External Nutrient Concentrations Research Articles

Analytic results of a double-layered radial tumor model with different consumption rates

This paper is concerned with the evolution of a double-layered spherically symmetric tumor model containing two kinds of living cells: quiescent cells and proliferating cells. The main feature of our work is to take different nutrient consumption rates λ1 and λ2 of quiescent cells and proliferating cells into rigorous analysis. Under the condition λ1⩽λ2 and some reasonable biological assumptions, we prove the transient solutions of the model exist globally in time and investigate the asymptotic behavior of these solutions towards different radial stationary solutions such as (σs,Rs) and (σs,ηs,Rs), where Rs is the steady radius of the dormant tumor and ηs is the steady width of the quiescent-cell layer. We also point out the potential of controlling the internal structure and development trend of tumors by adjusting the external nutrient concentration. This might have important implications for tumor research and cancer treatment.

Transporter networks can serve plant cells as nutrient sensors and mimic transceptor-like behavior

SummarySensing of external mineral nutrient concentrations is essential for plants to colonize environments with a large spectrum of nutrient availability. Here, we analyzed transporter networks in computational cell biology simulations to understand better the initial steps of this sensing process. The networks analyzed were capable of translating the information of changing external nutrient concentrations into cytosolic H+ and Ca2+ signals, two of the most ubiquitous cellular second messengers. The concept emerging from the computational simulations was confirmed in wet-lab experiments. We document in guard cells that alterations in the external KCl concentration were translated into cytosolic H+ and Ca2+ transients as predicted. We show that transporter networks do not only serve their primary task of transport, but can also take on the role of a receptor without requiring conformational changes of a transporter protein. Such transceptor-like phenomena may be quite common in plants.

Simulation of Bottom Phytocenosis in the Crimean Coastal Zone

Purpose. The work is aimed at simulating production processes in the bottom phytocenosis typical of the rocky soils in the Crimean coastal zone. The macroalgae uptake of the main nutrients (phosphates, ammonium and nitrates) and release of oxygen and dissolved compounds are planned to be estimated. Methods and Results. The study was based on the balance model of the macroalgae growth and interaction with the environment. The model empirical parameters were determined from the published data of the laboratory experiments. The bottom phytocenosis in the model was represented by seven algae species: Cystoseira barbata, Ulva lactuca, Ceramium tenuicorne, Cladophora glomerata, Polysiphonia nigrescens, Phyllophora truncata, Enteromorpha prolifera. The main simulation results were: annual estimates of the released and absorbed substances, seasonal dynamics of the macroalgae biomass and evaluation of the bottom phytocenosis response to the changing environmental conditions. The rate of the production processes in the model was influenced both by the external (water temperature, insolation and concentration of nutrients in water) and internal (intracellular quotas of nitrogen and phosphorus, and species-specific parameters determining efficiency of nutrient assimilation by the algae tissues) factors. According to the simulation results, the ratio of nitrogen to phosphorus in the algae tissues was significantly higher than that in the seawater and varied from 30 to 60 for different species depending on water temperature, light exposure and concentration of nutrients. Conclusions. During the periods of intensive macroalgae growth (from April to September), the rate of nitrogen and phosphorus uptake can increase by a factor of 1.5 in comparison with the winter period. Maximum contribution to water purification from the surplus nutrients was made by the green algae: Ulva lactuca, Cladophora glomerata and Enteromorpha prolifera. At that Cladophora glomerata has shown the best adaptive abilities (minimal decrease in the growth rate) under a sharp drop of nutrients concentration in water.

Photosynthetic activity of Prorocentrum donghaiense Lu acclimated to phosphorus limitation and its photosynthetic responses to nutrient depletion

Prorocentrum donghaiense blooms periodically occur in late spring and early summer in the phosphorus (P)-limited East China Sea; however, few studies have been conducted in nutrient conditions that resembled those in the East China Sea. To study the actual photosynthetic status of P. donghaiense in P-limited conditions, we established stable P-limited conditions using semi-continuous culture at two dilution rates (0.1 and 0.3 day−1). When the external nutrient concentrations remained steady after 26 days, time did not affect the maximum quantum yield (Fv/Fm), normalized Stern-Volmer quenching (NPQNSV), maximum light use efficiency (α), maximum relative electron transport rate (rETRmax), and light saturation intensity (Ik). Prorocentrum donghaiense acclimated to P limitation, and the high actual quantum yield of photosystem II (ΦPSII) and rETRmax, low NPQNSV at 0.3 day−1, and increased ΦPSII with time at 0.3 day−1 indicated the photosynthetic advantages of P. donghaiense at a high growth rate. To test the responses of P-limitation-acclimated P. donghaiense to nutrient inputs, nutrient-addition experiments were conducted. Nitrate and phosphate addition thoroughly depleted P or nitrogen (N). Greater decreases in Fv/Fm, ΦPSII, α and rETRmax, and higher NPQNSV derived from rapid light curves (RLCs) were found under the P-depletion condition than under the N depletion condition. These results showed that the P-limitation-acclimated P. donghaiense had a weaker photosynthetic tolerance to P depletion. Given the differences in the photosynthetic tolerance of P. donghaiense to nutrient depletion, our results suggested that reduced P inputs should be considered during the management of P. donghaiense blooms.

Qualitative analysis of a free boundary problem for tumor growth under the action of periodic external inhibitors

In this paper, a free boundary problem for a solid avascular tumor growth under the action of periodic external inhibitors with time delays in proliferation is studied. Sufficient conditions for the global stability of tumor-free equilibrium are given. Moreover, if external concentration of nutrients is large, we also prove that the tumor will not disappear and determine the conditions under which there exist periodic solutions to the model. The results show that the periodicity of the inhibitor may imply periodicity of the size of the tumor. More precisely, if [Formula: see text] (the concentration of external nutrients) is greater than [Formula: see text], where [Formula: see text] are two constants; [Formula: see text]; [Formula: see text] is a periodic function which can be interpreted as a treatment and [Formula: see text] is the period of [Formula: see text]. Results are illustrated by computer simulations.

Model Simulations of a Mesocosm Experiment Investigating the Response of a Low Nutrient Low Chlorophyll (LNLC) Marine Ecosystem to Atmospheric Deposition Events

Atmospheric deposition of nitrogen and phosphorus represents an important source of nutrients, enhancing the marine productivity in oligotrophic areas, e.g. the Mediterranean. A comprehensive biogeochemical model (ERSEM) was setup and customized to simulate a mesocosm experiment, where dissolved inorganic nitrogen and phosphorus by means of atmospheric dust (single addition/SA and repetitive addition/RA in three successive doses) was added in controlled tanks and compared with a control (blank), all with Cretan Sea (Eastern Mediterranean) water. Observations on almost all components of the pelagic ecosystem in a ten-day period allowed investigating the effect of atmospheric deposition and the pathways of the added nutrients. The model was able to reasonably capture the observed variability of different ecosystem components and reproduce the main features of the experiment. An enhancement of primary production and phytoplankton biomass with added nutrients was simulated, in agreement with observations. A significant increase of bacterial production was also reproduced, while the model underestimated the observed increase and variability in bacterial biomass, but this deviation could be partly removed considering a lower carbon conversion factor from cell abundance data. A slightly stronger overall response was simulated with the single dust addition, compared to the repetitive that showed a few days delay. The simulated carbon pathways indicated that nutrient additions did not modify the microbial food web structure, but just increased its trophic status. Changes in model assumptions and parameter set that were necessary to reproduce the observed variability in the mesocosm experiment were discussed through a series of sensitivity simulations. Bacterial production was assumed to be mostly affected by the in situ produced labile organic matter, while it was further stimulated by the addition of inorganic nutrients, adopting a function of external nutrient concentrations for bacteria nutrient limitation. The effective increase in phytoplankton nutrient uptake rate was necessary, in order to reproduce the observed primary production, under such low nutrient concentrations, as also the increase of the grazers growth rate. The model was thus tuned to better work under very low nutrient concentrations, such as those found in the Eastern Mediterranean.

Achieving global perfect homeostasis through transporter regulation.

Nutrient homeostasis—the maintenance of relatively constant internal nutrient concentrations in fluctuating external environments—is essential to the survival of most organisms. Transcriptional regulation of plasma membrane transporters by internal nutrient concentrations is typically assumed to be the main mechanism by which homeostasis is achieved. While this mechanism is homeostatic we show that it does not achieve global perfect homeostasis—a condition where internal nutrient concentrations are completely independent of external nutrient concentrations for all external nutrient concentrations. We show that the criterion for global perfect homeostasis is that transporter levels must be inversely proportional to net nutrient flux into the cell and that downregulation of active transporters (activity-dependent regulation) is a simple and biologically plausible mechanism that meets this criterion. Activity-dependent transporter regulation creates a trade-off between robustness and efficiency, i.e., the system's ability to withstand perturbation in external nutrients and the transporter production rate needed to maintain homeostasis. Additionally, we show that a system that utilizes both activity-dependent transporter downregulation and regulation of transporter synthesis by internal nutrient levels can create a system that mitigates the shortcomings of each of the individual mechanisms. This analysis highlights the utility of activity-dependent regulation in achieving homeostasis and calls for a re-examination of the mechanisms of regulation of other homeostatic systems.

Long-term trends in nutrient budgets of the western Dutch Wadden Sea (1976–2012)

Long-term field observations of nitrogen [N] and phosphorus [P] concentrations were used to construct nutrient budgets for the western Dutch Wadden Sea between 1976 and 2012. Nutrients come into the western Dutch Wadden Sea via river runoff, through exchange with the coastal zone of the North Sea, neighbouring tidal basins and through atmospheric deposition (for N). The highest concentrations in phosphorus and nitrogen were observed in the mid-1980s. Improved phosphorus removal at waste water treatment plants, management of fertilization in agriculture and removal of phosphates from detergents led to reduced riverine nutrient inputs and, consequently, reduced nutrient concentrations in the Wadden Sea. The budgets suggest that the period of the initial net import of phosphorus and nitrogen switched to a net export in 1981 for nitrogen and in 1992 for phosphorus. Such different behaviour in nutrient budgets during the rise and fall of external nutrient concentrations may be the result of different sediment-water exchange dynamics for P and N. It is hypothesized that during the period of increasing eutrophication (1976–1981) P, and to a lesser degree N, were stored in sediments as organic and inorganic nutrients. In the following period (1981–1992) external nutrient concentrations (especially in the North Sea) decreased, but P concentrations in the Wadden Sea remained high due to prolonged sediment release, whilst denitrification removed substantial amounts of N.From 1992 onwards, P and N budgets were closed by net loss, most probably because P stores were then depleted and denitrification continued. Under the present conditions (lower rates of sediment import and depleted P stores), nutrient concentrations in this area are expected to be more strongly influenced by wind-driven exchange with the North Sea and precipitation-driven discharge from Lake IJssel. This implies that the consequences of climate change will be more important, than during the 1970s and 1980s.

Synergistic Malaria Parasite Killing by Two Types of Plasmodial Surface Anion Channel Inhibitors.

Malaria parasites increase their host erythrocyte’s permeability to a broad range of ions and organic solutes. The plasmodial surface anion channel (PSAC) mediates this uptake and is an established drug target. Development of therapies targeting this channel is limited by several problems including interactions between known inhibitors and permeating solutes that lead to incomplete channel block. Here, we designed and executed a high-throughput screen to identify a novel class of PSAC inhibitors that overcome this solute-inhibitor interaction. These new inhibitors differ from existing blockers and have distinct effects on channel-mediated transport, supporting a model of two separate routes for solute permeation though PSAC. Combinations of inhibitors specific for the two routes had strong synergistic action against in vitro parasite propagation, whereas combinations acting on a single route produced only additive effects. The magnitude of synergism depended on external nutrient concentrations, consistent with an essential role of the channel in parasite nutrient acquisition. The identified inhibitors will enable a better understanding of the channel’s structure-function and may be starting points for novel combination therapies that produce synergistic parasite killing.

A Time-Delayed Mathematical Model for Tumor Growth with the Effect of a Periodic Therapy.

A time-delayed mathematical model for tumor growth with the effect of periodic therapy is studied. The establishment of the model is based on the reaction-diffusion dynamics and mass conservation law and is considered with a time delay in cell proliferation process. Sufficient conditions for the global stability of tumor free equilibrium are given. We also prove that if external concentration of nutrients is large the tumor will not disappear and the conditions under which there exist periodic solutions to the model are also determined. Results are illustrated by computer simulations.

Analysis of a free boundary problem for tumor growth in a periodic external environment

In this paper a free boundary problem for solid avascular tumor growth in a periodic external environment is studied. The periodic environment means that the supply of nutrient and inhibitors is periodic. Sufficient conditions for the global stability of tumor free equilibrium are given. We also prove that if external concentration of nutrients is large, the tumor will not disappear. The conditions under which there exists a unique periodic solution to the model are determined, and we also show that the unique periodic solution is a global attractor of all other positive solutions.

Physiological response of a submerged plant (Myriophyllum spicatum) to different NH4Cl concentrations in sediments

The physiological characteristics and resistance capacity of the submerged macrophyte, Myriophyllum spicatum, under different external NH4Cl concentrations in sediments, were investigated in lab-scale tanks. Experimental results indicated that the growth, bioremediation capacity and enzyme activity of M. spicatum were affected by external sediment NH4Cl concentrations, along with time. When external NH4Cl concentrations in sediments were <200mg/kg, the N in sediments was assimilated and improved plant growth. When external NH4Cl concentration was >400mg/kg, the RGR decreased markedly and the MDA content increased obviously. Simultaneously, the SOD and POD activity in plant remained constant, or even decreased in some cases, suggesting that the stress resistance of M. spicatum was inhibited. Currently, there is little information about the physiological response of submerged macrophytes at different external nutrient concentrations in sediments. Therefore, the present study can provide theoretical foundation and date support for the control and bioremediation of endogenous pollution in lake.

Solute Restriction Reveals an Essential Role forclag3-Associated Channels in Malaria Parasite Nutrient Acquisition

The plasmodial surface anion channel (PSAC) increases erythrocyte permeability to many solutes in malaria but has uncertain physiological significance. We used a PSAC inhibitor with different efficacies against channels from two Plasmodium falciparum parasite lines and found concordant effects on transport and in vitro parasite growth when external nutrient concentrations were reduced. Linkage analysis using this growth inhibition phenotype in the Dd2 × HB3 genetic cross mapped the clag3 genomic locus, consistent with a role for two clag3 genes in PSAC-mediated transport. Altered inhibitor efficacy, achieved through allelic exchange or expression switching between the clag3 genes, indicated that the inhibitor kills parasites through direct action on PSAC. In a parasite unable to undergo expression switching, the inhibitor selected for ectopic homologous recombination between the clag3 genes to increase the diversity of available channel isoforms. Broad-spectrum inhibitors, which presumably interact with conserved sites on the channel, also exhibited improved efficacy with nutrient restriction. These findings indicate that PSAC functions in nutrient acquisition for intracellular parasites. Although key questions regarding the channel and its biological role remain, antimalarial drug development targeting PSAC should be pursued.

Elemental and fatty acid composition of snow algae in Arctic habitats

Red, orange or green snow is the macroscopic phenomenon comprising different eukaryotic algae. Little is known about the ecology and nutrient regimes in these algal communities. Therefore, eight snow algal communities from five intensively tinted snow fields in western Spitsbergen were analysed for nutrient concentrations and fatty acid (FA) composition. To evaluate the importance of a shift from green to red forms on the FA-variability of the field samples, four snow algal strains were grown under nitrogen replete and moderate light (+N+ML) or N-limited and high light (−N+HL) conditions. All eight field algal communities were dominated by red and orange cysts. Dissolved nutrient concentration of the snow revealed a broad range of NH+4 (<0.005–1.2 mg N l−1) and only low PO3−4 (<18 μg P l−1) levels. The external nutrient concentration did not reflect cellular nutrient ratios as C:N and C:P ratios of the communities were highest at locations containing relatively high concentrations of NH+4 and PO3−4. Molar N:P ratios ranged from 11 to 21 and did not suggest clear limitation of a single nutrient. On a per carbon basis, we found a 6-fold difference in total FA content between the eight snow algal communities, ranging from 50 to 300 mg FA g C−1. In multivariate analyses total FA content opposed the cellular N:C quota and a large part of the FA variability among field locations originated from the abundant FAs C18:1n-9, C18:2n-6, and C18:3n-3. Both field samples and snow algal strains grown under −N+HL conditions had high concentrations of C18:1n-9. FAs possibly accumulated due to the cessation of growth. Differences in color and nutritional composition between patches of snow algal communities within one snow field were not directly related to nutrient conditions. We propose that the highly patchy distribution of snow algae within and between snow fields may also result from differences in topographical and geological parameters such as slope, melting water rivulets, and rock formation.

Dynamic model of flexible phytoplankton nutrient uptake.

The metabolic machinery of marine microbes can be remarkably plastic, allowing organisms to persist under extreme nutrient limitation. With some exceptions, most theoretical approaches to nutrient uptake in phytoplankton are largely dominated by the classic Michaelis-Menten (MM) uptake functional form, whose constant parameters cannot account for the observed plasticity in the uptake apparatus. Following seminal ideas by earlier researchers, we propose a simple cell-level model based on a dynamic view of the uptake process whereby the cell can regulate the synthesis of uptake proteins in response to changes in both internal and external nutrient concentrations. In our flexible approach, the maximum uptake rate and nutrient affinity increase monotonically as the external nutrient concentration decreases. For low to medium nutrient availability, our model predicts uptake and growth rates larger than the classic MM counterparts, while matching the classic MM results for large nutrient concentrations. These results have important consequences for global coupled models of ocean circulation and biogeochemistry, which lack this regulatory mechanism and are thus likely to underestimate phytoplankton abundances and growth rates in oligotrophic regions of the ocean.

MODELING GROWTH AND ION CONCENTRATION OF LILIUM IN RESPONSE TO NITROGEN:POTASSIUM:CALCIUM MIXTURE SOLUTIONS

Nutrient solution composition plays an important role in root uptake rate due to interactions among nutrients and internal regulation. Studies to determine the optimum nutrient solution concentration are focused on individual ions, ignoring the adaptation mechanisms triggered by plants when growing in a varying external nutrient concentration. The objective of the present study was to determine the response in growth and tissue ion concentration of lilium cv. ‘Navona’ to nutrient mixtures of varying proportions of nitrogen (N), potassium (K+), and calcium (Ca2+) in solution using mixture experiments methodology in order to determine the optimum concentration. Bulbs of lilium were transplanted in plastic crates and drip-irrigated with the treatment solutions, which consisted of a mixture of N, K+, and Ca2+ whose total concentration was 340 mg L−1 and minimum concentrations of each ion was 34 mg L−1. Chlorophyll concentration (SPAD), shoot fresh weight (FW), leaf FW, and leaf area were measured 60 days after transplanting and ion analysis was performed on shoot tissues from selected treatments. Lilium exhibited a moderate demand for N and K+ (136–170 mg L−1 N and 116–136 mg L−1 K+) and a very low demand for Ca2+ (34–88 mg · L−1). This low demand may be due to the remobilization of the nutrients stored in the bulbs. Integrating the predictions of the models estimated to produce >90% of maximum growth, the optimum nutrient solution should contain Ca2+ at a concentration between 34 and 126 mg · L−1, K+ between 119 and 211 mg · L−1, and N between 92 mg · L−1 and 211 mg · L−1. Increasing external N concentration affected internal N concentration but not internal K+ or Ca2+ concentrations, despite that the increase in external N was associated with a decrease in external K+ and Ca2+. Similar trends were observed for external K+ and Ca2+ concentration. In conclusion, lilium was able to maintain a relatively constant K+ and Ca2+ concentration regardless of the lower concentration in the nutrient solution when N was increased (similar response was observed for K+ and Ca2+) and it has a low Ca2+ demand and moderate N and K+ supply.

NUTRIENT CONCENTRATIONS OF PLANT TISSUES OF GREENHOUSE CROPS AS AFFECTED BY THE EC OF THE EXTERNAL NUTRIENT SOLUTION

NUTRIENT CONCENTRATIONS OF PLANT TISSUES OF GREENHOUSE CROPS AS AFFECTED BY THE EC OF THE EXTERNAL NUTRIENT SOLUTION

Nutrient Sensing and Signaling: NPKS

Plants often grow in soils that contain very low concentrations of the macronutrients nitrogen, phosphorus, potassium, and sulfur. To adapt and grow in nutrient-deprived environments plants must sense changes in external and internal mineral nutrient concentrations and adjust growth to match resource availability. The sensing and signal transduction networks that control plant responses to nutrient deprivation are not well characterized for nitrogen, potassium, and sulfur deprivation. One branch of the signal transduction cascade related to phosphorus-deprivation response has been defined through the identification of a transcription factor that is regulated by sumoylation. Two different microRNAs play roles in regulating gene expression under phosphorus and sulfur deprivation. Reactive oxygen species increase rapidly after mineral nutrient deprivation and may be one upstream mediator of nutrient signaling. A number of molecular analyses suggest that both short-term and longer-term responses will be important in understanding the progression of signaling events when the external, then internal, supplies of nutrients become depleted.

Influence of salinity, nutrients and light on the germination and growth of Enteromorpha sp. spores

In many shallow coastal areas worldwide, several species of opportunistic macroalgae (mainly Chlorophyta) have an excessive growth, as a consequence of eutrophication processes. Therefore, bloom-forming macroalgae become the dominant primary producers within these coastal systems. However, frequently the ecology and the ecophysiology of adult macroalgae have been insufficient to explain their seasonal abundances. Thus, it is essential to understand the factors that regulate the germination and growth of spores of opportunistic green macroalgae. In the present work, we assessed the effects of nutrients (N and P), salinity and light on the germination and growth of Enteromorpha spores. Overall, the results highlight the fact that, such as for adult macroalgae, spore germination and growth are adversely affected by low salinities. Growth of the spores is significantly decreased at 5 psu, while salinities of 20 and, especially of 35 psu, clearly promote the spore growth. Additionally, Enteromorpha spores seem to be particularly sensitive to PO 4–P limitation and to NH 4–N toxicity, which suggests a higher sensitivity to the variation of external nutrient concentrations than adult macroalgae. The present results contribute to increase the understanding about the factors that control macroalgal growth at its early phases of development. In particular, the results suggest that the growth of spores from opportunistic green macroalgae is strongly salinity-dependent. Consequently, in highly hydrodynamic systems such as most shallow estuaries, salinity variations may play a determinant role in the yearly abundances of green macroalgae, since it controls macroalgal growth from the spores to the adults.

Development of a robust marine ecosystem model to predict the role of iron in biogeochemical cycles: A comparison of results for iron-replete and iron-limited areas, and the SOIREE iron-enrichment experiment

A new mixed layer multi-nutrient ecosystem model, incorporating diatoms, non-diatoms and zooplankton, is described that models the role of iron in marine biogeochemical cycles. The internal cell biochemistry of the phytoplankton is modelled using the mechanistic model of Flynn [2001. A mechanistic model for describing dynamic multi-nutrient, light, temperature interactions in phytoplankton. Journal of Plankton Research 23, 977–997] in which the internal cell concentrations of chlorophyll, nitrogen, silica, and iron are all dynamic variables that respond to external nutrient concentrations and light levels. Iron stress in phytoplankton feeds back into chlorophyll synthesis and changes in photosynthetic unit (PSU) size, thereby reducing their growth rate. Because diatom silicon metabolism is inextricably linked with cell division, diatom population density (cell m −3) is modelled as well as C biomass. An optimisation technique was used to fit the model to three time-series datasets at Biotrans (47°N, 20°W) and Kerfix (50°40′S, 68°25′E) and the observations for the Southern Ocean Iron-Release Experiment (SOIREE) iron-enrichment experiment (61°S, 140°E). The model gives realistic simulations of the annual cycles of nutrients, phytoplankton, and primary production at Biotrans and Kerfix and can also accurately simulate an iron fertilisation experiment. Specifically, the model predicts the high values of diatom Si:N and Si:C ratios observed in areas where iron is a limiting factor on algal growth. In addition, the model results at Kerfix confirm previous suggestions that underwater light levels have a more limiting effect on phytoplankton growth than iron supply. The model is also used to calculate C budgets and C and Si export from the mixed layer. The implications of these results for developing biogeochemical models incorporating the role of iron are discussed.