Iron Limitation (iron + limitation)

Distribution by Scientific Domains


Selected Abstracts


IMPACT OF IRON LIMITATION ON THE PHOTOSYNTHETIC APPARATUS OF THE DIATOM CHAETOCEROS MUELLERI (BACILLARIOPHYCEAE)

JOURNAL OF PHYCOLOGY, Issue 6 2001
Margaret Davey
Iron starvation induced marked increases in flavodoxin abundance and decreases in light-saturated and light-limited photosynthesis rates in the diatom Chaetoceros muelleri. Consistent with the substitution of flavodoxin for ferredoxin as an early response to iron starvation, increases of flavodoxin abundance were observed before declines of cell division rate or chl a specific photosynthesis rates. Changes in the abundance of flavodoxin after the addition of iron to iron-starved cells indicated that flavodoxin was not actively degraded under iron-replete conditions. Greater declines in light-saturated oxygen evolution rates than dark oxygen consumption rates indicated that the mitochondrial electron transfer chain was not affected as greatly by iron starvation as the photosynthetic electron transfer chain. The carbon:nitrogen ratio was unaffected by iron starvation, suggesting that photosynthetic electron transfer was a primary target of iron starvation and that reductions in nitrate assimilation were due to energy limitation (the C:N ratio would be expected to rise under nitrogen-limited but energy-replete conditions). Parallel changes were observed in the maximum light-saturated photosynthesis rate and the light-limited initial slope of the photosynthesis-light curve during iron starvation and recovery. The lowest photosynthesis rates were observed in iron-starved cells and the highest values in iron-replete cells. The light saturation parameter, Ik, was not affected by iron starvation, nor was the chl-to-C ratio markedly reduced. These observations were consistent with iron starvation having a similar or greater effect on photochemical charge separation in PSII than on downstream electron transfer steps. Declines of the ratio of variable to maximum fluorescence in iron-starved cells were consistent with PSII being a primary target of iron starvation. The functional cross-section of PSII was affected only marginally (<20%) by iron starvation, with the largest values observed in iron-starved cells. The rate constant for electron transfer calculated from fast repetition rate fluorescence was found to covary with the light-saturated photosynthesis rate; it was lowest in the most severely starved cells. [source]


Iron limitation induces SpoT-dependent accumulation of ppGpp in Escherichia coli

MOLECULAR MICROBIOLOGY, Issue 4 2005
Daniel Vinella
Summary In Escherichia coli the ,-lactam mecillinam specifically inhibits penicillin-binding protein 2 (PBP2), a peptidoglycan transpeptidase essential for maintaining rod shape. We have previously shown that PBP2 inactivation, results, in, a, cell, division, block, and, that an increased concentration of the nucleotide ppGpp, effector of the RelA-dependent stringent response, confers mecillinam resistance and allows cells to divide as spheres in the absence of PBP2 activity. In this study we have characterized an insertion mutation which confers mecillinam resistance in wild-type and ,relA strains but not in ,relA,spoT strains, devoid of ppGpp. The mutant has an insertion in the fes gene, coding for enterochelin esterase. This cytoplasmic enzyme hydrolyses enterochelin,Fe3+ complexes, making the scavenged iron available to the cells. We show that inactivation of the fes gene causes iron limitation on rich medium plates and a parallel SpoT-dependent increase of the ppGpp pool, as judged by the induction of the iron-regulated fiu::lacZ fusion and the repression of the stringently controlled P1rrnB::lacZ fusion respectively. We further show, by direct ppGpp assays, that iron starvation in liquid medium produces a SpoT-dependent increase of the ppGpp pool, strongly suggesting a role for iron in the balance of the two activities of SpoT, synthesis and hydrolysis of (p)ppGpp. Finally, we present evidence that ppGpp exerts direct or indirect positive control on iron uptake, suggesting a simple homeostatic regulatory circuit: iron limitation leads to an increased ppGpp pool, which increases the expression of iron uptake genes, thereby alleviating the limitation. [source]


Gene transcript analysis of assimilatory iron limitation in Geobacteraceae during groundwater bioremediation

ENVIRONMENTAL MICROBIOLOGY, Issue 5 2008
Regina A. O'Neil
Summary Limitations on the availability of Fe(III) as an electron acceptor are thought to play an important role in restricting the growth and activity of Geobacter species during bioremediation of contaminated subsurface environments, but the possibility that these organisms might also be limited in the subsurface by the availability of iron for assimilatory purposes was not previously considered because copious quantities of Fe(II) are produced as the result of Fe(III) reduction. Analysis of multiple Geobacteraceae genomes revealed the presence of a three-gene cluster consisting of homologues of two iron-dependent regulators, fur and dtxR (ideR), separated by a homologue of feoB, which encodes an Fe(II) uptake protein. This cluster appears to be conserved among members of the Geobacteraceae and was detected in several environments. Expression of the fur-feoB-ideR cluster decreased as Fe(II) concentrations increased in chemostat cultures. The number of Geobacteraceae feoB transcripts in groundwater samples from a site undergoing in situ uranium bioremediation was relatively high until the concentration of dissolved Fe(II) increased near the end of the field experiment. These results suggest that, because much of the Fe(II) is sequestered in solid phases, Geobacter species, which have a high requirement for iron for iron-sulfur proteins, may be limited by the amount of iron available for assimilatory purposes. These results demonstrate the ability of transcript analysis to reveal previously unsuspected aspects of the in situ physiology of microorganisms in subsurface environments. [source]


Characterization of a nif-regulated flavoprotein (FprA) from Rhodobacter capsulatus

FEBS JOURNAL, Issue 3 2000
2S] ferredoxin, Redox properties, molecular interaction with a [2Fe
A flavoprotein from Rhodobacter capsulatus was purified as a recombinant (His)6 -tag fusion from an Escherichia coli clone over-expressing the fprA structural gene. The FprA protein is a homodimer containing one molecule of FMN per 48-kDa monomer. Reduction of the flavoprotein by dithionite showed biphasic kinetics, starting with a fast step of semiquinone (SQ) formation, and followed by a slow reduction of the SQ. This SQ was in the anionic form as shown by EPR and optical spectroscopies. Spectrophotometric titration gave a midpoint redox potential for the oxidized/SQ couple of Em1 = +20 mV (pH 8.0), whereas the SQ/hydroquinone couple could not be titrated due to the thermodynamic instability of SQ associated with its slow reduction process. The inability to detect the intermediate form, SQ, upon oxidative titration confirmed this instability and led to an estimate of Em2 , Em1 of > 80 mV. The reduction of SQ by dithionite was significantly accelerated when the [2Fe,2S] ferredoxin FdIV was used as redox mediator. The midpoint redox potential of this ferredoxin was determined to be ,275 ± 2 mV at pH 7.5, consistent with FdIV serving as electron donor to FprA in vivo. FdIV and FprA were found to cross-react when incubated together with the 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide, giving a covalent complex with an Mr of , 60 000. Formation of this complex was unaffected by the redox states of the two proteins. Other [2Fe,2S] ferredoxins, including FdV and FdVI from R. capsulatus, were ineffective as electron carriers to FprA, and cross-reacted poorly with the flavoprotein. The possible function of FprA with regard to nitrogen fixation was investigated using an fprA -deleted mutant. Although nitrogenase activity was significantly reduced in the mutant compared with the wild-type strain, nitrogen fixation was apparently unaffected by the fprA deletion even under iron limitation or microaerobic conditions. [source]


Iron limits primary productivity during spring bloom development in the central North Atlantic

GLOBAL CHANGE BIOLOGY, Issue 4 2006
C. MARK MOORE
Abstract We present in situ biophysical measurements and bioassay experiments that demonstrate iron limitation of primary productivity during the spring bloom in the central North Atlantic. Mass balance calculations indicate that nitrate drawdown is iron (Fe)-limited and that aeolian Fe supply to this region cannot support maximal phytoplankton growth during the bloom. Using a simple simulation model, we show that relief of Fe limitation during the spring bloom can increase nitrate drawdown and, hence, new primary production, by 70%. We conclude that the episodic nature of iron supplied by dust deposition is an important factor controlling the dynamics of the spring bloom. From this, we hypothesize that variability in the timing and magnitude of the spring bloom in response to aeolian Fe supply will affect carbon drawdown and food web dynamics in the central North Atlantic. [source]


FERRIC CHELATE REDUCTASE ACTIVITY AS AFFECTED BY THE IRON-LIMITED GROWTH RATE IN FOUR SPECIES OF UNICELLULAR GREEN ALGAE (CHLOROPHYTA)1

JOURNAL OF PHYCOLOGY, Issue 3 2002
Harold G. Weger
Four species of green algae (Chlorella kessleri Fott et Nováková, Chlorococcum macrostigmatum Starr, Haematococcus lacustris[Girod-Chantrans] Rostaf., Stichococcus bacillaris Näg.) were grown in iron-limited chemostats and under phosphate limitation and iron (nutrient) sufficiency. For all four species, steady-state culture density declined with decreasing degree of iron limitation (increasing iron-limited growth rate), whereas chl per cell or biovolume increased. Plasma membrane ferric chelate reductase activity was enhanced by iron limitation in all species and suppressed by phosphate limitation and iron sufficiency. These results confirm previous work that C. kessleri uses a reductive mechanism of iron acquisition and also suggest that the other three species use the same mechanism. Although imposition of iron limitation led to enhanced activities of ferric chelate reductase in all species, the relationship between ferric chelate reductase activity and degree of iron limitation varied. Ferric chelate reductase activity in C. macrostigmatum and S. bacillaris was an inverse function of the degree of iron limitation, with the most rapidly growing iron-limited cells exhibiting the highest ferric chelate reductase activity. In contrast, ferric chelate reductase activity was only weakly affected by the degree of iron limitation in C. kessleri and H. lacustris. Calculation of ferric reductase activity per unit chl allowed a clear differentiation between iron-limited and iron-sufficient cells. The possible extension of the ferric chelate reductase assay to investigate the absence or presence of iron limitation in natural waters may be feasible, but it is unlikely that the assay could be used to estimate the degree of iron limitation. [source]


Effects of non-steady-state iron limitation on nitrogen assimilatory enzymes in the marine diatom thalassiosira weissflogii (BACILLARIOPHYCEAE)

JOURNAL OF PHYCOLOGY, Issue 1 2000
Allen J. Milligan
Since the recognition of iron-limited high nitrate (or nutrient) low chlorophyll (HNLC) regions of the ocean, low iron availability has been hypothesized to limit the assimilation of nitrate by diatoms. To determine the influence of non-steady-state iron availability on nitrogen assimilatory enzymes, cultures of Thalassiosira weissflogii (Grunow) Fryxell et Hasle were grown under iron-limited and iron-replete conditions using artificial seawater medium. Iron-limited cultures suffered from decreased efficiency of PSII as indicated by the DCMU-induced variable fluorescence signal (Fv/Fm). Under iron-replete conditions, in vitro nitrate reductase (NR) activity was rate limiting to nitrogen assimilation and in vitro nitrite reductase (NiR) activity was 50-fold higher. Under iron limitation, cultures excreted up to 100 fmol NO2,·cell,1·d,1 (about 10% of incorporated N) and NiR activities declined by 50-fold while internal NO2, pools remained relatively constant. Activities of both NR and NiR remained in excess of nitrogen incorporation rates throughout iron-limited growth. One possible explanation is that the supply of photosynthetically derived reductant to NiR may be responsible for the limitation of nitrogen assimilation at the NO2, reduction step. Urease activity showed no response to iron limitation. Carbon:nitrogen ratios were equivalent in both iron conditions, indicating that, relative to carbon, nitrogen was assimilated at similar rates whether iron was limiting growth or not. We hypothesize that, diatoms in HNLC regions are not deficient in their ability to assimilate nitrate when they are iron limited. Rather, it appears that diatoms are limited in their ability to process photons within the photosynthetic electron transport chain which results in nitrite reduction becoming the rate-limiting step in nitrogenassimilation. [source]


Iron limitation induces SpoT-dependent accumulation of ppGpp in Escherichia coli

MOLECULAR MICROBIOLOGY, Issue 4 2005
Daniel Vinella
Summary In Escherichia coli the ,-lactam mecillinam specifically inhibits penicillin-binding protein 2 (PBP2), a peptidoglycan transpeptidase essential for maintaining rod shape. We have previously shown that PBP2 inactivation, results, in, a, cell, division, block, and, that an increased concentration of the nucleotide ppGpp, effector of the RelA-dependent stringent response, confers mecillinam resistance and allows cells to divide as spheres in the absence of PBP2 activity. In this study we have characterized an insertion mutation which confers mecillinam resistance in wild-type and ,relA strains but not in ,relA,spoT strains, devoid of ppGpp. The mutant has an insertion in the fes gene, coding for enterochelin esterase. This cytoplasmic enzyme hydrolyses enterochelin,Fe3+ complexes, making the scavenged iron available to the cells. We show that inactivation of the fes gene causes iron limitation on rich medium plates and a parallel SpoT-dependent increase of the ppGpp pool, as judged by the induction of the iron-regulated fiu::lacZ fusion and the repression of the stringently controlled P1rrnB::lacZ fusion respectively. We further show, by direct ppGpp assays, that iron starvation in liquid medium produces a SpoT-dependent increase of the ppGpp pool, strongly suggesting a role for iron in the balance of the two activities of SpoT, synthesis and hydrolysis of (p)ppGpp. Finally, we present evidence that ppGpp exerts direct or indirect positive control on iron uptake, suggesting a simple homeostatic regulatory circuit: iron limitation leads to an increased ppGpp pool, which increases the expression of iron uptake genes, thereby alleviating the limitation. [source]