambient nutrient concentrations. Although only relatively large difference in K₈ values could be easily distinguished, K₈ values for NO_3? and NH₄₊ were generally similar and low compared with NO_2?. There was also some suggestion that K₈ was reduced at lower temperatures. At 15 C. V_max for light and dark uptake for both NH₄+ and NO_3?, and light uptake of N0_2? were similar, suggesting comparable potential use at higher concentrations. Ammonium and NO_3?uptake decreased at lower temperatures giving Q_ro values of 1.8 and 1.6, respectively, between 5 and 15°C. Nitrate and NH₄+ were taken up together and high levels of NH₄+ did not inhibit NO_3? uptake. Light did not affect uptake of either but did stimulate NO_2? uptake. Ammonium and NO_3? uptake were highest in apical frond and whole young plants, and lowest in slower growing, older frond and stipe. On a relative basis. NO_3?, NH₄₊ and NO_2? were estimated to have contributed ca. 59, 39 and 2% respectively, to the yearly N uptake by apical frond. During winter, NO_3? would provide ca. twice the N to F. spiralis as would, NH₄+. From summer to early fall, when NO_3? levels are lower, NO_3? and NH₄+ would be used in comparable amounts.     

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.     

The kinetics of NH4 + and NO3 − uptake by Douglas fir from single N-solutions and from solutions containing both NH4 + and NO3 −

The kinetics of NH₄ ⁺ and NO₃ ⁻ uptake in young Douglas fir trees (Pseudotsuga menziesii [Mirb.] Franco) were studied in solutions, containing either one or both N species. Using solutions containing a single N species, the V_max of NH₄ ⁺ uptake was higher than that of NO₃ ⁻ uptake. The K_m of NH₄ ⁺ uptake and K_m of NO₃ ⁻ uptake differed not significantly. When both NH₄ ⁺ and NO₃ ⁻ were present, the V_max for NH₄ ⁺ uptake became slightly higher, and the K_m for NH₄ ⁺ uptake remained in the same order. Under these conditions the NO₃ ⁻ uptake was almost totally inhibited over the whole range of concentrations used (10–1000 μM total N). This inhibition by NH₄ ⁺ occurred during the first two hours after addition. ei]{gnA C}{fnBorstlap}     

Changes in the Activity of Antioxidant Enzymes in Wheat Leaves and Roots as a Function of Nitrogen Source and Supply

The activity of enzymes participating in the systems of antioxidant protection was assayed in the second leaf and roots of 21-day-old wheat seedlings (Triticum aestivum L.) grown in a medium with nitrate (NO^– ₃ treatment), ammonium (NH⁺ ₄ treatment), or without nitrogen added (N-deficiency treatment). The activities of superoxide dismutase (SOD), peroxidase, ascorbate peroxidase, glutathione reductase, and catalase in the leaves and roots of the NH⁺ ₄ plants was significantly higher than in the plants grown in the nitrate medium. The activity of SOD decreased and ascorbate peroxidase markedly increased in leaves, whereas the activity of ascorbate peroxidase increased in the roots of N-deficient plants, as compared to the plants grown in nitrate and ammonium. Low-temperature incubation (5°, 12 h) differentially affected the antioxidant activity of the studied plants. Whereas leaf enzyme activities did not change in the NH⁺ ₄ plants, the activities of SOD, peroxidase, ascorbate peroxidase, and catalase markedly increased in the NO^– ₃ plants. In leaves of the N-deficient plant, the activity of SOD decreased; however, the activity of other enzymes increased. In response to temperature decrease, catalase activity increased in the roots of NO^– ₃ and NH⁺ ₄-plants, whereas in the N-deficient plants, the activity of peroxidase increased. Thus, in wheat, both nitrogen form and nitrogen deficiency changed the time-course of antioxidant enzyme activities in response to low temperature.     

Effect of UV Irradiance on Utilization of Inorganic Nitrogen by the Antarctic Diatom Odontella weissflogii (Janisch) Grunow

Abstract: The impact of UV-B irradiation on unialgal cultures of the temperate marine diatom Odontella sinensis and of the Antarctic Odontella weissflogii was tested under controlled laboratory conditions. Uptake rates of inorganic nitrogen by Odontella sinensis were more affected by UV-B radiation than those of Odontella weissflogii. Utilization of ¹⁵N-ammonium was reduced after 3 h of UV-B exposure. Values of K_m from several marine phytoplankton species were estimated from algae not exposed to UV-B and from those after 3 h of UV-B radiation. Calculation of Lineweaver-Burk plot showed, under UV-B stress, a noncompetitive inhibition for ¹⁵NH⁺₄ uptake and a competitive inhibitory effect for ¹⁵NO^?₃ by Odontella weissflogii. The damage to ¹⁵NH⁺₄ uptake by UV-B was more pronounced under red than blue light; a contradictory result was obtained for ¹⁵NO^?₃ utilization by Odontella weissflogii. Reduction of the pigments by UV-B under white light was dependent on the exposure time; a strong depression of the contents of chlorophyllide a, chlorophyll c, diatoxanthin and the fucoxanthins was found. Contents of the chlorophylls are markedly affected by UV-B under red light whereas the chlorophyll a concentration is enhanced in blue and green light. UV-B exposure in conjunction with blue and green light led to a reduction of the protein content and an increase in the amino acid contents. The pattern of pool sizes of free amino acids varied after UV-B exposure. UV-A irradiance had no effect. The possible targets of UV-B irradiation on the uptake system of inorganic nitrogen are discussed in detail. Adaptation to the environmental conditions, e.g. via synthesis of UV stress proteins or mycosporine-like amino acids, were also considered.     

Dissolved Nitrogen Uptake by a Cyanobacterial Bloom (Anabaena flos-aquae) in a Subarctic Lake

Uptake of dissolved nitrogen (NH₄⁺ + NO₃^- + urea + N₂) by a cyanobacterial [Anabaena flos-aquae (Lyngb.)] De Brèb population in Smith Lake, Alaska, was measured every 2 to 4 days during the spring of 1990. Total dissolved nitrogen uptake ranged from 0.34 to 24.75 μmol liter^-1 h^-1, with a mean of 5.75 μmol liter^-1 h^-1; the euphotic zone accounted for 91% of the uptake. The mean turnover time for dissolved combined nitrogen (NH₄⁺ + NO₃^- + urea) in the euphotic zone was less than 14 h, and that for NH₄⁺ was only 3.6 h. The mean relative preference indices for NH₄⁺ (2.4), NO₃^- (0.4), and urea (0.5) established NH₄⁺ as the preferred nitrogenous nutrient. The uptake rates were apparently dependent on biomass, temperature, and light. Regeneration, probably due to zooplankton excretion and bacterial remineralization of dissolved organic nitrogen, was the main source of NH₄⁺ for the cyanobacterial growth. The high half-saturation constant for NH₄⁺ with low ambient NH₄⁺ concentration nevertheless resulted in the simultaneous utilization of several forms of nitrogen.     

Relationships between the kinetics of NH4+ and NO3− absorption and growth in the cultivated tomato (Lycopersicon esculentum Mill, cv T-5)

Tomato growth was examined in solution culture under constant pH and low levels of NH₄⁺ or NO₃^?. There were five nitrogen treatments: 20 mmoles m^?3 NH₄⁺, 50 mmoles m^?3 NO₃^?, 100 mmoles m^?3 NH₄⁺ 200 mmoles m^?3 NO₃^?, and 20 mmoles m^?3 NH₄₊+ 50 mmoles m^?3 NO₃^?. The lower concentrations (20 mmoles m^?3 NH₄⁺ and 50 mmoles m^?3 NO₃^?) were near the apparent K_m for net NH₄⁺ and NO₃^? uptake; the higher concentrations (100 mmoles m^?3 NH₄⁺ and 200 mmoles m^?3 NO₃^?) were near levels at which the net uptake of NH₄⁺ or NO₃^? saturate. Although organic nitrogen contents for the higher NO₃^? and the NH₄⁺+ NO₃^? treatments were 22.2–30.3% greater than those for the lower NO₃^? treatment, relative growth rates were initially only 10–15% faster. After 24 d, relative growth rates were similar among those treatments. These results indicate that growth may be only slightly nitrogen limited when NH₄⁺ or NO₃^? concentrations are held constant over the root surface at near the apparent K_m concentration. Relative growth rates for the two NH₄⁺ treatments were much higher than have been previously reported for tomatoes growing with NH₄⁺ as the sole nitrogen source. Initial growth rates under NH₄⁺ nutrition did not differ significantly (P≥ 0.05) from those under NO₃^? or under combined NH₄⁺+ NO₃^?. Growth rates slowed after 10–15 d for the NH₄⁺ treatments, whereas they remained more constant for the NO₃^? and mixed NH₄⁺+ NO₃^? treatments over the entire observation period of 24–33 d. The decline in growth rate under NH₄⁺ nutrition may have resulted from a reduction in Ca²⁺, K⁺, and/or Mg²⁺ absorption.     

Nitrate and ammonium as nitrogen sources for lupins prior to nodulation

Two cultivars of Lupinus angustifolius L. were grown in a glasshouse in solutions containing NO₃ ^-, NH₄ ⁺ or NH₄NO₃ with a total nitrogen concentration of 2.8 M m^-3 in each treatment. One cultivar chosen (75A-258) was relatively tolerant to alkaline soils whereas the other (Yandee) was intolerant to alkalinity. Controlled experiments were used to assess the impact of cationic vs. anionic forms of nitrogen on the relative performance of these cultivars. Relative growth rates (dry weight basis) were not significantly different between the two cultivars when grown in the presence of NO₃ ^-, NH₄ ⁺ or NH₄NO₃. However, when NO₃ ^- was supplied, there was a modest decline in relative growth rates in both cultivars over time. When plants grown on the three sources of nitrogen for 9 days were subsequently supplied with ¹⁵NH₄NO₃ or NH₄ ¹⁵NO₃ for 30 h, NH₄ ⁺ uptake was generally twice as fast as NO₃ ^- uptake, even for plants grown in the presence of NO₃ ^-. Low rates of NO₃ ^- uptake accounted for the decrease in growth rates over time when plants were grown in the presence of NO₃ ^-. It is concluded that the more rapid growth of 75A-258 than Yandee in alkaline conditions was not due to preferential uptake of NH₄ ⁺ and acidification of the external medium. In support of this view, acidification of the root medium was not significantly different between cultivars when NH₄ ⁺ was the sole nitrogen source.     

Stable isotopes as indicators for seasonally dominant nitrogen cycling processes in a subarctic lake

Nitrogen stable isotopes (δ¹⁵N) of dissolved inorganic nitrogen (DIN = NH₄⁺ and NO₃^–), dissolved organic nitrogen (DON), and particulate organic nitrogen (PON) were measured in Smith Lake, Alaska to assess their usefulness as proxies for the biological nitrogen cycling processes, nutrient concentration, and lake productivity. Large seasonal variations in δ¹⁵NH₄⁺, δ¹⁵NO₃^– and δ¹⁵N_PON occurred in response to different processes of nitrogen transformation that dominated a specific time period of the annual production cycle. In spring, ¹⁵N depletion in all three pools was closely related to the occurrences of a N₂‐fixing cyanobacterial bloom (Anabaena flos‐aquae). In summer, δ¹⁵N_PON increased as phytoplankton community shifted to use NH₄⁺ and decreased as a brief N₂‐fixing bloom (Aphanizomenon flos‐aquae) occurred in August. In early and mid‐winter, microbial nitrogen processes were dominated by nitrification that resulted in the largest isotope fractionation between NO₃^– and NH₄⁺ in the annual cycle. This was followed by denitrification that led to the highest ¹⁵N enrichment in NO₃^–. A peak of NH₄⁺ assimilation by phytoplankton along with the elevated δ¹⁵N_PON and Chl a concentration occurred just before the ice break due to increased light penetration. The δ¹⁵N_DON displayed little temporal and spatial variations. This suggests that the DON pool was not altered by biological transformations of nitrogen as the results of its large size and possibly refractory nature. There was a positive correlation between Chl a concentration and δ¹⁵N_PON, and a negative correlation between NH₄⁺ and δ¹⁵N_PON, suggesting that δ¹⁵N_PON is a useful proxy for nitrogen productivity and ammonium concentration. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

SHORT TERM UPTAKE OF NUTRIENTS BY ENTEROMORPHA PROLIFERA (CHLOROPHYCEAE)1

Rates of NH₄⁺ and NO₃^? uptake were determined by accumulation of ¹⁵N in plant tissue and by disappearance of nutrient from the medium. Agreement between rates calculated by the two methods was good, averaging 82.7% (SD = 15.8%) and 91.2% (SD = 13.7%) for NH₄⁺ and NO₃^? uptake, respectively. An average of 93.4 and 96.0% of added ¹⁵NH₄⁺ and ¹⁵NO₃^? was recovered from the medium and /or plant tissue at the end of the incubations. Both bacterial uptake and regeneration of NH₄⁺ may contribute to discrepancies between NH₄⁺ uptake rates calculated by ¹⁵N accumulation and disappearance of NH₄⁺ from the medium. The influence of tissue composition on uptake of NH₄⁺, NO₃^? and PO₄^3- by Enteromorpha prolifera (Müller) J. Agardh was examined. For NH₄⁺ uptake, V_max was 188 μmol NH₄^+. g dry wt^?1. h^?1 and K_s ranged from 9.3 to 13.4 μM, but there was no correlation between kinetic parameters and tissue nitrogen content. For NO₃^?, both kinetic parameters were higher for plants with low tissue nitrogen than for plants with high tissue nitrogen. Maximum rates were 169 and 75.4 μmol NO₃^?. g dry wt^?1. h^?1, and K_s was 13.3 and 2.31 μM for low and high tissue nitrogen plants, respectively. Estimates of uptake in the field suggested that NH₄⁺ accounted for 65% and NO₃^? for up to 35% of total nitrogen uptake during the summer. Nutrient uptake rates of field-collected plants also indicated that E. prolifera in Yaquina Bay, Oregon was not likely to have been nitrogen-limited, but may have been phosphorus-limited.     

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.