Rate Of Iron Uptake Research Articles

Carbon and Iron Uptake by Phytoplankton in the Amundsen Sea, Antarctica.

Freshwater components in the Southern Ocean, whether sea ice meltwater or meteoric water, influence the growth of phytoplankton by affecting water stability and supplying dissolved iron (DFe). In addition, melting sea ice stimulates phytoplankton blooms by providing ice algae. In this study, sea ice meltwater and meteoric water in the Amundsen Sea (AS) were differentiated by their stable oxygen isotopic compositions (δ18O), while the phytoplankton carbon fixation rate (CFR) and iron uptake rate (FeUR) values were determined using the 14C and 55Fe tracer assays, respectively. Our results showed that FeUR exhibits a significant positive response only to sea ice meltwater, suggesting that DFe and algae provided by sea ice melting may be the main cause. In addition, the CFR had a slightly positive response to the freshwater input and a stronger correlation with the phytoplankton biomass, suggesting that the freshwater input may have enhanced the CFR through the algae released from sea ice melting. The FeUR normalized to the phytoplankton biomass was significantly positively correlated with the mixed layer depth, suggesting that water stability regulates the phytoplankton growth and the resulting Fe demand. A higher Fe demand per unit of carbon fixation during sea ice formation leads to a higher Fe/C ratio in phytoplankton. Although no significant correlations were observed between the FeUR, CFR, and meteoric water, meteoric water may have an effect on larger phytoplankton sensitive to Fe deficiencies. The results of culture experiments with DFe addition showed that the added Fe significantly enhanced the Fe uptake, carbon fixation, and Fe/C ratio of the cells, especially for micro-phytoplankton. The more pronounced response of micro-phytoplankton means that the meteoric water input may affect the efficiency of carbon export. Our study provides the first measurements of phytoplankton Fe quotas in the AS in austral late summer and early autumn, providing insights into how meteoric water and sea ice meltwater affect seasonal changes in Antarctic ecosystems.

Chlorophyll fluorescence as a light signal enhances iron uptake by the marine diatom Phaeodactylum tricornutum under high-cell density conditions

BackgroundDiatoms usually dominate phytoplankton blooms in open oceans, exhibiting extremely high population densities. Although the iron uptake rate of diatoms largely determines the magnitude and longevity of diatom blooms, the underlying mechanisms regulating iron uptake remain unclear.ResultsThe transcription of two iron uptake proteins, ISIP2a and ISIP1, in the marine diatom Phaeodactylum tricornutum was enhanced with increasing cell density, whereas the cellular iron content showed the opposite trend. When compared with the wild-type strain, knockdown of ISIP2a resulted in 43% decrease in cellular iron content, implying the involvement of ISIP2a in iron uptake under high-cell density conditions. Incubation of the diatom cells with sonicated cell lysate conditioned by different cell densities did not affect ISIP2a and ISIP1 expression, ruling out regulation via chemical cues. In contrast, ISIP2a and ISIP1 transcription were strongly induced by red light. Besides, chlorophyll fluorescence excited from the blue light was also positively correlated with population density. Subsequently, a “sandwich” illumination incubator was designed to filter out stray light and ensure that the inner layer cells only receive the emitted chlorophyll fluorescence from outer layers, and the results showed that the increase in outer cell density significantly elevated ISIP2a and ISIP1 transcription in inner layer cells. In situ evidence from Tara oceans also showed positively correlated between diatom ISIP transcripts and chlorophyll content.ConclusionsThis study shows that chlorophyll fluorescence derived from neighboring cells is able to upregulate ISIP2a and ISIP1 expression to facilitate iron assimilation under high-cell density. These results provide novel insights into biotic signal sensing in phytoplankton, which can help to elucidate the underlying mechanisms of marine diatom blooms.

Advances in understanding the crosstalk between mother and fetus on iron utilization

A full-term pregnancy comes with significant demand for iron. Not meeting this demand has adverse effects on maternal health and on the intrauterine and postnatal development of the infant. In the infant, some of these adverse effects cannot be reversed by postnatal iron supplementation, highlighting the need to tackle iron deficiency in utero. Achieving this requires sound understanding of the pathways that govern iron transfer at the fetomaternal interface. Two pathways are emerging as key players in this context; the hepcidin/ferroportin axis pathway and the iron regulatory protein (IRPs) pathway. In late gestation, suppression of maternal hepcidin, by as yet unknown factors, is required for increasing iron availability to the growing fetus. In the placenta, the rate of iron uptake by transferrin receptor TfR1 at the apical/maternal side and of iron release by ferroportin FPN at the basal/fetal side is controlled by IRP1. In fetal hepatocytes, build up of fetal iron stores requires post-translational inhibition of FPN by the cell-autonomous action of hepcidin. In the fetal liver, FPN is also subject to additional control at the transcriptional level, possibly by the action of hypoxia-inducible factor HIF2α. The rates of apical iron uptake and basal iron release in the placenta are modulated according to iron availability in the maternal blood and the placenta's own needs. This placental modulation ensures that the amount of iron delivered to the fetal circulation is maintained within a normal range, even in the face of mild maternal iron deficiency or overload. However, when maternal iron deficiency or overload are extreme, placental modulation is not sufficient to maintain normal iron supply to the fetus, resulting in fetal iron deficiency and overload respectively. Thus, the rate of iron transfer at the fetomaternal interface is subject to several regulatory signals operating simultaneously in the maternal liver, the placenta and the fetal liver. These regulatory signals act in concert to maintain normal iron supply to the fetus within a wide range of maternal iron states, but fail to do so when maternal iron deficiency or overload are extreme. The limitations of existing experimental models must be overcome if we are to gain better understanding of the role of these regulatory signals in normal and complicated pregnancy. Ultimately, that understanding could help identify better markers of fetal iron demand and underpin novel iron replacement strategies to treat maternal and fetal iron deficiency.

Evaluation of root growth, antioxidant enzyme activity and mineral absorbability of carnation (Dianthus caryophyllus “Express golem”) plantlets cultured in two culture systems supplemented with iron nanoparticles

In this study, different concentrations (0, 0.025, 0.05, 0.075, 0.1 and 0.2 mM) of iron nanoparticles (FeNPs) were used to replace 0.1 mM Fe-EDTA in half-strength MS medium to assess their effect on root growth, antioxidant enzyme activity and mineral absorbability of carnation plantlets cultured in in vitro system (MO) and microponic system (MP). The results showed that MP supplemented with 0.075 mM FeNPs was the most optimal culture condition for not only rooting stage, but also antioxidant activities and mineral absorption of carnation. The 3-week-old shoots cultured in this condition obtained the highest growth values with plant height (6.67 cm), root number (9.00 roots/plantlet), average root length (1.80 cm), fresh weight (70.33 mg), dry weight (5.67 mg), dry mass ratio (8.05 %), chlorophyll a (1.96 mg g−1 FW) and chlorophyll b (0.87 mg g−1 FW). Furthermore, the antioxidant activity analysis results showed that the activity of superoxide dismutase (28.52 U g-1 prot), catalase (540.98 U g-1 prot), and ascorbate peroxidase (0.55 U g-1 prot) in plantlets grown in MP system supplemented with 0.075 mM FeNPs were higher than those in MO system (14.15, 426.66 and 0.46 U g-1 prot; respectively) and also in the control (7.84, 309.08, 0.35 U g-1 prot; respectively). In addition, the absorption of potassium (205.51 mg g−1 DW), calcium (27.70 mg g−1 DW), magnesium (25.55 mg g−1 DW) and the iron-uptake rate (79.37 %) were highest in the MP system added with 0.075 mM FeNPs, as compared with those of the others. Plantlets derived from culture in MP system supplemented with FeNPs gave higher survival rate, better growth and development than that of the control treatment (used 0.1 mM Fe-EDTA). The plantlets were investigated after six weeks transplanting into the greenhouse.

Using Fe chemistry to predict Fe uptake rates for natural plankton assemblages from the Southern Ocean

A recent study using Fe-limited phytoplankton strains, showed that iron (Fe) uptake rates normalized by cellular surface area were best related to dissolved iron (dFe) concentrations as the inorganic Fe (Fe’) supply rates were not sufficient to satisfy the Fe biological demand. Short-term (24 h) shipboard incubations with the in-situ phytoplankton community were used to measure Fe uptake rates that were normalized per biomass (as particulate organic carbon, POC). Fe uptake rates measured following 55FeCl3 additions (0.05 to 0.9 nM) were fitted to different Fe pools (dFe, Felabile, and Fe’) using the Michaelis-Menten equation. Data showed a similar high conditional stability constant for biological transporters across all sites and phytoplankton size classes, with only a 2-fold variation in the concentrations of cellular transporters. These observations are in line with previous reports that eukaryotic phytoplankton takes up Fe close to the limit imposed by transporters cellular density and uses similar high-affinity Fe uptake systems. To further explore the link between Fe uptake rates and Fe chemistry, we also studied the effect of Fe additions preequilibrated with different Fe-binding ligands (L) including: the siderophore desferrioxamine B, two carbohydrates (glucuronic acid and carrageenan) and two different bacterial exopolycarbohydrates (L6 and L22, referred as EPS). For all stations, phytoplankton were able to acquire Fe associated to DFB as previously reported, however, different Fe:L ratios prevent quantitative comparison with other studies. Iron bound to carbohydrates, glucuronic acid, carrageenan and EPS could enhance or decrease Fe uptake rates in comparison to equimolar FeCl3 addition. These results illustrate that the effect of such L on Fe uptake rates will depend on the in-situ plankton community and their chemical structure. The variation of the Fe’ concentrations was able to explain up to 69% of the Fe uptake rates observed for the Antarctic communities. This relationship with Fe’ was related to the fact that the Fe’ maximal supply, due to the dissociation of FeL, was enough to satisfy the measured Fe uptakes rates. Calculations using previous reports in contrasted regions of the Southern Ocean, showed that Fe’ maximal supply was greater than Fe uptake rates measured in 80% of the cases. Moreover, considering photo- and redox-chemistry as well as kinetical situations prevailing in the field, Fe’ should not be overlooked as a pool able to satisfy most of the Fe biological demand. Finally, this study points towards the potential that the GEOTRACES Fe chemical speciation data represent to explore Fe uptake rates at a larger scale in this vast Fe-limited oceanic region.

Salp Fecal Pellets Release More Bioavailable Iron to Southern Ocean Phytoplankton than Krill Fecal Pellets

Over the last decades, the habitat of the Southern Ocean (SO) key species Antarctic Krill (Euphausia superba) has contracted to high latitudes due to reduced winter sea ice coverage, while salps as Salpa thompsoni, have extended their dispersal to the former krill habitats. To date, the potential implications of this population shift on the biogeochemical cycling of the limiting micronutrient iron and its bioavailability to SO phytoplankton has never been tested. In this study, we determined how efficiently krill and salps recycle iron and how bioavailable this iron is to SO phytoplankton. To test this, we collected fecal pellets (FP) of natural populations of salps and krill, added them to the same SO phytoplankton community and measured their iron uptake rates. Our result reveal that both FP additions yielded similar dissolved iron concentrations in the seawater. Per FP carbon added to the seawater, 4.8 ± 1.5 times more Fe was taken up by the same phytoplankton community from salp FP than from krill FP, suggesting that salp FP increased the iron bioavailability, possibly through the release of ligands. With respect to the ongoing shift from krill to salps, the potential for carbon fixation of the iron limited SO could be strengthened in the future, representing a negative feedback to climate change.

Characterization of the Potential Probiotic Vibrio sp. V33 Antagonizing Vibrio Splendidus Based on Iron Competition.

Background: Vibrio Splendidus Vs is an important aquaculture pathogen that can infect a broad host of marine organisms. In our previous study, an antagonistic bacterium Vibrio sp. V33 that possessed inhibitory effects on the growth and virulence of a pathogenic isolate V. splendidus Vs was identified.Objectives: Here, we further explored the antagonistic substances and antagonistic effects from the viewpoint of iron competition.Materials and Methods: The main antagonistic substances in the supernatants from Vibrio sp. V33 were identified using the bioassay-guided method. The response of V. splendidus Vs under the challenge of cell-free supernatant from Vibrio sp. V33 was determined via sodium dodecyl sulfate-polyacrylamide gel electrophoresis and real-time reverse-transcription PCR.Results: The main antagonistic substances produced by Vibrio sp. V33 have low molecular weights, are water soluble, and are heat-stable substances. Meanwhile, the iron uptake rate of Vibrio sp. V33 was higher than that of V. splendidus Vs. In the presence of cell-free supernatant from Vibrio sp. V33, expressions of two functional genes, viuB and asbJ related to ferric uptake processes in V. splendidus Vs, were up-regulated, whereas furVs coding the ferric uptake repressor was suppressed below 0.5-fold. One gene coding phosphopyruvate hydratase does not change at mRNA level, but was up-regulated at protein level. Conclusions: Our results suggested that antagonistic effect of Vibrio sp. V33 on the pathogenic isolate V. splendidus Vs was partially due to the stronger ability of Vibrio sp. V33 to seize iron. This cell-free supernatant from Vibrio sp. V33 created an iron-limited milieu for V. splendidus Vs, which led to the changed expression profiles of genes that were related to iron uptake in V. splendidus Vs.

Interaction effect of nitrogen form and planting density on plant growth and nutrient uptake in maize seedlings

High planting density is essential to increasing maize grain yield. However, single plants suffer from insufficient light under high planting density. Ammonium (NH4+) assimilation consumes less energy converted from radiation than nitrate (NO3−). It is hypothesized that a mixed NO3−/NH4+ supply is more important to improving plant growth and population productivity under high vs. low planting density. Maize plants were grown under hydroponic conditions at two planting densities (low density: 208 plants m−2 and high density: 667 plants m−2) and three nitrogen forms (nitrate only, 75/25NO3−/NH4+ and ammonium only). A significant interaction effect was found between planting density and N form on plant biomass. Compared to nitrate only, 75/25NO3−/NH4+ increased per-plant biomass by 44% under low density, but by 81% under high density. Treatment with 75/25NO3−/NH4+ increased plant ATP, photosynthetic rate, and carbon amount per plant by 31, 7, and 44% under low density, respectively, but by 51, 23, and 95% under high density. Accordingly, carbon level per plant under 75/25NO3−/NH4+ was improved, which increased leaf area, specific leaf weight and total root length, especially for high planting density, increased by 57, 17 and 63%, respectively. Furthermore, under low density, 75/25NO3−/NH4+ increased nitrogen uptake rate, while under high density, 75/25NO3−/NH4+ increased nitrogen, phosphorus, copper and iron uptake rates. By increasing energy use efficiency, an optimum NO3−/NH4+ ratio can improve plant growth and nutrient uptake efficiency, especially under high planting density. In summary, an appropriate supply of NH4+ in addition to nitrate can greatly improve plant growth and promote population productivity of maize under high planting density, and therefore a mixed N form is recommended for high-yielding maize management in the field.

Uptake of iron by Kluyveromyces marxianus DSM 5422 cultivated in a whey-based medium.

The ability of Kluyveromyces marxianus for converting lactose into ethyl acetate offers a chance for the economical reuse of whey. Iron plays a significant role in this process as ester synthesis requires a low intracellular iron content, xFe . The iron content in turn is decreased by growth due to cell expansion and increased by iron uptake. Thus, the iron-uptake rate, ψ, is important for the considered process. Iron uptake by K. marxianus DSM 5422 was studied in aerobic cultivation on a whey-borne medium with varied initial iron content, in part combined with a feed of iron under intensive growth conditions. A possible precipitation of iron that would pretend iron uptake was verified not to have occurred. Regularly measured dissolved iron concentrations, CFe,L , allowed the xFe and ψ parameters to be obtained by model-based iron balancing. The achieved data were used for establishing a ψ(CFe,L , xFe ) model. Mathematical simulations based on this iron-uptake model reproduced the performed cultivation processes. The created iron-uptake model allows for a future predictive system to be developed for the optimization of biotechnological ester production.

Cytoplasmic glycosylation of clathrin-mediated endocytosis signalling components alters the rate of iron uptake by placenta of mothers with type 2 diabetes

Cytoplasmic glycosylation of clathrin-mediated endocytosis signalling components alters the rate of iron uptake by placenta of mothers with type 2 diabetes

Increased iron availability resulting from increased CO2 enhances carbon and nitrogen metabolism in the economical marine red macroalga Pyropia haitanensis (Rhodophyta)

Ocean acidification caused by rising CO2 is predicted to increase the concentrations of dissolved species of Fe(II) and Fe(III), leading to the enhanced photosynthetic carbon sequestration in some algal species. In this study, the carbon and nitrogen metabolism in responses to increased iron availability under two CO2 levels (390 μL L−1 and 1000 μL L−1), were investigated in the maricultivated macroalga Pyropia haitanensis (Rhodophyta). The results showed that, elevated CO2 increased soluble carbonhydrate (SC) contents, resulting from enhanced photosynthesis and photosynthetic pigment synthesis in this algae, but declined its soluble protein (SP) contents, resulting in increased ratio of SC/SP. This enhanced photosynthesis performance and carbon accumulation was more significant under iron enrichment condition in seawater, with higher iron uptake rate at high CO2 level. As a key essential biogenic element for algae, Fe-replete functionally contributed to P. haitanensis photosynthesis. Increased SC fundamentally provided carbon skeletons for nitrogen assimilation. The significant increase of carbon and nitrogen assimilation finally contributed to enhanced growth in this alga. This was also intuitively reflected by respiration that provided energy for cellular metabolism and algal growth. We propose that, in the predicted scenario of rising atmospheric CO2, P. haitanensis is capable to adjust its physiology by increasing its carbon and nitrogen metabolism to acclimate the acidified seawater, at the background of global climate change and simultaneously increased iron concentration due to decreased pH levels.

An epirubicin-peptide conjugate with anticancer activity is dependent upon the expression level of the surface transferrin receptor.

Epirubicin (EPI) is one of the most widely used anticarcinogens; however, serious side effects, including cardiomyopathy and congestive heart failure, limit its long‑term administration. To overcome this problem, the HAIYPRH peptide ligand was used with EPI in the synthesis of a HAIYPRH‑EPI conjugate. The anticancer activity and cellular uptake of the conjugate were measured and evaluated. The results of the present study indicated that the cytotoxicity of HAIYPRH‑EPI was correlated with the expression of the cell surface transferrin receptor (TfR). The conjugate exerted high cytotoxicity and proapoptotic function when in an LN229 glioma cell line, which overexpresses surface TfR. It was hypothesized that transferrin (Tf) can promote cytotoxicity. Conversely, the conjugate exhibited very low cytotoxicity and proapoptotic function in a U87 glioma cell line, in which surface TfR expression was undetectable. In addition, fluorescence microscopy and flow cytometry methods were used to evaluate cellular uptake, and the results of these methods were consistent with the present hypotheses. The conjugate cellular uptake of the conjugate in LN229 cells was markedly higher compared with that in U87 cells, and it was hypothesized that Tf can enhance the uptake in LN229 cells. The cytotoxicity of HAIYPRH‑EPI was dependent upon the expression of surface TfR. Considering that the majority of cancer cells have high rates of iron uptake and surface TfR is generally overexpressed on cancer cells, it was speculated by the authors that HAIYPRH‑EPI may form part of an effective strategy for increasing the selectivity of EPI for cancer cells, as well as reducing its systemic toxicity. To confirm the hypothesis, the effects of HAIYPRH‑EPI on non‑cancerous cell lines were investigated. A future study will examine the side effects of HAIYPRH‑EPI, using a suitable delivery system in an animal model.

Laboratory culture experiments to study the effect of lignite humic acid fractions on iron solubility and iron uptake rates in phytoplankton

The major fractions of dissolved iron in seawater exist as a complex with organic ligands. A high bioavailability of iron bound to humic acid (HA) compared to the other model ligands, such as desferrioxamine B or ferrichrome, has been reported, which implies the importance of HA to control the geochemical behavior and the transfer of Fe to marine phytoplankton, particularly in estuarine and coastal waters. In the current work, the effect of different HA fractions (>100, 100–30, 30–10, 10–5, and 5–3 kDa), which were extracted from lignite, on the comparative solubility of iron in seawater and the corresponding influence on iron uptake and growth rate of the phytoplankton Prymnesium parvum (Haptophyta) was studied using laboratory cultures. The lower-molecular-weight (MW) HA fractions, such as 30–10, 10–5, and 5–3 kDa, remained soluble in the simulated seawater medium for a longer time span compared to the higher MW fractions. The lower MW fractions facilitated higher iron solubility and assisted in achieving a better phytoplankton growth rate. However, a reciprocal impact on phytoplankton growth rates was observed when the HA concentration increased to a higher range (0.18 to 18 mg-C L−1). The highest intracellular Fe uptake in phytoplankton occurred with 30–10 kDa HA in seawater, and the extracellular dissolved Fe concentrations were higher for smaller-sized HA fractions. In summary, our study showed that the controlled addition of lower MW fractions of HA (up to 30–10 kDa) in estuarine waters could ensure the accelerated uptake of Fe in phytoplankton as well as a better growth rate.

Identification of an iron permease, cFTR1, in cyanobacteria involved in the iron reduction/re-oxidation uptake pathway.

Cyanobacteria are globally important primary producers and abundant in many iron-limited aquatic environments. The ways in which they take up iron are largely unknown, but reduction of Fe3+ is an important step in the process. Here we report a special iron permease in Synechocystis, cFTR1, that is required for Fe3+ uptake following Fe2+ re-oxidation. The expression of cFTR1 is induced by iron starvation, and a mutant lacking the gene is abnormally sensitive to iron starvation. The cFTR1 protein localizes to the plasma membrane and contains the iron-binding motif "REXXE". Point-directed mutagenesis of the REXXE motif results in a sensitivity to Fe-deficiency. Measurements of iron (55 Fe) uptake rate show that cFTR1 takes up Fe3+ rather than Fe2+ . The function of cFTR1 in Synechocystis could be genetically complemented by the iron permease, Ftr1p, of Saccharomyces cerevisiae, that is known to transport Fe3+ produced by the oxidation of Fe2+ via a multicopper oxidase. Unlike yeast Ftr1p, cyanobacterial cFTR1 probably obtains Fe3+ primarily from the oxidation of Fe2+ by oxygen. Growth assays show that the cFTR1 is required during oxygenic, photoautotrophic growth but not when oxygen production is inhibited during photoheterotrophic growth. In cyanobacteria, iron reduction/re-oxidation uptake pathway may represent their adaptation to oxygenated environments.

Multiplicity and specificity of siderophore uptake in the cyanobacterium Anabaena sp. PCC 7120.

Many cyanobacteria secrete siderophores to sequester iron. Alternatively, mechanisms to utilize xenosiderophores have evolved. The overall uptake systems are comparable to that of other bacteria involving outer membrane transporters energized by TonB as well as plasma membrane-localized transporters. However, the function of the bioinformatically-inferred components is largely not established and recent studies showed a high diversity of the complexity of the uptake systems in different cyanobacteria. Thus, we approached the systems of the filamentous Anabaena sp. PCC 7120 as a model of a siderophore-secreting cyanobacterium. Anabaena sp. produces schizokinen and uptake of Fe-schizokinen involves the TonB-dependent transporter, schizokinen transporter (SchT), and the ABC-type transport system FhuBCD. We confirm that this system is also relevant for the uptake of structurally similar Fe-siderophore complexes like Fe-aerobactin. Moreover, we demonstrate a function of the TonB-dependent transporter IutA2 in Fe-schizokinen uptake in addition to SchT. The iutA2 mutant shows growth defects upon iron limitation, alterations in Fe-schizokinen uptake and in the transcription profile of the Fe-schizokinen uptake system. The physiological properties of the mutant confirm the importance of iron uptake for cellular function, e.g. for the Krebs cycle. Based on the relative relation of expression of schT and iutA2 as well as of the iron uptake rate to the degree of starvation, a model for the need of the co-existence of two different outer membrane transporters for the same substrate is discussed.

Iron-induced reactive oxygen species mediate transporter DMT1 endocytosis and iron uptake in intestinal epithelial cells.

Recent evidence shows that iron induces the endocytosis of the iron transporter dimetal transporter 1 (DMT1) during intestinal absorption. We, and others, have proposed that iron-induced DMT1 internalization underlies the mucosal block phenomena, a regulatory response that downregulates intestinal iron uptake after a large oral dose of iron. In this work, we investigated the participation of reactive oxygen species (ROS) in the establishment of this response. By means of selective surface protein biotinylation of polarized Caco-2 cells, we determined the kinetics of DMT1 internalization from the apical membrane after an iron challenge. The initial decrease in DMT1 levels in the apical membrane induced by iron was followed at later times by increased levels of DMT1. Addition of Fe(2+), but not of Cd(2+), Zn(2+), Cu(2+), or Cu(1+), induced the production of intracellular ROS, as detected by 2',7'-dichlorofluorescein (DCF) fluorescence. Preincubation with the antioxidant N-acetyl-l-cysteine (NAC) resulted in increased DMT1 at the apical membrane before and after addition of iron. Similarly, preincubation with the hydroxyl radical scavenger dimethyl sulfoxide (DMSO) resulted in the enhanced presence of DMT1 at the apical membrane. The decrease of DMT1 levels at the apical membrane induced by iron was associated with decreased iron uptake rates. A kinetic mathematical model based on operational rate constants of DMT1 endocytosis and exocytosis is proposed. The model qualitatively captures the experimental observations and accurately describes the effect of iron, NAC, and DMSO on the apical distribution of DMT1. Taken together, our data suggest that iron uptake induces the production of ROS, which modify DMT1 endocytic cycling, thus changing the iron transport activity at the apical membrane.

Competitive Effects of Calcium and Magnesium Ions on the Photochemical Transformation and Associated Cellular Uptake of Iron by the Freshwater Cyanobacterial Phytoplankton Microcystis aeruginosa.

Photochemical reduction of iron and iron uptake by Microcystis were investigated in a freshwater medium (pH 8) containing a range of calcium (Ca) and magnesium (Mg) ion concentrations (0.002-20 mM). In a medium containing the chelator ethylenediaminetetraacetic acid (EDTA), 50-fold increases in net photochemical formation rates of unchelated ferrous iron (Fe(II)') were observed as the concentration of calcium or magnesium metal (Me) was increased to exceed the concentration of EDTA. Kinetic modeling of iron transformation processes indicated that the facilitated Fe(II)' formation is attributed to Me-promoted photoreductive dissociation of the ferric iron-EDTA complex. In the medium containing Suwanee River fulvic acid, in contrast, the competitive effect of Me on photochemical Fe(II)' formation appears to be negligible due to the weak binding affinities of fulvic acid to Me. The cellular iron uptake rate in the EDTA-buffered system increased by ∼3-fold in the excess Me condition where the increased rate of photochemical Fe(II)' formation was observed, whereas the presence of Me resulted in a decrease in iron uptake rate in the fulvic acid system (by up to 5-fold). The decrease in iron uptake is likely caused by Me binding to iron transporters and other entities involved in intracellular iron transport. The findings of this study indicate a significant effect of Ca and Mg concentrations in natural waters on iron uptake by Microcystis, with the magnitude of effect depending strongly on ligand type.

Oxidative damage and iron effects following RBC transfusion

It is controversial whether transfusion of red blood cells (RBCs) after prolonged refrigerated storage induces adverse effects. Nonetheless, due to the RBC ‘storage lesion’, at outdate, the 24‐h post‐transfusion RBC recovery averages 75% in healthy recipients; alternatively, ~25% of these RBCs are cleared, mostly within 1–2 h post‐transfusion. Clearance is even greater in hospitalized patients. In addition, oxidative damage is, at least partly, responsible for the storage lesion. We hypothesized that donors with RBCs that are less, or more, able to withstand oxidative stress could constitute ‘poor storers’, or ‘super storers’, respectively. We also hypothesized that delivering a bolus of haemoglobin‐derived iron, resulting from macrophage catabolism of these rapidly cleared RBCs, is responsible for, at least some of, the adverse effects of these transfusions. For example, increased intracellular iron levels enhance the virulence of intracellular bacterial pathogens. Furthermore, in this setting, excess iron is exported to plasma through ferroportin at a pace exceeding transferrin's rate of iron uptake, thereby producing circulating non‐transferrin‐bound iron, which enhances the virulence of extracellular pathogens. To address the ‘iron hypothesis’, we are using cell culture models in vitro, murine and canine models in vivo, and transfusion studies in healthy human volunteers. Although experimental results demonstrate the effects of the post‐transfusion release of iron on infection, several unresolved questions remain.

Microbial iron uptake in the naturally fertilized waters in the vicinity of the Kerguelen Islands: phytoplankton–bacteria interactions

Abstract. Iron (Fe) uptake by the microbial community and the contribution of three different size fractions was determined during spring phytoplankton blooms in the naturally Fe-fertilized area off the Kerguelen Islands (KEOPS2). Total Fe uptake in surface waters was on average 34 ± 6 pmol Fe L-1 d-1, and microplankton (> 25 μm size fraction; 40–69%) and pico-nanoplankton (0.8–25 μm size fraction; 29–59%) were the main contributors. The contribution of heterotrophic bacteria (0.2–0.8 μm size fraction) to total Fe uptake was low at all stations (1–2%). Iron uptake rates normalized to carbon biomass were highest for pico-nanoplankton above the Kerguelen Plateau and for microplankton in the downstream plume. We also investigated the potential competition between heterotrophic bacteria and phytoplankton for the access to Fe. Bacterial Fe uptake rates normalized to carbon biomass were highest in incubations with bacteria alone, and dropped in incubations containing other components of the microbial community. Interestingly, the decrease in bacterial Fe uptake rate (up to 26-fold) was most pronounced in incubations containing pico-nanoplankton and bacteria, while the bacterial Fe uptake was only reduced by 2- to 8-fold in incubations containing the whole community (bacteria + pico-nanoplankton + microplankton). In Fe-fertilized waters, the bacterial Fe uptake rates normalized to carbon biomass were positively correlated with primary production. Taken together, these results suggest that heterotrophic bacteria are outcompeted by small-sized phytoplankton cells for the access to Fe during the spring bloom development, most likely due to the limitation by organic matter. We conclude that the Fe and carbon cycles are tightly coupled and driven by a complex interplay of competition and synergy between different members of the microbial community.

A Comparative Study of Iron Uptake Rates and Mechanisms amongst Marine and Fresh Water Cyanobacteria: Prevalence of Reductive Iron Uptake.

In this contribution, we address the question of iron bioavailability to cyanobacteria by measuring Fe uptake rates and probing for a reductive uptake pathway in diverse cyanobacterial species. We examined three Fe-substrates: dissolved inorganic iron (Fe') and the Fe-siderophores Ferrioxamine B (FOB) and FeAerobactin (FeAB). In order to compare across substrates and strains, we extracted uptake rate constants (kin = uptake rate/[Fe-substrate]). Fe' was the most bioavailable Fe form to cyanobacteria, with kin values higher than those of other substrates. When accounting for surface area (SA), all strains acquired Fe' at similar rates, as their kin/SA were similar. We also observed homogeneity in the uptake of FOB among strains, but with 10,000 times lower kin/SA values than Fe'. Uniformity in kin/SA suggests similarity in the mechanism of uptake and indeed, all strains were found to employ a reductive step in the uptake of Fe' and FOB. In contrast, different uptake pathways were found for FeAB along with variations in kin/SA. Our data supports the existence of a common reductive Fe uptake pathway amongst cyanobacteria, functioning alone or in addition to siderophore-mediated uptake. Cyanobacteria combining both uptake strategies benefit from increased flexibility in accessing different Fe-substrates.