Effects of Iron Starvation on the Physiology of the Cyanobacterium Agmenellum quadruplicatum

Fe (III)-Mediated Antioxidant Response of the Acidotolerant Microalga Coccomyxa onubensis

Coccomyxa onubensis (C. onubensis) is an acidotolerant microalga isolated from Tinto River (Huelva), which contains high levels of metal cations in solution, mainly Fe (II) and (III), and Cu (II). Fe is more bioavailable at low pH, mainly because Fe (II) and Fe (III) are far more soluble, especially Fe (III). For this reason, this study aims to evaluate both physiological and biochemical responses of C. onubensis when subjected to Fe (III)-induced stress. Changes in growth, photosynthetic viability and antioxidant responses to the induced oxidative stress were determined. The results obtained suggest that the addition of moderate Fe (III) levels to C. onubensis cultures results in improved growth and photosynthetic viability. Increases in the intracellular levels of the enzyme superoxide dismutase (SOD) and flavonoids, used as antioxidant response biomarkers, a point at Fe (III)-mediated oxidative stress induction. The apparent decrease in the content of other phenolic molecules and polyunsaturated fatty acids might be understood as a sign of antioxidant molecules' involvement in reactive oxygen species (ROS) scavenging. In conclusion, a noticeable antioxidant capacity displayed by C. onubensis allows the use of moderate Fe (III) levels to trigger the accumulation of valuable antioxidant molecules, allowing the production of cell extracts with potential anti-inflammatory activity.

Acclimation and Characterization of Marine Cyanobacterial Strains Euryhalinema and Desertifilum for C-Phycocyanin Production

This study involves evaluation of two native cyanobacterial strains Euryhalinema and Desertifilum isolated from a mangrove pond in Haikou (China) for their possible phycocyanin (C-PC) production. Maximal growth rate with highest chlorophyll and C-PC accumulation were observed at 28°C and 60 μmol photons m^-2 s^-1 photon flux density for Euryhalinema sp., while for Desertifilum sp. at 32°C and 80 μmol photons m^-2 s^-1. Nitrogen and iron concentration trails revealed that double strength concentration of sodium nitrate and ferric ammonium citrate in original BG11 media increased growth rate and accumulation of C-PC for both strains. Three different C-PC extraction methods were tested. The combined extraction protocol of freeze-thaw and ultrasonication markedly increased the C-PC extraction efficiency and attained the food grade purity (A ₆₂₀/A ₂₈₀ ratio >0.7), whereas a higher C-PC yield was found with Na-phosphate buffer. Furthermore, the clarified crude extract was used to purify C-PC by fractional ammonium sulfate [(NH₄)₂SO₄] precipitation, Sephadex G-25 gel filtration chromatography, and DEAE-sephadex ion exchange chromatography and attained analytical grade purity (A ₆₂₀/A ₂₈₀ ratio >3.9). Taken together, both strains showed their potential to be domesticated for valuable phycocyanin production.

Evidence for a role of nitric oxide in iron homeostasis in plants

Nitric oxide (NO), once regarded as a poisonous air pollutant, is now understood as a regulatory molecule essential for several biological functions in plants. In this review, we summarize NO generation in different plant organs and cellular compartments, and also discuss the role of NO in iron (Fe) homeostasis, particularly in Fe-deficient plants. Fe is one of the most limiting essential nutrient elements for plants. Plants often exhibit Fe deficiency symptoms despite sufficient tissue Fe concentrations. NO appears to not only up-regulate Fe uptake mechanisms but also makes Fe more bioavailable for metabolic functions. NO forms complexes with Fe, which can then be delivered into target cells/tissues. NO generated in plants can alleviate oxidative stress by regulating antioxidant defense processes, probably by improving functional Fe status and by inducing post-translational modifications in the enzymes/proteins involved in antioxidant defense responses. It is hypothesized that NO acts in cooperation with transcription factors such as bHLHs, FIT, and IRO to regulate the expression of enzymes and proteins essential for Fe homeostasis. However, further investigations are needed to disentangle the interaction of NO with intracellular target molecules that leads to enhanced internal Fe availability in plants.

Responses to iron limitation are impacted by light quality and regulated by RcaE in the chromatically acclimating cyanobacterium Fremyella diplosiphon

Photosynthetic organisms adapt to environmental fluctuations of light and nutrient availability. Iron is critical for photosynthetic organismal growth, as many cellular processes depend upon iron cofactors. Whereas low iron levels can have deleterious effects, excess iron can lead to damage, as iron is a reactive metal that can result in the production of damaging radicals. Therefore, organisms regulate cellular iron levels to maintain optimal iron homeostasis. In particular, iron is an essential factor for the function of photosystems associated with photosynthetic light-harvesting complexes. Photosynthetic organisms, including cyanobacteria, generally respond to iron deficiency by reduced growth, degradation of non-essential proteins and in some cases alterations of cellular morphology. In response to fluctuations in ambient light quality, the cyanobacterium Fremyella diplosiphon undergoes complementary chromatic adaptation (CCA). During CCA, phycobiliprotein composition of light-harvesting antennae is altered in response to green light (GL) and red light (RL) for efficient utilization of light energy for photosynthesis. We observed light-regulated responses to iron limitation in F. diplosiphon. RL-grown cells exhibited significant reductions in growth and pigment levels, and alterations in iron-associated proteins, which impact the accumulation of reactive oxygen species under iron-limiting conditions, whereas GL-grown cells exhibited partial resistance to iron limitation. We investigated the roles of known CCA regulators RcaE, RcaF and RcaC in this light-dependent iron-acclimation response. Through comparative analyses of wild-type and CCA mutant strains, we determined that photoreceptor RcaE has a central role in light-induced oxidative stress associated with iron limitation, and impacts light-regulated iron-acclimation responses, physiologically and morphologically.

Microalgal carbohydrates: an overview of the factors influencing carbohydrates production, and of main bioconversion technologies for production of biofuels

Microalgal biomass seems to be a promising feedstock for biofuel generation. Microalgae have relative high photosynthetic efficiencies, high growth rates, and some species can thrive in brackish water or seawater and wastewater from the food- and agro-industrial sector. Today, the main interest in research is the cultivation of microalgae for lipids production to generate biodiesel. However, there are several other biological or thermochemical conversion technologies, in which microalgal biomass could be used as substrate. However, the high protein content or the low carbohydrate content of the majority of the microalgal species might be a constraint for their possible use in these technologies. Moreover, in the majority of biomass conversion technologies, carbohydrates are the main substrate for production of biofuels. Nevertheless, microalgae biomass composition could be manipulated by several cultivation techniques, such as nutrient starvation or other stressed environmental conditions, which cause the microalgae to accumulate carbohydrates. This paper attempts to give a general overview of techniques that can be used for increasing the microalgal biomass carbohydrate content. In addition, biomass conversion technologies, related to the conversion of carbohydrates into biofuels are discussed.

Gross morphological changes in thylakoid membrane structure are associated with photosystem I deletion in Synechocystis sp. PCC 6803

Cells of Synechocystis sp. PCC 6803 lacking photosystem I (PSI-less) and containing only photosystem II (PSII) or lacking both photosystems I and II (PSI/PSII-less) were compared to wild type (WT) cells to investigate the role of the photosystems in the architecture, structure, and number of thylakoid membranes. All cells were grown at 0.5μmol photons m(-2)s(-1). The lumen of the thylakoid membranes of the WT cells grown at this low light intensity were inflated compared to cells grown at higher light intensity. Tubular as well as sheet-like thylakoid membranes were found in the PSI-less strain at all stages of development with organized regular arrays of phycobilisomes on the surface of the thylakoid membranes. Tubular structures were also found in the PSI/PSII-less strain, but these were smaller in diameter to those found in the PSI-less strain with what appeared to be a different internal structure and were less common. There were fewer and smaller thylakoid membrane sheets in the double mutant and the phycobilisomes were found on the surface in more disordered arrays. These differences in thylakoid membrane structure most likely reflect the altered composition of photosynthetic particles and distribution of other integral membrane proteins and their interaction with the lipid bilayer. These results suggest an important role for the presence of PSII in the formation of the highly ordered tubular structures.

A pair of iron-responsive genes encoding protein kinases with a Ser/Thr kinase domain and a His kinase domain are regulated by NtcA in the Cyanobacterium Anabaena sp. strain PCC 7120

The filamentous cyanobacterium Anabaena sp. strain PCC 7120 can fix N(2) when combined nitrogen is not available in the growth medium. It has a family of 13 genes encoding proteins with both a Ser/Thr kinase domain and a His kinase domain. The function of these enzymes is unknown. Two of them are encoded by pkn41 (alr0709) and pkn42 (alr0710). These two genes are separated by only 72 bp on the chromosome, and our results indicate that they are cotranscribed. The expression of pkn41 and pkn42 is induced by iron deprivation irrespective of the nature of the nitrogen source. Mutants inactivating either pkn41, pkn42, or both grow similarly to the wild type under normal conditions, but their growth is impaired either in the presence of an iron chelator or under conditions of nitrogen fixation and iron limitation, two situations where the demand for iron is particularly strong. Consistent with these results, these mutants display lower iron content than the wild type and a higher level of expression for nifJ1 and nifJ2, which encode pyruvate:ferredoxin oxidoreductases. Both nifJ1 and nifJ2 are known to be induced by iron limitation. NtcA, a global regulatory factor for different metabolic pathways, binds to the putative promoter region of pkn41, and the induction of pkn41 in response to iron limitation no longer occurs in an ntcA mutant. Our results suggest that ntcA not only regulates the expression of genes involved in nitrogen and carbon metabolism but also coordinates iron acquisition and nitrogen metabolism by activating the expression of pkn41 and pkn42.

Functional analysis of the iron-stress induced CP 43' polypeptide of PS II in the cyanobacterium Synechococcus sp. PCC 7942

Under conditions of iron-stress, the Photosystem II associated chlorophyll a protein complex designated CP 43', which is encoded by the isiA gene, becomes the major pigment-protein complex in Synechococcus sp. PCC 7942. The isiB gene, which is located immediately downstream of isiA, encodes the protein flavodoxin, which can functionally replace ferredoxin under conditions of iron stress. We have constructed two cyanobacterial insertion mutants which are lacking (i) the CP 43' apoprotein (designated isiA (-)) and (ii) flavodoxin (designated isiB (-)). The function of CP 43' was studied by comparing the cell characteristics, PS II functional absorption cross-sections and Chl a fluorescence parameters from the wild-type, isiA (-) and isiB (-) strains grown under iron-stressed conditions. In all strains grown under iron deprivation, the cell number doubling time was maintained despite marked changes in pigment composition and other cell characteristics. This indicates that iron-starved cells remained viable and that their altered phenotype suggests an adequate acclimation to low iron even in absence of CP 43' and/or flavodoxin. Under both iron conditions, no differences were detected between the three strains in the functional absorption crossection of PS II determined from single turnover flash saturation curves of Chl a fluorescence. This demonstrates that CP 43' is not part of the functional light-harvesting antenna for PS II. In the wild-type and the isiB (-) strain grown under iron-deficient conditions, CP 43' was present in the thylakoid membrane as an uncoupled Chl-protein complex. This was indicated by (1) an increase of the yield of prompt Chl a fluorescence (Fo) and (2) the persistence after PS II trap closure of a fast fluorescence decay component showing a maximum at 685 nm.

Roles of trace metals in transcriptional control of microbial secondary metabolism

Secondary metabolism in bacteria and fungi requires a much narrower range of environmental concentrations of key trace metals than that permitted for primary metabolism. The cells acquire appropriate quantities of the key metals at the initiation of the shift from primary to secondary metabolism. Evidence suggests that these essential micronutrients participate in regulating the expression of genes responsible for synthesis of secondary metabolites and/or morphological alterations associated with cellular differentiation.

Cloning, nucleotide sequence, and mutagenesis of a gene (irpA) involved in iron-deficient growth of the cyanobacterium Synechococcus sp. strain PCC7942

We describe the cloning and sequencing of a gene from the cyanobacterium Synechococcus sp. strain PCC7942, designated irpA (iron-regulated protein A), that encodes for a protein involved in iron acquisition or storage. Polyclonal antibodies raised against proteins which accumulate during iron-deficient growth were used as probes to isolate immunopositive clones from a lambda gt11 genomic expression library. The clone, designated lambda gtAN26, carried a 1.7-kilobase (kb) chromosomal DNA insert and was detected by cross-reactivity with antibody against a 36-kilodalton protein. It was possible to map a 20-kb portion of the chromosome with various DNA probes from lambda gt11 and lambda EMBL-3 clones, and Southern blot analysis revealed that the irpA gene was present in a single copy and localized within a 1.7-kb PstI fragment. DNA sequencing revealed an open reading frame of 1,068 nucleotides capable of encoding 356 amino acids which yields a protein with a molecular weight of 38,584. The hydropathy profile of the polypeptide indicated a putative N-terminal signal sequence of 44 amino acid residues. IrpA is a cytoplasmic membrane protein as determined by biochemistry and electron microscopy immunocytochemistry. The upstream region of the irpA gene contained a consensus sequence similar to the aerobactin operator in Escherichia coli. This fact, plus a mutant with a mutation in irpA that is unable to grow under iron-deficient conditions, led us to suggest that irpA is regulated by iron and that the gene product is involved in iron acquisition or storage.

Structure and regulation of genes encoding phycocyanin and allophycocyanin from Anabaena variabilis ATCC 29413

Gene clones encoding phycocyanin and allophycocyanin were isolated from an Anabaena variabilis ATCC 29413-Charon 30 library by using the phycocyanin (cpc) genes of Agmenellum quadruplicatum and the allophycocyanin (apc) genes of Cyanophora paradoxa as heterologous probes. The A. variabilis cpcA and cpcB genes occur together in the genome, as do the apcA and apcB genes; the two sets of genes are not closely linked, however. The cpc and apc genes appear to be present in only one copy per genome. DNA-RNA hybridization analysis showed that expression of the cpc and apc genes is greatly decreased during nitrogen starvation; within 1 h no cpc or apc mRNA could be detected. The source of nitrogen for growth did not influence expression of the genes; vegetative cells from nitrogen-fixing and ammonia-grown cultures had approximately the same levels of cpc and apc mRNAs. Heterocysts had less than 5% as much cpc mRNA as vegetative cells from nitrogen-fixing cultures. Northern hybridization (RNA blot) analysis showed that the cpc genes are transcribed to give an abundant 1.4-kilobase (kb) RNA as well as two less prominent 3.8- and 2.6-kb species. The apc genes gave rise to two transcripts, a 1.4-kb predominant RNA and a minor 1.75-kb form.

Chlorophyll-Proteins and Electron Transport during Iron Nutrition-Mediated Chloroplast Development

Chlorophyll-protein complexes and electron transport activities were measured during iron nutrition-mediated chloroplast development in sugar beet (Beta vulgaris L. cv F58-554H1). Results showed that the chlorophyll-protein complexes associated with the reaction centers of photosystem I (CP1) and photosystem II (CPa) and the electron transport activities of these two photosystems per leaf area increased rapidly during the first 24 to 48 hours of iron resupply to iron-deficient sugar beet plants. Bulk chlorophyll and the amounts of light-harvesting chlorophyll-proteins increased after a lag period of 24 hours. The changes in chlorophyll-proteins with time were apparently the cause of an initial increase, then decrease, in the chlorophyll a/b ratio during iron resupply. There was evidence that iron deficiency diminished photosystem I more than photosystem II. We propose that there are two distinct phases in iron nutrition-mediated chloroplast development: (a) the commencement of the synthesis of the lipid matrix of the thylakoid membrane, including a fully functioning electron transport (and photosynthetic) system, during the first 24 hours of iron resupply; and (b) after 24 to 48 hours, the formation of the bulk of the thylakoid proteins, including the light-harvesting chlorophyll-proteins with which the large increase in total chlorophyll is associated.

Effects of Iron Starvation on the Ultrastructure of the Cyanobacterium Agmenellum quadruplicatum

The effects of iron starvation on the ultrastructure of the unicellular cyanobacterium Agmenellum quadruplicatum were studied by using thin sectioning and transmission electron microscopy. Intracellular polysaccharide began to accumulate at the onset of iron limitation. This was followed by degradation of ribosomes and (later) degradation of the thylakoid membranes, both of which were virtually absent by 200 h. The thylakoids underwent structural modifications and rearrangements before they actually began to break down. Iron starvation did not appear to affect carboxysomes or the extracellular glyocalyx. On the other hand, polyphosphate bodies may have been partially degraded, and an electrontransparent gap eventually appeared between the cell wall and the cytoplasmic membrane. All of these changes were reversed when iron was added back to 200-h starved cultures. The sequence of ultrastructural changes observed during iron starvation clearly differed from those previously reported to occur during nitrogen, phosphorous, or carbon limitation.