pubs.acs.org/Langmuir © 2009 American Chemical Society
Photosynthetic Microorganism-Mediated Synthesis of Akaganeite (β-FeOOH) Nanorods Roberta Brayner,*,† Claude Yepremian,‡ Chakib Djediat,‡ Thibaud Coradin,§ Frederic Herbst,† Jacques Livage,§ Fernand Fievet,† and Alain Coute‡ † Universit e Paris Diderot (Paris 7), CNRS, UMR 7086, Interfaces, Traitements, Organisation et Dynamique des Syst emes (ITODYS), 15 rue Jean de Baif, F-75205 Paris Cedex 13, France, ‡Mus eum National d’Histoire Naturelle D epartement RDDM, USM 505, 57 rue Cuvier, F-75005 Paris, France, and §UPMC Universit e Pierre et Marie Curie (Paris 6), CNRS and Coll ege de France, UMR 7574, Chimie de la Mati ere Condens ee de Paris, 11 place Marcellin Berthelot, F-75005 Paris, France
Received March 24, 2009. Revised Manuscript Received June 16, 2009 Common Anabaena and Calothrix cyanobacteria and Klebsormidium green algae are shown to form intracellularly akaganeite β-FeOOH nanorods of well-controlled size and unusual morphology at room temperature. X-ray diffraction (XRD), transmission electron microscopy (TEM), and scanning electron microscopy X-ray energy dispersive spectrometry (SEM-EDS) analyses are used to investigate particle structure, size, and morphology. A mechanism involving iron-siderophore complex formation is proposed and compared with iron biomineralization in magnetotactic bacteria.
1. Introduction The development of reliable, eco-friendly processes for the synthesis of nanomaterials urges chemists to find new synthetic routes with limited ecological impact, involving natural, renewable resources.1 Indeed, there are innumerable examples of living organisms that synthesize exquisite organized inorganic structures under mild pH, pressure, and temperature conditions,2 such as magnetotactic bacteria (Fe3O4 nanoparticles),3 fungi and plant cells (ZrO2, Au, Ag nanoparticles),4-7 cyanobacteria (Au, Ag, Pd, Pt nanoparticles),8,9 S-layer bacteria (CaSO4, CaCO3 layers and CdS),10 and diatoms (siliceous materials).11,12 Focusing on biological iron-based inorganic structures, it is wellknown that ferrous complexes are the active centers of hemoglobin and ferrodoxins. Various biomineralization processes involve ferrous and ferric species for the regulation of iron concentration *To whom correspondence should be addressed. E-mail: roberta.brayner @univ-paris-diderot.fr. (1) Dahl, J. A.; Maddux, B. L. S.; Hutchison, J. E. Chem. Rev. 2007, 107, 2269. (2) (a) Mann, S. Biomineralization; Oxford University Press: Oxford, 2001. (b) Mann, S. Angew. Chem., Int. Ed. 2008, 47, 5306. (3) (a) Matsunaga, T.; Sakaguchi, T. J. Biosci. Bioeng. 2000, 1, 1. (b) Matsunaga, T.; Okamura, Y. Trends Microbiol. 2003, 11, 536. (4) (a) Southam, G.; Beveridge, T. J. Geochim. Cosmochim. Acta 1994, 58, 4527. (b) Southam, G.; Beveridge, T. J. Geochim. Cosmochim. Acta 1996, 60, 4369. (5) (a) Mukherjee, P.; Ahmad, A.; Mandal, D.; Senapati, S.; Saikar, S. R.; Khan, M. I.; Ramani, R.; Parischa, R.; Ajaykumar, P. V.; Alam, M.; Sastry, M.; Kumar, R. Angew. Chem., Int. Ed. 2001, 40, 3585. (b) Mukherjee, P.; Senapati, S.; Mandal, D.; Ahmad, A.; Khan, M. I.; Kumar, R.; Sastry, M. ChemBioChem 2002, 5, 461. (c) Ahmad, A.; Senapati, S.; Khan, M. I.; Kumar, R.; Sastry, M. Langmuir 2003, 19, 3550. (d) Shankar, S. S.; Ahmad, A.; Pasricha, R.; Sastry, M. J. Mater. Chem. 2003, 13, 1822. (6) Nair, B.; Pradeep, T. Cryst. Growth Des. 2002, 2, 293. (7) Bhainsa, K. C.; D’Souza, S. F. Colloids Surf., B 2006, 47, 160. (8) (a) Lengke, M. F.; Fleet, M. E.; Southam, G. Langmuir 2006, 22, 2780. (b) Lengke, M. F.; Fleet, M. E.; Southam, G. Langmuir 2006, 22, 7318. (c) Lengke, M. F.; Fleet, M. E.; Southam, G. Langmuir 2007, 23, 2694. (d) Lengke, M. F.; Fleet, M. E.; Southam, G. Langmuir 2007, 23, 8982. (9) Brayner, R.; Barberousse, H.; Hemadi, M.; Djediat, C.; Yepremian, C.; Coradin, T.; Livage, J.; Fievet, F.; Coute, A. J. Nanosci. Nanotechnol. 2007, 7, 2696. (10) (a) Sleytr, U. B.; Messner, P.; Pum, D; Sara, M. Angew. Chem., Int. Ed. 1999, 38, 1034. (b) Shenton, W.; Pum, D.; Sleytr, U. B.; Mann, S. Nature 1997, 389, 585. (11) Sumper, M.; Brunner, E. Adv. Funct. Mater. 2006, 16, 17. (12) Lopez, P. J.; Gautier, C.; Livage, J.; Coradin, T. Curr. Nanosci. 2005, 1, 73.
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in organisms (ferritin) and also to produce different oxides and oxyhydroxides, such as goethite, lepidocrocite, akaganeite, magnetite, and green rusts.13 For example, magnetite (Fe3O4) has been reported as a biomineralization product in bacteria and many other multicellular organisms.14,15 In magnetotactic bacteria, magnetite nanoparticles are formed intracellularly in membrane vesicles (magnetosomes) and have dimensions ranging from 40 to 120 nm with specific morphologies.14b These nanoparticles are often arranged in a chain conferring a magnetic dipole moment to the cell that is responsible for the magnetotactic response.14c Iron was probably a much more common ion in primordial anoxic oceans than in today’s oxygen-rich surface waters. Thus, early cells most likely had an abundant supply of Fe2þ ions and became dependent on iron as a cofactor for many enzymes.16 When iron is relatively abundant, some algae can store it within protein aggregates known as phytoferritin. Some small cyanobacteria, certain dinoflagelates, and also some diatoms are known to harvest iron ions at low concentrations in seawater by producing surface iron-binding organic molecules, known as siderophores.17 In this context, we demonstrate here that three filamentous microorganisms have the capability to form akaganeite βFeOOH nanorods of well-controlled size at room temperature. Akaganeite (β-FeOOH) has been the subject of numerous studies since its first synthesis by Bohm in 1928,18 due to its sorption, ionexchange, and catalytic properties.19,20 Moreover, the β-FeOOH (13) Bauerlein, E. Angew. Chem., Int. Ed. Engl. 2003, 42, 614. (14) (a) Frankel, R. B.; Blakemore, R. P.; Wolfe, R. S. Science 1979, 203, 1355. (b) Mann, S.; Frankel, R. B.; Blakemore, R. P. Nature 1984, 310, 405. (c) Frankel, R. B.; Blakemore, R. P. J. Magn. Magn. Mater. 1980, 15, 1562. (15) Lowenstam, H. A.; Kirschvink, J. L. In Magnetite biomineralization and Magnetoreception in organisms; Kirschvink, H. A., Jones, D. S., MacFadden, B. J., Eds.; Plenum Publishing Corp.: New York, 1985; pp 3-13. (16) Graham, L.; Wilcox, L. W. Algae; Prentice Hall Inc.: New Jersey, 2000. (17) Chan, C. S.; de Stasio, G.; Welch, S. A.; Girasole, M.; Frazer, B. H.; Nesterova, M. V.; Fakra, S.; Banfield, J. F. Science 2004, 303, 1656. (18) Bohm, J. Z. Kristallogr. 1928, 68, 567. (19) (a) Cai, J.; Liu, J.; Gao, Z.; Navrotsky, A.; Suib, S. L. Chem. Mater. 2001, 13, 4595. (b) Mazeina, L.; Deore, S.; Navrotsky, A. Chem. Mater. 2006, 18, 1830. (20) (a) Deliyanni, E. A.; Bakoyannakis, D. N.; Zouboulis, A. I.; Matis, K. A. Chemosphere 2003, 50, 155. (b) Lazaridis, N. K.; Bakoyannakis, D. N.; Deliyanni, E. A. Chemosphere 2004, 58, 65.
Published on Web 07/02/2009
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phase is widely used as a precursor in the preparation of ferromagnetic materials such as maghemite (γ-Fe2O3).21 The formation of sub-micrometer iron oxyhydroxide filaments containing polysaccharides (between pH 7 and 8.6) was observed inside underground tunnels of the Piquette Mine, Tennyson, WI.17 In this case, the filaments are typically several micrometers long with diameters ranging from 20 to 200 nm and they contain a mixed akaganeite, ferrihydrite and goethite iron oxyhydroxides encrusted sheaths and stalks characteristic of Leptothrix spp. and Gallionella ferruginea. However, all filaments were found outside the cells17 and, to the best of our knowledge, no intracellular synthesis of β-FeOOH nanorods has been reported until now.
2. Experimental Section 2.1. Photosynthetic Microorganism Description and Culture. All photosynthetic microorganisms were selected on the basis of previous reports demonstrating that they are able to synthesize some metallic nanoparticles (Au, Ag, Pt, and Pd) by an enzymatic route.9 The epilithic cyanobacteria Calothrix pulvinata, strain ALCP 745A (MNHN culture collection), was isolated from a sample of a black soiling developing on a building near Geneve (Switzerland). Anabaena flos-aquae planktonic cyanobacteria, strain ALCP B24, came from MNHN Culture Collection. Klebsormidium flaccidum benthic green-algae, strain ALCP 749 (MNHN culture collection), was isolated from a sample of a black soiling developing on buildings near Paris (France). All microorganisms were grown in 250 mL Erlenmeyer flasks, in sterile Bold’s basal medium (BB medium), and buffered with 3.5 mM phosphate buffer at a controlled temperature of 20.0 ( 0.5 °C and luminosity (50-80 μmol m-2 s-1 photosynthetic photon flux (PPF) for Klebsormidium flaccidum and 30-60 μmol m-2 s-1 PPF for Calothrix pulvinata and Anabaena flosaquae) under ambient CO2 conditions. The pH of the medium was adjusted to 7.0 using 1 M NaOH solution. Before addition of iron salts in concentrations ranging from 10-3 to 10-1 M, the culture was transferred (20% (v/v) of inoculum) into the culture medium and grown for 4 weeks. Morphologically, the two cyanobacteria genera used for our experimentation are composed of one linear series of vegetative cells. Moreover, Calothrix possesses one heterocyt at its basal apex. (Heterocyt has a thick wall, and its interior appears yellowish and empty. Its peculiar physiology is atmospheric dinitrogen gas (N2) fixation to ammonia that is catalyzed by nitrogenase enzyme generated by the cell itself.) On the other hand, Anabaena may have a lot of heterocyts dispersed all along the trichome. Klebsormidium green-algae possess cells, cylindrical to barrelshaped, forming long filaments slightly constricted at cross-walls, one parietal and cup-shaped chloroplast, covering approximately 2/3 of the cells, with one clearly visible pyrenoid (centers of carbon dioxide fixation). 2.2. Microalgae and Nanoparticle Characterization. Optical microscopy was performed with a Nikon EFD3 interferential contrast microscope. The photosynthetic activity of the microalgae was measured using the pulsed amplitude modulation (PAM) method with a Handy PEA (Hansatech instruments) fluorometer. This method uses the saturation pulse method, in which a phytoplankton sample is subjected to a short beam of light that saturates the photosystem II (PS II) reaction centers of the active (21) Sugimoto, T.; Murumatsu, A. J. Colloid Interface Sci. 1996, 184, 626. (22) Rohacek, K.; Bartak, M. Photosynthetica 1999, 37, 339. (23) Antal, T. K.; Krendeleva, T. E.; Rubin, A. B. Photosynth. Res. 2007, 94, 13. (24) Schmitt-Jansen, M.; Altenburger, R. Aquat. Toxicol. 2008, 86, 49. (25) Wright, A. H.; DeLong, J. M.; Franklin, J. L.; Lada, R. R.; Prange, R. K. Photosynth. Res. 2008, 97, 205. (26) Ritchie, R. J. Photosynth. Res. 2008, 96, 201.
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Figure 1. Photosynthetic activity of microalgae as monitored from PAM measurement (a) in the absence of iron salts, (b) in the presence of Fe3þ, and (c) in the presence of Fe2þ/Fe3þ. chlorophyll molecules.22-26 This process suppresses photochemical quenching, which might otherwise reduce the maximum fluorescence yield. A ratio of variable over maximal fluorescence (Fv/Fm) can then be calculated which approximates the potential quantum yield of PS II. Biomass transmission electron microscopy (TEM) imaging was performed with a Hitachi H-700 operating at 75 kV equipped with a Hamatsu camera. For TEM studies, the microalgae were fixed with a mixture containing 2.5% glutaraldehyde, 1.0% acrolein, and 0.1% ruthenium red in a phosphate S€ orengen buffer (0.1 M, pH 7.4). Dehydration was then achieved in a series of ethanol baths, and the samples were processed for flat embedding in a Spurr resin. Ultrathin sections were made using a Reicherd E Young Ultracut ultramicrotome (Leica). Sections were contrasted with ethanolic uranyl acetate before visualization. β-FeOOH nanorods prepared in vitro were observed in a JEOL 100CXII transmission electron microscope operating at 100 kV. Mean particle diameters were estimated from image analysis using a digital camera and the SAISAM and TAMIS software (Microvision Instruments). DOI: 10.1021/la9010345
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Figure 2. Optical micrographs of microalgae before (top) and after (bottom) iron salts addition for (a,d) Anabaena, (b,e) Calothrix, and (c,f) Klebsormidium. X-ray energy dispersive spectrometry (EDS) was performed using a (EDXD) EDAX system equipped with a super ultrathin window (SUTW) connected to a JEOL JSM 6100 scanning electron microscope operating at 25 kV. Atomic compositions (%) were obtained with Genesis software. X-ray powder diffraction (XRD) patterns were recorded using an X’Pert PRO (PANalytical) diffractometer with Co KR radiation. The diffractometer was calibrated using a standard Si sample. Magnetic measurements were performed using a Quantum Design MPMS-5S SQUID magnetometer in the temperature range 4-300 K.
3. Results In a first stage, the photosynthetic activity of Calothrix pulvinata, Anabaena flos-aquae, and Klebsormidium flaccidum was measured using a PAM fluorimeter (Figure 1), before and after addition of iron salts either as a FeCl3 3 4H2O solution ([Fe3þ] = 10-2 M) or as a mixed FeCl3 3 4H2O and FeSO4(NH4)2SO4 3 6H2O (Mohr’s salt) solution in equimolar concentration ([Fe2þ; Fe3þ]=10-2 M). Mohr’s salt was chosen because it is much less affected by atmospheric oxygen than iron(II) sulfate, chloride, or nitrate solutions which tend to be oxidized to iron(III). The oxidation of iron(II) solutions is highly pH-dependent, occurring much more readily at high pH (eq 1). 4Fe2þ þO2 þð4þ2xÞH2 O S 2Fe2 O3 xH2 Oþ8Hþ
ð1Þ
The ammonium ions make Mohr’s salt aqueous solutions slightly acidic, thus preventing oxidation. Before addition of iron salts, the three genera present a stable photosynthetic activity during more than 2 months (Figure 1a). On the other hand, after addition of Fe3þ ions, a progressive decrease of photosynthetic activities followed by cellular death was observed after 18 days of incubation (Figure 1b). This behavior was due to the progressive formation of HCl after FeCl3 addition, as indicated by a decrease of the pH of the culture medium from 7.0 to 4.0. A better resistance of orga nisms was found when an equimolar solution of Fe2þ and Fe3þ ions was used (Figure 1c). In this case, an increase of the photosynthetic activities was observed between 5 and 15 days after Mohr’s salt addition, with Fv/Fm ratios being higher than those for microorganisms studied in the absence of iron 10064 DOI: 10.1021/la9010345
salts. This behavior may be explained by the algae adaptation that allows them to harvest and store iron when it is available. Hence, due to the here-observed increase in photosynthetic activity, all following studies were carried out using this Mohr’s salt solution. Before reaction, for all experiments, the culture medium is a transparent yellow-colored solution due to the presence of iron salts. After 1 week, all solutions containing microorganisms and iron salts underwent a color change to clear brown. Immediately after iron salts addition (Figure 2a-c), all vegetative cells turned blue-green (Anabaena and Calothrix) or green (Klebsormidium). Calothrix genus also presented a yellow-green heterocyt at its basal apex (Figure 2b). After reaction, all cell colors turned to brown (Figure 2d-f). For Calothrix genus, an increase of cell wall thickness and sheath production was observed (Figure 2e), while the Klebsormidium chloroplasts size was reduced (Figure 2f). The chemical composition of the samples was quantified by EDS measurements before and after reaction (Supporting Information). All samples presented C, O, Si, P, S, K, and Fe corresponding to algae nutriments. The atomic percentage (atom %) of Fe added in the culture medium corresponds to 0.2%. After reaction, the atom % of Fe increases from 0.2% to 2.8% for Calothrix, 3.8% for Klebsormidium, and 8.3% for Anabaena, suggesting significant intracellular iron accumulation by the cells. On ultrathin sections of these microorganisms after reaction with Mohr’s salt, it was possible to observe numerous dark nanorods, ca. 100 nm in length, randomly distributed inside the cells (Figure 3a-c). No specific orientation or organization could be observed. High-resolution TEM (HRTEM) images of these anisotropic materials show that nanorods synthesized by Anabaena strain (Figure 3d) present a complex arrangement of pores forming a spongelike structure. Such morphology was not observed for nanorods formed by Calothrix and Klebsormidium genera (Figure 3e, f). XRD patterns of whole cells indicate the presence of the β-FeOOH akaganeite tetragonal structure (Figure 4b-d),27 together with an amorphous phase attributed to the presence of algae biomass. (27) Cornell, R. M.; Schwertmann, U. The iron oxides: structure, properties, reactions, occurrence and uses; VCH: Weinheim, 1996.
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Figure 3. TEM micrographs of (a) Anabaena, (b) Calothrix, and (c) Klebsormidium microalgae after iron salts addition. HRTEM micrographs of microalgae-based nanorods: (d) Anabaena-β-FeOOH, (e) Calothrix-β-FeOOH, and (f) Klebsormidium-β-FeOOH.
As a comparison, akaganeite nanorods were synthesized in aqueous solution from FeCl3 3 9H2O salt at 80 °C during 5 h. The synthetic particles were very similar to biogenic samples in terms of morphology, at least for Calothrix and Klebsormidium genera, and dimensions (Table 1) but showed a better crystallinity, as assessed by XRD (Figure 4a) and HRTEM (Supporting Information), The dependence of the magnetization with applied field for akaganeite-containing cells revealed a hysteresis loop at 4.5 K (Supporting Information), indicating that the biogenic colloids exhibit a superparamagnetic behavior. In fact, bulk akaganeite exhibits an antiferromagnetic ordering with a Neel temperature Langmuir 2009, 25(17), 10062–10067
lying in the range 270-296 K, but the superparamagnetic behavior of akaganeite nanoparticles has been recently reported.28,29 This behavior can attributed to an incomplete magnetic compensation between antiferromagnetically coupled magnetic sublattices corresponding to the core and surface spins of the particle.30 The latter (28) Zhang, L.-Y.; Xue, D.-S.; Fen, J. J. Magn. Magn. Mater. 2006, 305, 228. (29) Millan, A.; Urtizberea, A.; Natividad, E.; Luis, F.; Silva, N. J. O.; Palacio, F.; Mayoral, I.; Ruiz-Gonzalez, M. L.; Gonzalez-Calbet, J. M.; Lecante, P.; Serin, V. Polymer 2009, 50, 1088. (30) (a) Makhlouf, S. A.; Parker, F. T.; Spada, F. E.; Berkowitz, A. E. J. Appl. Phys. 1997, 81, 5561. (b) Makhlouf, S. A.; Parker, F. T.; Berkowitz, A. E. Phys. Rev. B 1997, 55, R14717.
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Figure 4. XRD patterns of (a) in vitro synthesized β-FeOOH and whole cells of (b) Anabaena, (c) Calothrix, and (d) Klebsormidium microalgae after iron salts addition. Peak attribution corresponds to the akaganeite structure. Asterisk symbol (/) corresponds to algal biomass, in particular cellulose.
could reverse coherently and randomly under thermal activation, leading to a fluctuation of the resulting moment of the uncompensated surface spins above a blocking temperature. In the case of akaganeite, it was also proposed that a Cl- deficiency in the structure could also lead to an imperfect compensation of spin sublattices.29 Noticeably, synthetic akaganeite nanorods exhibit a similar magnetic behavior but with a higher coercivity (Hc) value (Table 1 and Supporting Information), that may be explained by its better degree of crystallinity when compared to biogenic particles.
4. Discussion Usually, the simultaneous presence of ferrous and ferric ions in solution drives the condensation process toward the formation of specific phases, namely, the green rusts (GR), of hydrotalcite structural type, and magnetite or maghemite, of spinel structural type.27,31 The formation of these mixed phases depends on many factors: (i) iron concentration, (ii) pH, and more significantly (iii) the composition of the system defined as x=[Fe3þ/(Fe3þ þ Fe2þ)]. (31) Wells, A. F. Structural Inorganic Chemistry, 5th ed.; Oxford University Press: Oxford, U.K., 1991.
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The condensation process at pH=7.0 (20 °C) allows the formation of the goethite (R-FeOOH) phase. In our case, the formation of β-FeOOH tetragonal structure was not expected. However, it is known that an akaganeite metastable phase is first formed instead of ferrihydrite at high concentration of chloride anions (higher than 4 10-2 M).32 These anions are not included in stoichiometric amounts in the channels of the structure, and they seem to act as a template allowing the specific organization of the anisotropic akaganeite-like double chains of octahedrons. In higher plants, two basic strategies for iron acquisition have been observed.33 In grasses (strategy II plants), iron is taken up as a Fe3þ-siderophore complex. In presumably all other plants, a second mechanism (strategy I) is used, which involves soil acidification, formation of lateral roots and specific transfer cells in the rhizodermis, as well as induction of an Fe3þ-chelate reductase and of Fe2þ transporters.34 In the rhizosphere of strategy I plants, extracellular Fe3þ-chelates are reduced to (32) Remazeilles, C.; Refait, P. Corros. Sci. 2007, 49, 844. (33) Guerinot, M. L.; Ly, L. Plant Physiol. 1994, 104, 815. (34) Moog, P. R.; Br€uggemann, W. Plant Soil 1994, 165, 241.
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Table 1. Size (O, L), Length Standard Deviation (σ), and Coercivity (Hc) of β-FeOOH sample
Hc (Oe)
φ (nm)a
L (nm)a
β-FeOOH (in vitro) 100 22 111 Anabaena-β-FeOOH 10 20 103 Calothrix-β-FeOOH 15 11 128 Klebsormidium-β-FeOOH 25 8 93 a φ (diameter) and L (length) from TEM analyses.
σ (nm) 0.5 1.2 1.5 1.7
Fe2þ which is subsequently transported into the root cells35 by either iron-specific or nonspecific divalent transporters.36 It is well-established that cyanobacteria use a “strategy II” siderophore-based iron acquisition mechanism.37,38 In strategy II organisms, which also include many species of cyanobacteria, fungi, bacteria,39 and all grasses,40 iron limitation leads to enhanced cellular release of siderophores. Siderophores are organic Fe3þ-metal chelating molecules that serve to solubilize and scavenge Fe3þ from the environment. The Fe3þsiderophore complex is subsequently imported into the cell. In the case of Anabaena flos-aquae strain, iron-limited cells are able to use a wide variety of chelated/bound iron sources,41 involving ligands such as ethylenediamine di(o-hydroxyphenyl acetic acid) (EDDHA) and desferrioxamine B (DFB). It was shown that the rate of iron uptake was a function of the chelator strength.41 Relatively little is known about iron assimilation in green algae. In the case of Chlorella vulgaris, the uptake of siderophore-bound iron occurs via reduction (strategy I).42 In contrast, the mechanism of iron accumulation in Scenedesmus was shown to resemble strategy II.43 In our study, only the presence of β-FeOOH (Fe3þ-oxyhydroxide) was observed inside the cells, showing that the iron accumulation mechanism is strategy II. Under oxic conditions, iron is generally in the 3þ oxidation state and forms various insoluble minerals. To obtain iron from such minerals, cells produce iron-binding siderophores that bind iron and transport it into the cell. Usually, the BB medium used in this work contains 5 10-5 M Fe2þ-EDTA complex, and 10-2 M [Fe2þ, Fe3þ] equimolar solution was added to synthesize β-FeOOH nanorods inside the cells. The Fe2þ ions can be oxidized to Fe3þ by dioxygen from the photosynthesis process. Concomitantly, Fe3þ ions (less soluble than Fe2þ) may then be chelated by the siderophores produced by the microorganisms and subsequently imported into the cells. Part of these complexes can be consumed by the microorganisms, and the excess can precipitate to form β-FeOOH nanorods (Scheme 1). In addition, the presence of high concentration of Cl- anions may have favored the formation of akaganeite to the detriment of goethite at pH=7.0.
5. Conclusion Although the ability of several living organisms to elaborate nanoparticles and/or nanostructured materials has been known for a long time, several reports3-9 suggest that the diversity (35) Chaney, R. L.; Brown, J. C.; Tiffin, L. O. Plant Physiol. 1972, 50, 208. (36) Eide, D.; Broderius, M.; Fett, J.; Guerinot, M. L. Proc. Natl. Acad. Sci. U. S.A. 1996, 93, 5624. (37) Goldman, S. J.; Lammers, P. J.; Berman, M. S.; Sanders-Loehr, J. J. Bacteriol. 1983, 156, 1144. (38) Wihelm, S. W.; Trick, C. G. Limnol. Oceanogr. 1994, 39, 1979. (39) Winkelmann, W. Biochem. Soc. Trans. 2002, 30, 691. (40) Mori, S. Curr. Opin. Plant Biol. 1999, 2, 250. (41) Gress, C. D.; Treble, R. G.; Matz, C. J.; Weger, H. G. J. Phycol. 2004, 40, 879. (42) (a) Alnutt, F. C. T.; Bonner, W. D., Jr. Plant Physiol. 1987, 85, 746. (b) Alnutt, F. C. T.; Bonner, W. D., Jr. Plant Physiol. 1987, 85, 751. (43) Benderliev, K. M.; Ivanova, N. I. Planta 1994, 193, 163.
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Scheme 1. Suggested Mechanism of High-Affinity Iron Acquisition by Cyanobacteria in Aquatic Systemsa
a Siderophores are released from the cell which, by complexing with Fe3þ ions, increase iron availability to the cell. Here, we show that only one example of the production of hydroxamate-type siderophores (H) functions similarly to eubacterial systems. The Fe3þ-siderophore complex is subsequently imported into the cell. One part of the Fe3þ precipitate forms the akaganeite phase due to the presence of Cl- ions.
of organisms that exhibit such an ability and the variety of chemical compositions and shapes of these nanomaterials may be much wider than one could have expected. In the present work, we show that common microalgae and cyanobacteria can form magnetic akaganeite nanorods. Importantly, this process does not appear detrimental to the photosynthetic activity of the organisms It is interesting to note that, although some magnetotactic bacteria also use siderophores for iron uptake,3 the intracellular synthesis of akaganeite by these organisms has not been reported so far. Indeed, iron-based biominerals within magnetotactic bacteria have a precise function, that is, orientation. These biominerals must therefore have a well-controlled composition, size, morphology, and organization that have been optimized over evolution. In this context, akaganeite is not a good candidate due to its needlelike morphology and its magnetic properties. In contrast, the here-studied experimental conditions cannot be considered as “natural” for the cyanobacteria and algae, so that it is not expected that these organisms have developed a specific strategy to deal with them. It is therefore not surprising that the mineralization process occurs randomly in the cell and leads to particles very similar to synthetic ones, at least for two of the species. In that sense, the here-described algae-mediated process of akaganeite formation may be considered as a typical example of a bioinduced mineralization reaction when compared to the biocontrolled growth of iron oxide in magnetotactic bacteria.2a Acknowledgment. The authors gratefully acknowledge Professor Souad Ammar for fruitful discussions on magnetic studies. Supporting Information Available: EDS analysis of organisms after iron salt addition; TEM, HRTEM, and SAED pattern of synthetic akaganeite nanorods; hysteresis curves at 4.5 K for synthetic and biogenic akaganeite nanorods. This material is available free of charge via the Internet at http://pubs.acs.org. DOI: 10.1021/la9010345
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