Nitrate Uptake Kinetics Research Articles

Assessing the nitrate and phosphate uptake kinetics potential and growth performance of Gracilaria corticata var. cylindrica in shrimp farm water (SFW)

Gracilaria corticata var. cylindrica (GCC) was studied as a biofilter in shrimp farm water (SFW) and its growth and biochemical attributes in the medium were evaluated. GCC showed significant nutrient absorption from SFW, with maximum nitrate and phosphate uptake observed after the first 48 h. Nitrate depletion happened faster than phosphate, thus showing a higher affinity for nitrate uptake (78.4% removal) by GCC. Nutrient uptake kinetics followed the Michaelis–Menten curve, with Vmax values of 43.16 μM g dw−1 h−1 for nitrate and 67.50 μM g dw−1 h−1 for phosphate. Also, GCC’s low Km values (12.53 µM for nitrate, 1.91 µM for phosphate) indicated efficient nutrient utilization. GCC showed rapid growth in SFW, with a daily growth rate of 2.93 ± 0.19% d−1, comparable to the commercial MP1 media (3.54 ± 0.19% d−1) and greater to seawater (2.23 ± 0.13% d−1). Proximate analysis revealed consistent biochemical compositions across all three media, with carbohydrate and protein contents of 2.24 ± 0.41% and 0.76 ± 0.12%, respectively, in SFW. Pigment analysis revealed that pigment concentrations varied, with SFW having the greatest R-PE concentration (182.25 ± 32.77 µg g−1), followed by MP1 media and saltwater. R-PC concentrations followed a similar pattern, with SFW having the greatest concentration (85.73 ± 19.33 µg g−1). These findings highlight GCC’s potential as an efficient biofilter for reducing nitrate and phosphate levels in SFW, therefore offering a sustainable solution for aquaculture effluent management and the potential integration of GCC into shrimp farming practices.

Polymer-Clay Nanocomposites for the Uptake of Hazardous Anions.

Polymer intercalated clay nanocomposites were prepared from various montmorillonites (Mt) and a polymer, polydiallyldimethylammonim (PDDA) chloride. X-ray diffraction (XRD) analysis of the above polymer intercalated nanocomposites showed either no crystalline peaks or very broad peaks with the intercalation of PDDA polymer in the interlayers, probably as a result of exfoliation of the clay layers. Infrared spectroscopy revealed the presence of PDDA in all the clay nanocomposite materials. The maximum adsorption capacities of nitrate, perchlorate, and chromate by one of the polymer intercalated nanocomposite materials prepared from montmorillonite, Kunipea were 0.40 mmol·g-1, 0.44 mmol·g-1 and 0.299 mmol·g-1, respectively. The other two polymer intercalated nanocomposites prepared with montmorillonites from Wyoming and China showed very good adsorption capacities for perchlorate but somewhat lower uptake capacities for chromate and nitrate compared to the nanocomposite prepared from montmorillonite from Kunipea. The uptake of nitrate, perchlorate and chromate by the polymer intercalated nanocomposites could be well described using the Freundlich isotherm while their uptake kinetics fitted well to the pseudo-second-order model. The uptake kinetics of nitrate, perchlorate, and chromate were found to be fast as equilibrium was reached within 4 h. Moreover, the uptakes of chromate by polymer intercalated nanocomposites were found to be highly selective in the presence of Cl-, SO42- and CO32-, the most abundant naturally occurring anions.

The Role of Cymodocea nodosa and Caulerpa prolifera Meadows as Nitrogen Sinks in Temperate Coastal Lagoons

In coastal lagoons, marine benthic macrophyte meadows can be an important element in the resistance to eutrophication of the ecosystem, as they can function as temporary nitrogen sinks, limiting the availability of this nutrient for opportunistic organisms. The role of nitrogen sinks for two dominant macrophyte species of Mediterranean coastal lagoons, the seagrass Cymodocea nodosa and the seaweed Caulerpa prolifera, was analysed by two different approaches: (i) studying nitrate (NO3-) and ammonium (NH4+) uptake kinetics of aboveground tissues in a laboratory and (ii) estimating nitrogen stocks and demands of meadows under natural conditions. The studies were carried out in the coastal lagoon of the Mar Menor, which has been subjected to high anthropogenic nitrogen inputs for decades. While both macrophytes were efficient in exploiting NH4+ from the water column, only C. prolifera showed a high NO3- uptake capacity. Large N pools in the C. nodosa and C. prolifera meadows of the Mar Menor were detected, suggesting that these habitats may have the potential to be essential reservoirs for this nutrient. However, the major role of belowground tissues of the seagrasses in nitrogen accumulation may determine important differences between the two species in temporary N storage and sequestration. The data on N demands for the meadows of both macrophytes in the Mar Menor suggest an important contribution of these habitats in controlling the inputs of this nutrient into the lagoon. We conclude that C. nodosa and C. prolifera meadows can play a key role as a sink for dissolved inorganic nitrogen in temperate coastal lagoons, being an important mechanism of resistance to eutrophication.

Growth, nitrate uptake kinetics, and biofiltration potential of eucheumatoids with different thallus morphologies.

The declining production of commercially important eucheumatoids related to serious problems, like increasing susceptibility to ice-ice disease and epiphytism, may be ameliorated by nutrition. This ushered an increasing interest in incorporating seaweeds into an integrated multi-trophic aquaculture (IMTA) setup to take up excess inorganic nutrients produced by fish farms for their nourishment. In this regard, it is important to understand the nutrient uptake capacity of candidate seaweeds for incorporation into an IMTA system. Here, we examined the growth, nitrate ( ) uptake kinetics, and biofiltration potential of Eucheuma denticulatum and three strains of Kappaphycus alvarezii (G-O2, TR-C16, and SW-13) with distinct thallus morphologies. The uptake rates of the samples were determined under a range of concentrations (1-48 µM) and uptake rates were fitted to the Michaelis-Menten saturation equation. Among the examined eucheumatoids, only SW-13 had a linear response to concentration while other strains had uptake rates that followed the Michaelis-Menten saturation equation. Eucheuma denticulatum had the lowest Km (9.78 ± 1.48 µM) while G-O2 had the highest Vmax (307 ± 79.3 µmol · g-1 · min-1 ). The efficiency in uptake (highest Vmax /Km and α) was translated into the highest growth rate (3.41 ± 0.58% · d-1 ) measured in E. denticulatum. Our study provided evidence that eucheumatoids could potentially take up large amount of and fix CO2 when cultivated proximate to a fish farm as one component of an IMTA system. During a 45 -d cultivation period of eucheumatoids, as much as 370 g can be sequestered by every 1 kg initial biomass of E. denticulatum growing at 3% · d-1 . Furthermore, based on our unpublished photosynthetic measurements, the congeneric K. striatus can fix 27.5 g C · kg-1 DW during a 12 h daylight period.

Differential Nutrient Uptake by Saltmarsh Plants Is Modified by Increasing Salinity.

In Southern European estuaries and associated salt marshes, the anthropogenic nutrient inputs, together with longer drought periods, are leading to increasing eutrophication and salinization of these coastal ecosystems. In this study, uptake kinetics of ammonium, nitrate, and phosphate by three common plants in Palmones salt marsh (Southern Spain), Sarcocornia perennis ssp. alpini, Atriplex portulacoides, and Arthrocnemum macrostachyum were measured in hydroponic cultures. We also determined how these uptakes could be modified by increasing salinity, adding NaCl to the incubation medium (from 170 to 1,025 mM). Kinetic parameters are analyzed to understand the competition of the three species for nutrient resources under realistic most frequent concentrations in the salt marsh. These results may also be useful to predict the possible changes in the community composition and distribution if trends in environmental changes persist. Atriplex portulacoides showed the highest Vmax for ammonium, the most abundant nutrient in the salt marsh, while the highest affinity for this nutrient was observed in A. macrostachyum. Maximum uptake rates for nitrate were much lower than for ammonium, without significant differences among species. The highest Vmax value for phosphate was observed in A. macrostachyum, whereas A. portulacoides presented the highest affinity for this nutrient. High salinity drastically affected the physiological response of these species, decreasing nutrient uptake. Sarcocornia perennis ssp. alpini and A. macrostachyum were not affected by salinity up to 510 mM NaCl, whereas A. portulacoides notably decreased its uptake capacity at 427 mM and even withered at 1,025 mM NaCl. At current most frequent concentrations of ammonium and phosphate in the salt marsh, S. perennis ssp. alpini is the most favored species, from the nutritional point of view. However, A. portulacoides could enhance its presence if the increasing ammonium load continues, although a simultaneous salinization would negatively affect its nutritional physiology.

Elevated CO2 differentially regulates root nitrate transporter kinetics in a genotype and nitrate dose-dependent manner

The nitrogen (N) and protein concentration of wheat crop and grain often decline as a result of exposure of the crop to elevated CO2 (EC). In our earlier studies, it was found that the exacerbated production of nitric oxide (NO) represses the transcription of nitrate reductase (NR) and high affinity nitrate transporters (HATS) in EC grown wheat seedlings receiving high N. High N supply under EC also resulted in accumulation of reactive oxygen species (ROS), and reactive nitrogen species (RNS; NO and S- nitrosothiols) ensuing faster senescence and reduced N metabolite concentration in wheat. In this study, the effect of short-term exposure to EC on nitrate uptake kinetics was studied. The impact of EC on constitutive and inducible components of high affinity and low affinity nitrate uptake systems (HATS and LATS) were delineated in two wheat genotypes diverse in terms of nitrate uptake and assimilation capacities. Nitrate dose-response of NR was suppressed by EC in both leaf and root tissues. Plants grown under EC displayed a marked reduction in nitrate uptake kinetic components of LATS. Wheat genotype with high leaf nitrate assimilation capacity was able to maintain considerably higher nitrate uptake rate under EC albeit at a lower rate in comparison to ambient CO2. Wheat leaves exposed to EC displayed a comparatively higher abundance of NO and showed incremental abundance depending on increase in nitrate supply. Exogenous NO supply significantly suppressed the nitrate uptake rate of EC grown plants. Hence, EC-induced production of NO downregulates LATS kinetics in a genotype and nitrate dose-dependent manner.

Initial short-term nitrate uptake in juvenile, cultivated Saccharina latissima (Phaeophyceae) of variable nutritional state

Abstract To reach the goal of large-scale seaweed cultivation in Norway, new knowledge concerning commercially important species like the kelp Saccharina latissima is essential. This includes fundamental understanding of physiological mechanisms like nutrient uptake kinetics to better understand its ecological niche and nutritional requirements. The initial short-term nitrate (NO3−) uptake kinetics in cultivated S. latissima juvenile sporophytes were evaluated under nitrogen-saturated and nitrogen-limited conditions. The uptake was measured for concentrations in a gradient from 2 to 18 μM NO3− which is representative of Norwegian coastal waters in the main growth season of winter and early spring. Preconditioning treatments led to internal nitrogen pools (total tissue nitrogen and intracellular nitrate) that were significantly lower for the N-limited than for the N-saturated sporophytes prior to the experiment. Nitrate uptake rates, related to biomass (V) and intracellular nitrogen content (U), were linearly related to the substrate concentrations for both N-limited and N-saturated sporophytes, indicating that S. latissima requires high ambient nitrate concentrations to maintain rapid growth. The sporophytes with deficient internal nitrogen pools exhibited higher uptake rates of NO3− than sporophytes with higher internal pools of nitrogen. Mathematical modelling was used to investigate the temporal development of total tissue nitrogen (QN) based on the nitrogen-specific uptake rates (U) and revealed a near linear response of U to changes in QN. The model also found that a maximum estimated value for QN was only approached after more than 20 days at external NO3- concentrations of 8 μM. These results expand the physiological understanding of cultivated S. latissima and are important for a sustainable upscaling of seaweed farm production.

Nitrate Transporter Gene Expression and Kinetics of Nitrate Uptake by Populus × canadensis 'Neva' in Relation to Arbuscular Mycorrhizal Fungi and Nitrogen Availability.

Plants and other organisms in the ecosystem compete for the limited nitrogen (N) in the soil. Formation of a symbiotic relationship with arbuscular mycorrhizal fungi (AMF) may influence plant competitiveness for N. However, the effects of AMF on plant nitrate (NO3–) uptake capacity remain unknown. In this study, a pot experiment was conducted to investigate the effects of N application and Rhizophagus irregularis inoculation on the root absorbing area, uptake kinetics of NO3–, and the expression of NO3– transporter (NRT) genes in Populus × canadensis ‘Neva’. The results showed that R. irregularis colonized more than 70% of the roots of the poplar and increased root active absorbing area/total absorbing area. The uptake kinetics of NO3– by poplar fitted the Michaelis–Menten equation. Mycorrhizal plants had a higher maximum uptake rate (Vmax) value than non-mycorrhizal plants, indicating that R. irregularis enhanced the NO3– uptake capacity of poplar. The expression of NRTs in roots, namely, NRT1;2, NRT2;4B, NRT2;4C, NRT3;1A, NRT3;1B, and NRT3;1C, was decreased by R. irregularis under conditions of 0 and 1 mM NH4NO3. This study demonstrated that the improved NO3– uptake capacity by R. irregularis was not achieved by up-regulating the expression of NRTs in roots. The mycorrhizal pathway might repress root direct pathway in the NO3– uptake by mycorrhizal plants.

Specific uptake kinetics of glucose and nitrate in carbon-limited and nitrogen-limited C:N ratio under photoheterotrophic cultural conditions for Botryococcus brauniigrowth and lipid production

Carbon and nitrogen are the most significant parameters for microalgal growth and an effective C:N ratio is critical to assess the biomass and lipid productivities of Botryococcus braunii. In this study, the effect of the C:N ratio on specific uptake kinetics of glucose and nitrate was evaluated using substrate kinetic equation, under glucose limited and nitrate limited photoheterotrophic conditions. In glucose limited conditions, the maximum specific growth rate, biomass and lipid productivity of 0.0873 h−1, 1.11 g L−1 d−1 and 0.390 g L−1 d−1 were observed respectively for C:N ratio of 29:1. Maximum specific nitrate uptake rate of 0.190 mM gb−1 h−1 was observed under glucose limited C:N ratio of 61:1 (i.e. carbon sufficient), while the maximum glucose uptake rate of 0.74 mM gb−1 h−1 was observed in nitrate limited C:N ratio of 13:1 (i.e. nitrogen sufficient). This suggests that carbon and nitrogen metabolism is linked together and significantly affects the specific uptake kinetics of the each other.

Identification and Expression Analyses of the Nitrate Transporter Gene (NRT2) Family Among Skeletonema species (Bacillariophyceae).

High-affinity nitrate transporters are considered to be the major transporter system for nitrate uptake in diatoms. In the diatom genus Skeletonema, three forms of genes encoding high-affinity nitrate transporters (NRT2) were newly identified from transcriptomes generated as part of the marine microbial eukaryote transcriptome sequencing project. To examine the expression of each form of NRT2 under different nitrogen environments, laboratory experiments were conducted under nitrate-sufficient, ammonium-sufficient, and nitrate-limited conditions using three ecologically important Skeletonema species: S.dohrnii, S.menzelii, and S.marinoi. Primers were developed for each NRT2 form and species and Q-RT-PCR was performed. For each NRT2 form, the three Skeletonema species had similar transcriptional patterns. The transcript levels of NRT2:1 were significantly elevated under nitrogen-limited conditions, but strongly repressed in the presence of ammonium. The transcript levels of NRT2:2 were also repressed by ammonium, but increased 5- to 10-fold under nitrate-sufficient and nitrogen-limited conditions. Finally, the transcript levels of NRT2:3 did not vary significantly under various nitrogen conditions, and behaved more like a constitutively expressed gene. Based on the observed transcript variation among NRT2 forms, we propose a revised model describing nitrate uptake kinetics regulated by multiple forms of nitrate transporter genes in response to various nitrogen conditions in Skeletonema. The differential NRT2 transcriptional responses among species suggest that species-specific adaptive strategies exist within this genus to cope with environmental changes.

Nitrate Uptake Kinetics and Correlation Analysis in an Agricultural Drainage Ditch

To investigate the whole-reach nitrate (NO3--N) uptake dynamics in a headwater agricultural stream, we performed five pulse tracer additions of a reactive solute (as KNO3) and a conservative solute (as NaBr) in an agricultural drainage ditch in Hefei district, Chaohu Lake basin, from October 2016 to April 2017. The TASCC (tracer additions for spiraling curve characterization) approach and Michaelis-Menten (M-M) method were applied for the simulation of NO3--N uptake dynamics. Results showed that the ambient areal rate of total NO3--N uptake Uamb varied from 11.40 to 69.13 μg ·(m2 ·s)-1 with an average of 34.45 μg ·(m2 ·s)-1, and the ambient uptake velocity Vf-amb averaged 0.24 mm ·s-1 and varied from 0.07 to 0.43 mm ·s-1 across three well-mixed sub-reaches in the study. The ambient uptake length Sw-amb averaged 199.06 m with a range from 92.51 to 405.74 m, which was much smaller than the length of the drainage ditch (about 2.5 km), suggesting that the agricultural drainage ditch had a high potential for NO3--N retention. Generally, the M-M model fit the NO3--N uptake dynamics well, and the maximum uptake Umax ranged from 158 to 1280 μg ·(m2 ·s)-1 with a mean of 631.13 μg ·(m2 ·s)-1. The half saturation constant Km ranged from 0.16 to 5.52 mg ·L-1 with a mean of 1.46 mg ·L-1. According to correlation analysis, Sw-amb was negatively correlated with NO3--Namb, and Uambwas significantly positively correlated with NO3--Namb, while other nutrient spiraling metrics were not correlated with the NO3--N ambient concentration. Hydrological conditions had no distinct effect on the NO3--N retention, but both the width variability Фw and variability in cross-sectional area ФA were significantly correlated with most of the nutrient spiraling metrics, indicating that geomorphic features in the drainage ditch evidently impacted NO3--N uptake.

Uptake kinetics of nitrate, ammonia and phosphate by the dinoflagellate Heterocapsa circularisquama isolated from Hiroshima Bay, Japan

SUMMARY Heterocapsa circularisquama is a harmful dinoflagellate whose first bloom in Hiroshima Bay, Japan, appeared in 1992. As suggested by the authors’ group, in the Seto Inland Sea including Hiroshima Bay, oligotrophication particularly the reduction of phosphate starting 1980 is severe. The bloom caused serious damage to the bay's extensive oyster culture. In the present study, the uptake kinetics of nitrate, ammonia, and phosphate by this species were experimentally investigated. The maximum uptake rate (ρmax) and the half‐saturation constant (Ks) were 0.41 pmol cell−1 h−1 and 4.45 μM, respectively, for nitrate, 2.02 pmol cell−1 h−1 and 11.1 μM for ammonium, and 0.079 pmol cell−1 h−1 and 1.79 μM for phosphate. The maximum specific uptake rates (Vmax) for nitrate, ammonia, and phosphate were estimated to be 8.95, 44.1, and 21.3 day−1, respectively. A comparison of Vmax/Ks, which is also an index of affinity to nutrients, between this species and others suggested that H. circularisquama can utilize nitrate and ammonia efficiently, but not phosphate. Considering both reports describing that H. circularisquama has the ability to utilize dissolved organic phosphorus (DOP) and the DOP concentration is higher than phosphate in Hiroshima Bay, it was concluded that H. circularisquama became dominant due to the phosphate reduction measure.

Spatiotemporal variation of nitrate uptake kinetics within the maize (Zea mays L.) root system is associated with greater nitrate uptake and interactions with architectural phenes.

Increasing maize nitrogen acquisition efficiency is a major goal for the 21st century. Nitrate uptake kinetics (NUK) are defined by I max and K m, which denote the maximum uptake rate and the affinity of transporters, respectively. Because NUK have been studied predominantly at the molecular and whole-root system levels, little is known about the functional importance of NUK variation within root systems. A novel method was created to measure NUK of root segments that demonstrated variation in NUK among root classes (seminal, lateral, crown, and brace). I max varied among root class, plant age, and nitrate deprivation combinations, but was most affected by plant age, which increased I max, and nitrate deprivation time, which decreased I max K m was greatest for crown roots. The functional-structural simulation SimRoot was used for sensitivity analysis of plant growth to root segment I max and K m, as well as to test interactions of I max with root system architectural phenes. Simulated plant growth was more sensitive to I max than K m, and reached an asymptote near the maximum I max observed in the empirical studies. Increasing the I max of lateral roots had the largest effect on shoot growth. Additive effects of I max and architectural phenes on nitrate uptake were observed. Empirically, only lateral root tips aged 20 d operated at the maximum I max, and simulations demonstrated that increasing all seminal and lateral classes to this maximum rate could increase plant growth by as much as 26%. Therefore, optimizing I max for all maize root classes merits attention as a promising breeding goal.

English

Nitrogen use efficiency (NUE) is an important characteristic for increasing the yield and quality of rice grains and reducing the use of nitrogen fertilizers. This study evaluated parameters that contribute to NUE in two tropical japonica rice varieties contrasting in nitrogen remobilization efficiency (NRE) and nitrate-uptake kinetics: IAC-47 (bred for high-input farming), and Piaui (a landrace from the state of Maranhao, Brazil). The two varieties were grown with and without N supplementation at anthesis stage. Both varieties received urea equivalent to 60 kg N ha -1 in the first ten days after planting (DAP) and a subset of the two varieties received a supplementation with 40 kg N ha -1 at anthesis stage, composing a treatment with supplementation (Ts). The control treatment (Tc) was constituted of the plants that did not receive nitrogen supplementation at anthesis. We analyzed the nitrogen soluble fractions, soluble sugars, dry matter production, total N, crude protein in grains, NUE parameters, and expression and activity of glutamine synthetase (GS). The N uptake after anthesis affected the total N content in the shoots and grains of both rice varieties. In treatments with and without N supplementation, the nitrogen uptake efficiency (NUpE) was higher in IAC-47 than in Piaui plants. The NRE, a desirable characteristic for plants grown with low N input, was around two times higher in the Piaui plants that were not supplemented with N during anthesis. panicles of piaui plants grouwn without N supplementation presented higher expression of the glutamine synthetase gene OsGS1.3 post-anthesis. The results indicate that NRE is the primary factor contributing to NUE in rice varieties adapted to a low N supply, whereas improved varieties exhibit a higher NUpE.

DOC:NO3− ratios and NO3− uptake in forested headwater streams

AbstractThe underlying mechanisms driving the coupled interactions between inorganic nitrogen uptake and dissolved organic matter are not well understood, particularly in surface waters. To determine the relationship between dissolved organic carbon (DOC) quantity and nitrate (NO3−) uptake kinetics in streams, we performed a series of NO3− Tracer Additions for Spiraling Curve Characterization experiments in four streams within the Lamprey River Watershed, New Hampshire, across a range in background DOC concentrations (1–8 mg C/L). Experiments were performed throughout the 2013 and 2014 growing seasons. Across streams and experimental dates, ambient uptake velocity (Vf) correlated positively with increasing DOC concentrations and DOC:NO3− ratios but was only weakly negatively associated with NO3− concentrations. Ambient NO3− Vf was unrelated to pH, light, temperature, dissolved oxygen, and Specific Ultraviolet Absorbance at 254 nm. Although there were general tendencies across the entire Lamprey River Watershed, individual sites behaved differently in their uptake kinetics. NO3− uptake dynamics in the Lamprey River Watershed are most strongly influenced by DOC concentrations rather than NO3− concentrations or physicochemical parameters, which have been identified as regional‐ to continental‐scale drivers in previous research. Understanding the fundamental relationships between dissolved organic matter and inorganic nutrients will be important as global and climatic changes influence the delivery and production of DOC and NO3− in aquatic ecosystems.

Uptake and uptake kinetics of nitrate, ammonium and glycine by pakchoi seedlings (Brassica Campestris L. ssp. Chinensis L. Makino)

Plants not only absorb inorganic nitrogen (N), but also have the ability to absorb intact amino acids. Studies about pakchoi N uptake (Brassica Campestris L. ssp. Chinensis L. Makino) mainly focused on nitrate (NO3−) uptake and its accumulation in plant edible portion. This study investigated pakchoi seedlings uptake and uptake kinetics of NO3−, ammonium (NH4+) and glycine (Gly) using a mixed equimolar concentrations of the three N forms, and examined passive and active uptake ratios of the three N forms under sterile hydroponics.Our results revealed that pakchoi N uptake was positively related to its substrate N concentration (R=0.89–0.99, p<0.11), while N uptake efficiency showed a negative relation with the N concentration (R=−0.64–−0.77, p<0.36). At the N concentration from 25 to 1500μmolL−1, pakchoi took up significantly more NO3− than NH4+, but no significant difference was detected between uptakes of Gly and NO3− except at 250μmolL−1. At 7500μmolL−1, Gly uptake and uptake rate were significantly more than those for NO3− and NH4+. Regression analysis showed uptake rates for NO3−, NH4+ and Gly were well fitted to the Michaelis–Menten kinetics, and their affinity constant (Km) was in the range of 177–2000μmolL−1 and maximal velocity (Vmax) 18.2–46.8μmolg−1DW h−1. Pakchoi uptake for NO3−, NH4+ and Gly was dominated by active uptake. Gly active uptake at low concentration indicated plants have the ability to uptake amino acids that is relevant to field condition. However, passive uptake should not be overlooked in future studies, especially at high N concentration.

Expression of the Genes OsNRT1.1, OsNRT2.1, OsNRT2.2, and Kinetics of Nitrate Uptake in Genetically Contrasting Rice Varieties

Four genetically contrasting rice varieties (IAC-47, Bico Ganga, Arroz de Revenda and Manteiga) according to Random Amplified Polymorphic DNA (RAPD) analysis were assessed regarding expression of the genes OsNRT1.1, OsNRT2.1 and OsNRT2.2 and the nitrate uptake kinetics parameters (Km and Vmax). Up to 250-fold increases in the induction of gene expression after nitrate resupply were observed for the high-affinity transporter (OsNRT2.1 and OsNRT2.2). However, no significant variations in Vmax among the varieties were obtained. The lower value of Km of the IAC-47 cultivar in relation to the Arroz de Revenda variety suggests a greater role of high-affinity transporter genes. These results indicate that closer attention should be paid to the expression levels of these genes in selecting varieties aiming to enhance nitrogen uptake efficiency.

Effects of mixing-induced irradiance fluctuations on nitrogen uptake in size-fractionated coastal phytoplankton communities

In coastal waters subjected to strong tidal forcing, phytoplankton populations are exposed to highly variable light regimes. To grow under such fluctuating light environments, phytoplankton adjust their physiological properties. Here, we investigated nitrogen (N) uptake patterns in the western English Channel to determine whether phytoplankton modify their physiological processes involved in N uptake in response to changing irradiance conditions induced by spring-neap tidal cycles. Nitrate (NO3−) and ammonium (NH4+) uptake kinetics as a function of irradiance (VN-E curves) were assessed using 15N tracer techniques on two size fractions (<10 and >10 μm) of phytoplankton collected at 50% and 1% of surface irradiance during two spring-neap tidal cycles. Overall, the results showed that both small and large phytoplankton, whatever their vertical position in the water column, increased their maximum uptake capacity and their light utilization efficiency for the two N substrates following the decrease in vertical mixing intensity. Moreover, the improvement of irradiance conditions at neap tides was of greater benefit for the larger cells than for the smaller ones and was more favorable for NO3− uptake than for NH4+ uptake. These findings show that the light regime fluctuation resulting from the relaxation of tidal mixing during spring-neap tidal cycle leads to profound physiological adjustments of N uptake processes in phytoplankton communities. They suggest that the changes in NO3− uptake by large phytoplankton associated with the fortnightly spring-neap tidal cycle can account for most of the deviation in background productivity in the western English Channel which is based on NH4+ and is dominated by small cells. The dynamic light regime inherent to macrotidal coastal ecosystems could therefore determine, to a large extent, the importance of new vs. regenerated production as well as the size structure of the phytoplankton community.

Uptake and elimination kinetics of silver nanoparticles and silver nitrate by Raphidocelis subcapitata: The influence of silver behaviour in solution

Raphidocelis subcapitata is a freshwater algae species that constitutes the basis of many aquatic trophic chains. In this study, R. subcapitata was used as a model species to investigate the kinetics of uptake and elimination of silver nanoparticles (AgNP) in comparison to silver nitrate (AgNO3) with particular focus on the Ag sized-fractions in solution. AgNP used in this study were provided in a suspension of 1 mg Ag/l, with an initial size of 3–8 nm and coated with an alkane material. Algae was exposed for 48 h to both AgNP and AgNO3 and sampled at different time points to determine their internal Ag concentration over time. Samples were collected and separated into different sized fractions: total (Agtot), water column Ag (Agwater), small particulate Ag (Agsmall.part.) and dissolved Ag (Agdis). At AgNO3 exposures algae reached higher bioconcentration factor (BCF) and lower elimination rate constants than at AgNP exposures, meaning that Ag is more readily taken up by algae in its dissolved form than in its small particulate form, however slowly eliminated. When modelling the kinetics based on the Agdis fraction, a higher BCF was found. This supports our hypothesis that Ag would be internalised by algae only in its dissolved form. In addition, algae images obtained by Coherent Anti-stokes Raman Scattering (CARS) microscopy demonstrated large aggregates of nanoparticles external to the algae cells with no evidence of its internalisation, thus providing a strong suggestion that these AgNP were not able to penetrate the cells and Ag accumulation happens through the uptake of Ag ions.

Highly selective removal of nitrate and perchlorate by organoclay

An organoclay was prepared using montmorillonite and hexadecylpyridinium chloride (HDPyCl) and tested for the removal of nitrate and perchlorate anions from aqueous solutions. The cationic surfactant modified organoclay was prepared at room temperature using HDPyCl corresponding to 4 times the cation exchange capacity (CEC) of Na-montmorillonite. Powder X-ray diffraction (XRD) analysis of the above organoclay showed a large basal spacing of 40.27Å with the intercalation of HDPy cations in the interlayers probably as a result of a paraffin-type bilayer arrangement. The adsorption data of nitrate and perchlorate were fitted to the Langmuir and Freundlich adsorption isotherms and pseudo-first order and pseudo-second order kinetic models to better understand their adsorption mechanisms. The nitrate and perchlorate uptakes by this organoclay could be described well using the Langmuir isotherm while their uptake kinetics fitted well to the pseudo-second order model. The maximum adsorption capacities of nitrate and perchlorate by the organoclay, HDPy-montmorillonite were calculated at 0.67 and 1.11mmolg−1, respectively. Nitrate and perchlorate uptake kinetics were found to be fast as equilibrium was reached within 4h. Furthermore, the uptakes of nitrate and perchlorate by HDPy-montmorillonite were found to be highly selective in the presence of Cl−, SO42− and CO32−, the most abundant naturally occurring anions. Therefore, the HDPy-montmorillonite could be used as a highly efficient adsorbent for the separation of nitrate and perchlorate from drinking or waste water and ground water.