μM Nitrate Research Articles

Optimizing Nitrate and Tryptone to Enhance Growth and Triacylglycerol Accumulation in Phaeodactylum tricornutum.

Phaeodactylum tricornutum, a unicellular diatom, is considered a potential feedstock for the production of biofuel and a promising producer for high-value products eicosapentaenoic acid and fucoxanthin. However, a high-efficient cultivating strategy to achieve commercial production of triacylglycerol (TAG) from the diatom is an urgent demand. In this study, we optimized the content and ratio of nitrate and tryptone in the medium to enhance biomass and TAG accumulation simultaneously. Growth with tryptone as the sole nitrogen gave rise to the lowest cell density but the highest TAG content in P. tricornutum relative to nitrate, nitrite, ammonium or urea cultures. In 500 μM NaNO3 cultures, the growth of P. tricornutum increased with the increasing concentration (from 294 to 7056 μM nitrogen) of supplemented tryptone, however supplementation of high tryptone (≥882 μM nitrogen) decreased the neutral lipid content. Elevating nitrogen concentration from 294 to 882 μM via tryptone addition in 250 μM nitrate culture increased cell densities from day 6 to 10 and neutral lipid content on day 10. In particular, supplementing 588 μM nitrogen of tryptone in the 250 μM nitrate culture gave rise to the highest neutral lipid content on days 8 and 10 (increased by 109% and 62% relative to 500 μM nitrate-sole) with a comparable growth to that in 500 μM nitratesole culture from day 2 to 8. In conclusion, we optimized nitrate/tryptone ratio and found that a suitable tryptone addition to a relatively low nitrate culture was favourable to the biomass and TAG accumulation simultaneously in P. tricornutum.

Nitrate Uptake and Primary Production Along the Amazon River Plume Continuum

AbstractThe Amazon River plume (ARP) has been shown to support high rates of nitrogen fixation and primary production. However, nitrogen fixation alone cannot account for total primary production determined in the region, hinting that other nitrogen uptake processes might play a role. For the first time, we measured nitrate uptake rates in the ARP during three cruises in May 2018, June 2019 and April/May 2021, along with primary production rates and an analysis of phytoplankton community composition via high performance liquid chromatography. Based on a classification according to the salt content the region was divided into estuarine (ES), mesohaline (MH) and oceanic (OC) stations. Primary production was light limited near the river mouth at ES stations and was maximal off the coasts of French Guiana and Suriname, where also nitrate uptake was highest with rates of 11.4 mmol m−2 d−1. The role of eddies pinching off a deflecting plume are discussed as possible reason for higher nutrient concentrations at the MH stations. Surprisingly, at most MH stations north of 5°N, nitrate uptake rates were low despite the presence of sufficient substrate concentration (up to 1.44 μM nitrate). Diatoms, dinoflagellates or Synechococcus sp. dominated phytoplankton communities. OC stations showed lowest productivity rates in accordance with oligotrophic conditions. However, rates seem to be sufficient to completely deplete the remaining riverine nitrate, preventing its export to the open ocean.

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.

Synergies of chemodenitrification and denitrification in a saline inland lake

The interconnection between biotic and abiotic pathways involving the nitrogen and iron biogeochemical cycles has recently gained interest. While lacustrine ecosystems are considered prone to the biotic nitrate reduction (denitrification), their potential for promoting the abiotic nitrite reduction (chemodenitrification) remains unclear. In the present study, batch incubations were performed to assess the potential for chemodenitrification and denitrification in the saline inland lake Gallocanta. Sulfidic conditions are found in top sediments of the system while below (5–9 cm), it presents low organic carbon and high sulfate and ferrous iron availability. Anoxic incubations of sediment (5–9 cm) and water from the lake with nitrite revealed potential for chemodenitrification, especially when external ferrous iron was added. The obtained isotopic fractionation values for nitrite (ɛ15NNO2) were −6.8 and −12.3 ‰ and therefore, fell in the range of those previously reported for the nitrite reduction. The more pronounced ɛ15NNO2 (−12.3 ‰) measured in the experiment containing additional ferrous iron was attributed to a higher contribution of the chemodenitrification over biotic denitrification. Incubations containing nitrate also confirmed the potential for denitrification under autotrophic conditions (low organic carbon, high ferrous iron). Higher reaction rate constants were found in the experiment containing 100 μM compared to 400 μM nitrate. The obtained ɛ15NNO3 values (−8.5 and −15.1 ‰) during nitrate consumption fell in the range of those expected for the denitrification. A more pronounced ɛ15NNO3 (−15.1 ‰) was determined in the experiment presenting a lower reaction rate constant (400 μM nitrate). Therefore, in Gallocanta lake, nitrite generated during nitrate reduction can be further reduced by both the abiotic and biotic pathways. These findings establish the significance of chemodenitrification in lacustrine systems and support further exploration in aquatic environments with different levels of C, N, S, and Fe. This might be especially useful in predicting nitrous oxide emissions in natural ecosystems.

Distribution and biogeochemical perspectives of nutrients in the Eastern Equatorial Indian Ocean

The seasonal reversal of ocean circulation associated with seasonal change in the direction of prevailing winds and the occurrence of several anomalous events in the Eastern Equatorial Indian Ocean (EEIO) make this region dynamic and complex in terms of its biogeochemical characteristics. Two multidisciplinary cruises were conducted to measure nutrients and associated physicochemical parameters across the water column (up to 1000 m) of the EEIO during boreal summer and winter monsoons to understand the distribution of nutrients and their spatio-temporal variability from a biogeochemical perspective. The seasonality in the thermohaline structure of the region is indistinct except for surface salinity drop during summer monsoon due to more precipitation on-site and in adjoining areas. Low concentrations of chlorophyll at the surface and in the deep chlorophyll maxima represent the oligotrophic nature of this region. Surface water was found nutrient-depleted (0.03–0.4 µM Nitrate, 0.02–0.13 µM Phosphate). The maxima of vertical profiles of nitrate and phosphate were recorded at a shallower depth (150–200 m) when compared to its maxima in usual oceanic conditions, but a silicate maximum was recorded in deeper water. In the surface and upper mixed layer paucity of nutrients resulted in low N:P and N:Si ratios. Therefore, nitrogen limitation is evident. The overall ratio of N:P yielded a mean value of 15.33 and matched with the representative literature value for the Indian Ocean. The minimum oxygen values (<50 µM) in the deep water (150–200 m) indicated a hypoxic condition. No signature of denitrification and a moderate nitrate deficit were observed in deep waters. The negative values of Nitrate anomaly (N-tracer) at 50–100 m depth were attributed to a Watermass influenced by denitrification. The prevailing oligotrophic condition caused limited synthesis of organic matter and subsequently little decomposition in deep water. The maxima in the apparent oxygen utilization (AOU) profile are confined to 150 to 200 m depth and represent the most active zone for regeneration that is limited to shallow depth. Regenerated nutrients reached maxima at shallower depth and primarily control material cycling in this region. Supply of nitrate to the surface water based on the preformed values of prevailing water mass was primarily by Bay of Bengal water. According to the findings of this study, preformed nitrate concentrations between 100 and 200 metres below the surface were found very low, indicating that Indonesian Through Flow (ITF) has little impact on the distribution of nutrients in this area.

Spatial and Temporal Variability in Water Quality and Phytoplankton Community Composition in Charleston Harbor

The Charleston Harbor estuary is a dynamic ecosystem draining three rivers that surround the rapidly urbanizing greater Charleston area. Projected climate change impacts include elevated sea surface temperature and local changes in water quality that will likely alter biogeochemical cycling as well as phytoplankton abundance and community composition. Partnering with the local non-profit organization Charleston Waterkeeper, surface water samples were collected from late April through October 2021 at 20 sites in the Charleston Harbor estuary system. Water quality parameters measured included sea surface temperature (SST), salinity, pH, dissolved inorganic carbon, chromophoric dissolved organic matter (CDOM), phytoplankton pigments and nitrate and phosphate concentrations. Relatively high (6—16 μM) nitrate values were observed throughout the year and the nitrate to phosphate (N:P) ratios were consistently elevated (50—200) relative to the Redfield ratio of 16. Analysis of variance indicated that the delivery of nitrate, phosphate, and CDOM into the estuary came from upriver of the Charleston Harbor. For instance, CDOM was significantly impacted by tidal stage (p &lt; 0.05) and was negatively correlated with salinity at low salinity sites, but no correlation was observed at high salinity sites. Seasonal patterns in phytoplankton community abundance and composition were driven by changes in SST, as overall phytoplankton biomass increased with a community shift from diatoms during colder months to flagellated cells such as prasinophytes during the warmer summer months. Phytoplankton diversity was greatest during the early summer and lowest in October. This study provides a reference baseline for water quality parameters and phytoplankton community composition in an estuarine ecosystem that is changing rapidly due to dredging and climate change processes.

Exploiting Laser-Induced Graphene Composites as Substrates for Copper-Mediated Nitrate Reduction

The development of a nanostructured copper–laser-induced graphene (LIG) composite that can catalyze the reduction of nitrate is described. The system was characterized using a range of surface analytical methods (SEM, Raman, DekTak profilometry). The electrochemical performance of the copper mesh in reducing nitrate was investigated, the nature of the catalytic response was elucidated, and the influence of potential interferences was critically appraised. The adaptation of the system as the basis of an electrochemical sensor for nitrate was assessed, which displayed a limit of detection of 4.7 μM nitrate. The analytical applicability in authentic media was evaluated through the analysis of two surface water samples and validated by standard spectroscopic (nitrate reductase–Griess methods). The LIG substrate offers a simple, scalable route towards the reduction of nitrate with a construction simplicity and sensitivity that is competitive with much more complex nanomaterials.

Electrochemical properties modulation of Zinc oxide nanoparticles

The influence of zinc salts precursors (nitrate, acetate, chloride) on the electroactivity of the synthesised Zinc oxide nanoparticles (ZnONPs) was investigated using Glassy carbon ZnONPs modified electrode (ZnONPs films). The precipitation synthesised ZnONPs ‘s optical, morphological, size and Structural properties were assessed by usual spectroscopical technique. The XRD data confirmed synthesis of The ZnONPs which were predominately Zincite except for the nitrate product (zincite, ZHNH). All the NPs were of smaller size with the highest diameter size of 0.25 μM and lowest 0.066 μM for nitrate and chloride NPs respectively. All had good electroactivity, with rate constants (ks) above 1 × 10−2 indicating fast reversible reactions, values ranged from 0.6152 to 0.7515 for Zn(NO3) and Zn(CH3COO)2 respectively. Excellent opto-electric properties were observed with the nitrate product that had the highest band gaps (∼3.7 eV), DPV current(,7.57 × 10−7 A) ΔEp, diffusion coefficient (De) (3.120 × 10−1(cm2/s (Ariyanta et al., 2022) [5]) and rate constant (0.62 S-1). In addition it had the lowest resistance as indicated by the Rs value (1.13 kΩ) below the bare electrode value lowest Rct (444.77 kΩ) and CPE (3.00 × 10−6 F). For fabrication of sensors and batteries where low resistance and conductivity are essential, ZnONPs using Nitrate is ideal.

The Phaseolus vulgaris Receptor-Like Kinase PvFER1 and the Small Peptides PvRALF1 and PvRALF6 Regulate Nodule Number as a Function of Nitrate Availability.

Legumes associate with Gram-negative soil bacteria called rhizobia, resulting in the formation of a nitrogen-fixing organ, the nodule. Nodules are an important sink for photosynthates for legumes, so these plants have developed a systemic regulation mechanism that controls their optimal number of nodules, the so-called autoregulation of nodulation (AON) pathway, to balance energy costs with the benefits of nitrogen fixation. In addition, soil nitrate inhibits nodulation in a dose-dependent manner, through systemic and local mechanisms. The CLE family of peptides and their receptors are key to tightly controlling these inhibitory responses. In the present study, a functional analysis revealed that PvFER1, PvRALF1, and PvRALF6 act as positive regulators of the nodule number in growth medium containing 0 mM of nitrate but as negative regulators in medium with 2 and 5 mM of nitrate. Furthermore, the effect on nodule number was found to be consistent with changes in the expression levels of genes associated with the AON pathway and with the nitrate-mediated regulation of nodulation (NRN). Collectively, these data suggest that PvFER1, PvRALF1, and PvRALF6 regulate the optimal number of nodules as a function of nitrate availability.

A new in vitro monitoring system reveals a specific influence of Arabidopsis nitrogen nutrition on its susceptibility to Alternaria brassicicola at the seedling stage.

Seedling growth is an early phase of plant development highly susceptible to environmental factors such as soil nitrogen (N) availability or presence of seed-borne pathogens. Whereas N plays a central role in plant-pathogen interactions, its role has never been studied during this early phase for the interaction between Arabidopsis thaliana and Alternaria brassicicola, a seed-transmitted necrotrophic fungus. The aim of the present work was to develop an in vitro monitoring system allowing to study the impact of the fungus on A. thaliana seedling growth, while modulating N nutrition. The developed system consists of square plates placed vertically and filled with nutrient agar medium allowing modulation of N conditions. Seeds are inoculated after sowing by depositing a droplet of conidial suspension. A specific semi-automated image analysis pipeline based on the Ilastik software was developed to quantify the impact of the fungus on seedling aerial development, calculating an index accounting for every aspect of fungal impact, namely seedling death, necrosis and developmental delay. The system also permits to monitor root elongation. The interest of the system was then confirmed by characterising how N media composition [0.1 and 5mM of nitrate (NO3-), 5mM of ammonium (NH4+)] affects the impact of the fungus on three A. thaliana ecotypes. Seedling development was strongly and negatively affected by the fungus. However, seedlings grown with 5mM NO3- were less susceptible than those grown with NH4+ or 0.1mM NO3-, which differed from what was observed with adult plants (rosette stage). The developed monitoring system allows accurate determination of seedling growth characteristics (both on aerial and root parts) and symptoms. Altogether, this system could be used to study the impact of plant nutrition on susceptibility of various genotypes to fungi at the seedling stage.

The pulsating soft coral Xenia umbellata shows high resistance to warming when nitrate concentrations are low

The resistance of hard corals to warming can be negatively affected by nitrate eutrophication, but related knowledge for soft corals is scarce. We thus investigated the ecophysiological response of the pulsating soft coral Xenia umbellata to different levels of nitrate eutrophication (control = 0.6, medium = 6, high = 37 μM nitrate) in a laboratory experiment, with additional warming (27.7 to 32.8 °C) from days 17 to 37. High nitrate eutrophication enhanced cellular chlorophyll a content of Symbiodiniaceae by 168%, while it reduced gross photosynthesis by 56%. After additional warming, polyp pulsation rate was reduced by 100% in both nitrate eutrophication treatments, and additional polyp loss of 7% d−1 and total fragment mortality of 26% was observed in the high nitrate eutrophication treatment. Warming alone did not affect any of the investigated response parameters. These results suggest that X. umbellata exhibits resistance to warming, which may facilitate ecological dominance over some hard corals as ocean temperatures warm, though a clear negative physiological response occurs when combined with nitrate eutrophication. This study thus confirms the importance of investigating combinations of global and local factors to understand and manage changing coral reefs.

Nitrogen dynamics in small watersheds in the Atlantic Forest and Cerrado in Northeastern Brazil

The aim of this study was to determine concentrations of nitrogen (N) in two biomes, Atlantic Forest (AF) and Cerrado (CR), at the interface between terrestrial and aquatic ecosystems. This involved evaluating N in vegetation (litterfall), soil and water in small watersheds in environmental protection areas in northeastern Brazil. Soil chemical and physical analyzes were performed and mineralization and nitrification rates were determined. The forms of organic, inorganic and particulate nitrogen in water were determined by spectrophotometry. Two collections were carried out, one in the dry season (October and November 2019) and another in the rainy season (February and March 2020), in two environmental preservation areas, Estação Veracel Private Natural Heritage Reserve - (Atlantic Forest) and Chapada Diamantina National Park (Cerrado), with five small watersheds sampled in each biome. The N concentration in litterfall was similar in both biomes, with averages of 0.69%. In relation to soil and water, N concentrations were higher in AF compared to CR, with concentrations of 0.95 ± 0.40 and 0.59 ± 0.14 µg.Ng-1 of nitrate in the soils of AF and CR, respectively, and 3.53 ± 2.51 µM of nitrate in water in AF and 0.76 ± 0.78 µM in CR.

Optimisation of Biomass Production and Nutritional Value of Two Marine Diatoms (Bacillariophyceae), Skeletonema costatum and Chaetoceros calcitrans.

Simple SummaryOne of the key constraints that is associated with the production of microalgae biomass and products, is the low yields that are associated with high production costs in microalgae cultivation units. Therefore, the aim of the present work was to improve the biomass productivity of two high-value diatom species, Skeletonema costatum and Chaetoceros calcitrans. To do so, the culture medium that was supplied to the cultures was optimised in a stepwise process, regarding the nutrient’s silicate, nitrate, phosphorus, iron, and micronutrients. For both diatoms, the results that were obtained revealed a significant increase in biomass productivity as well as an improved biochemical profile regarding increased omega-3 fatty acids contents. With this work, the optimise culture media was established for each diatom, thus providing a strategy for lower production costs that were reflected in higher productivities with higher biomass quality. Ultimately this will help improve the application of S. costatum and C. calcitrans in the aquaculture and nutraceutical industries.S. costatum and C. calcitrans are two cosmopolitan high-value centric diatoms, with a rich nutritional profile. The following work optimised the culture medium of S. costatum and C. calcitrans cultures, respectively, in a stepwise process as follows: 2.4 mM and 1.2 mM of silicate, 4 mM of nitrate, 100 µM of phosphate, 20 and 80 µM iron, and 0.5 mL L−1 of micronutrients. The results that were obtained revealed an increase in biomass productivity with a 1.8- and 3.2-fold increase in biomass that was produced by S. costatum and C. calcitrans, respectively. The biochemical profile showed an increase in high-value PUFAs such as 2.6-fold and 2.3-fold increase in EPA for S. costatum and C. calcitrans, respectively, whilst a 2.6-fold increase in DHA was detected in S. costatum cultures. The present work provides the basic tools for the industrial cultivation of S. costatum and C. calcitrans with enhanced productivity as well as improved biomass quality, two factors which are highly relevant for a more effective application of these diatoms to aquaculture and nutraceutical production.

Lab-on-Chip for In Situ Analysis of Nutrients in the Deep Sea

Microfluidic reagent-based nutrient sensors offer a promising technology to address the global undersampling of ocean chemistry but have so far not been shown to operate in the deep sea (>200 m). We report a new family of miniaturized lab-on-chip (LOC) colorimetric analyzers making in situ nitrate and phosphate measurements from the surface ocean to the deep sea (>4800 m). This new technology gives users a new low-cost, high-performance tool for measuring chemistry in hyperbaric environments. Using a combination of laboratory verification and field-based tests, we demonstrate that the analyzers are capable of in situ measurements during profiling that are comparable to laboratory-based analyses. The sensors feature a novel and efficient inertial-flow mixer that increases the mixing efficiency and reduces the back pressure and flushing time compared to a previously used serpentine mixing channel. Four separate replicate units of the nitrate and phosphate sensor were calibrated in the laboratory and showed an average limit of detection of 0.03 μM for nitrate and 0.016 μM for phosphate. Three on-chip optical absorption cell lengths provide a large linear range (to >750 μM (10.5 mg/L-N) for nitrate and >15 μM (0.47 mg/L-P) for phosphate), making the instruments suitable for typical concentrations in both ocean and freshwater aquatic environments. The LOC systems automatically collected a series of deep-sea nitrate and phosphate profiles in the northeast Atlantic while attached to a conductivity temperature depth (CTD) rosette, and the LOC nitrate sensor was attached to a PROVOR profiling float to conduct automated nitrate profiles in the Mediterranean Sea.

Nitrogen-limitation exacerbates the impact of ultraviolet radiation on the coccolithophore Gephyrocapsa oceanica.

To investigate effects of UV radiation (UVR, 280-400 nm) on coccolithophorids under nutrient-limited conditions, we grew Gephyrocapsa oceanica to determine its resilience to consecutive daily short-term exposures to +UVR (irradiances >295 nm) under a range of nitrate availabilities (100, 24, 12, 6 and 3 μM). +UVR alone significantly hampered the growth of G. oceanica, with the synergistic negative effects of +UVR and N-limitation being about 58% and 22% greater than under UVR or N-limitation alone, respectively. Most 3 μM nitrate cultures died, but those exposed to UVR succumbed sooner. This was due to a failure of photoprotection and repair mechanisms under low N-availability with exposures to UVR. Additionally, the UVR-induced inhibition of the effective quantum yield of photosystem II (PSII) was significantly higher and was further aggravated by N limitation. The algal cells increased photoprotective pigments and UV-absorbing compounds as a priority rather than using calcification for defense against UVR, indicating a trade-off in energy and resource allocation. Our results indicate the negative effects of UVR on coccolithophorid growth and photosynthesis, and highlight the important role of N availability in defense against UVR as well as high PAR. We predict that enhanced N-limitation in future surface oceans due to warming-induced stratification will exacerbate the sensitivity of G. oceanica to UVR, while coccolithophores can be potentially more susceptible to other environmental stresses due to increased levels of nutrient limitation.

Assessment of Water Quality and Phytoplankton Structure of Eight Alexandria Beaches, Southeastern Mediterranean Sea, Egypt

This study aims to investigate the abundance, community, and structure of phytoplankton, physicochemical parameters, and some eutrophication state indices, to estimate the water quality of eight selected beaches along the Alexandria Coast, in the southeast of the Mediterranean Sea. The samples were collected monthly from 2019 to 2020. Nutrient values ranged from 1.54 to 33.21 µM for nitrate, 0.01 to 1.98 µM for nitrite, 0.12 to 9.45 µM for ammonia, 0.01 to 1.54 µM for phosphate, and 0.67 to 29.53 µM for silicate. Phytoplankton biomass was characterized by chlorophyll-a concentration, which fluctuated between 0.12 and 12.31 µg L−1. The annual phytoplankton average was 63.85 ± 17.83 × 103 cells L−1. Phytoplankton was highly diversified (228 taxa), and the most diversified group was diatoms (136 taxa), followed by a remarkably low number of Dinophyta (36 taxa). Diatoms reached maximum abundance in December. Meanwhile, a dense bloom of microalga Chlorella marina occurred in June on some beaches. High temperature, high dissolved inorganic nitrogen, and less-saline waters have supported green algal proliferation. The Shannon–Wiener diversity index (H’) showed that there was a qualitative seasonal difference in the composition of the phytoplankton community. Waters of beaches 1–3 were classified as between clean and moderately polluted; and beaches 4–8 varied between moderately and heavily polluted. The study revealed that human activities might have triggered the algal bloom and may be responsible for alterations in the Alexandria coast ecosystem.

DIFFERENT NITRATE AND AMMONIUM LEVELS MEDIA ON CHANGES OF NITROGEN ASSIMILATION ENZYMES IN RICE

Nitrogen (N) is an important nutrient for the growth and development of rice plants, required in large quantity and often limiting factor of rice yields. The research was to understand the different sources and levels of nitrogen in rice plant on the activity of N assimilation enzymes, including nitrate reductase (NR), glutamine synthase (GS) content, glutamate synthase (Gogat) content, content, ammonium (NH4+) and nitrate (NO3-) content on the leaves. Paddy (Ciherang variety) was grown in sand media containing Hoagland solution with different sources (ammonium and nitrate) and levels (0.4, 0.8, 1.6, 3.2, 6.4, and 12.8 mM) of nitrogen. Nitrogen assimilation was observed from leaves at one month of age. The NR activity increased on both Nitrogen sources, it was a higher activity in media contained nitrate. Also, the activity of GS showed higher in media contains nitrate, but its activity was decreased after application 1.6 mM of nitrate and 3.2 mM of ammonium. Western blot analysis of GS1 and GS2 showed that the band pattern of protein was similar to these enzyme activities. Nitrate content in leaves gradually increased in both sources of nitrogen and higher than 3.2 mM ammonium application caused an increase in ammonium content in leaves, but the nitrate content decreased. This research resulted that the available source of N for rice was in nitrate form, easily by the rice plants during the growth stage.

High plasticity of nitrogen fixation and denitrification of common coral reef substrates in response to nitrate availability

Nitrogen cycling in coral reefs may be affected by nutrient availability, but knowledge about concentration-dependent thresholds that modulate dinitrogen fixation and denitrification is missing. We determined the effects of different nitrate concentrations (ambient, 1, 5, 10 μM nitrate addition) on both processes under two light scenarios (i.e., light and dark) using a combined acetylene assay for two common benthic reef substrates, i.e., turf algae and coral rubble. For both substrates, dinitrogen fixation rates peaked at 5 μM nitrate addition in light, whereas denitrification was highest at 10 μM nitrate addition in the dark. At 10 μm nitrate addition in the dark, a near-complete collapse of dinitrogen fixation concurrent with a 76-fold increase in denitrification observed for coral rubble, suggesting potential threshold responses linked to the nutritional state of the community. We conclude that dynamic nitrogen cycling activity may help stabilise nitrogen availability in microbial communities associated with coral reef substrates.

NRT2.1, a major contributor to cadmium uptake controlled by high-affinity nitrate transporters

Management of nitrogen fertilizer is a good strategy for controlling cadmium (Cd) accumulation in plants. Some progress has already been made but much remains to be done. Here, we show that mutants with loss of function of nitrate transporter1.1 (NRT1.1) or nitrate transporter2.1 (NRT2.1) had lower Cd concentrations than wild-type plants under low-nitrate conditions. However, this was eliminated when plants were cultivated in nitrate-free medium or supplied with Cd and nitrate alternately. These findings indicate that inhibition of NRT1.1 or NRT2.1 activity reduces Cd accumulation in plants, and depends on the presence of nitrate. The results showing that nrt2.1–2 mutants had the lowest Cd concentrations compared with Col-0, nrt1.1 and nrt2.4 plants, proves that NRT2.1 is the major contributor to Cd uptake controlled by nitrate high-affinity transporters. NRT2.1 acts as the major contributor to nitrate uptake under Cd stress in low-nitrate conditions, and contributes about 50% to nitrate uptake, while NRT1.1 contributes only 10%, and little is known regarding the role of NRT2.2 and NRT2.4 on nitrate uptake in medium with 200 μM nitrate. Positive correlations between nitrate uptake and Cd concentration in plants were also observed. Collectively, NRT2.1 acts as the major contributor to Cd uptake by controlling nitrate uptake in nitrate high-affinity systems.

A new perspective of nutrient management of subtropical coastal stress-tolerant scleractinian coral communities

Elevated nutrients decrease the healthy status of coral communities, which can be stressful on reef corals, always resulting in decreased live scleractinian coral cover (LSCC). Here, we tested the relationship between benthic communities (scleractinian coral, macroalgae, and soft coral) and corresponding environmental parameters to illustrate the current state and potential threats of coral communities in Dongshan, Southern China. These stations were influenced to varying degrees by nutrient enrichment due to severe anthropogenic pressures. Both correlation and regression analysis indicated that increasing concentration of nitrate, nitrite and dissolved inorganic phosphate (DIP) was adversely correlated with LSCC. Conversely, a positive relationship between LSCC and aragonite saturation state (Ωarag) in the field was also confirmed. Moreover, when the regression models incorporated historical data, potential nutrient tolerance thresholds of the scleractinian coral in Dongshan were identified across the evidences of sharply LSCC decreasing (~ 2.3 μM for nitrate, ~ 0.2 μM for nitrite, and ~ 0.15 μM for DIP). This work demonstrates the environmental parameters varied under frequent anthropogenic disturbances, which in our case shaped LSCC in a potential future reef “refuge” for tropical scleractinian corals. We also confirm the forms-dependent effects of nitrogen on LSCC in a eutrophic environment, suggesting that proper management of reef associated waters, and interpretation of nutrient tolerance thresholds in coral may take nitrogen forms into consideration.