Oxygen Dependency of Neutrophilic Fe(II) Oxidation by Leptothrix Differs from Abiotic Reaction

Neutrophilic Fe(II) oxidizing microorganisms are found in many natural environments. It has been hypothesized that, at low oxygen concentrations, microbial iron oxidation is favored over abiotic oxidation. Here, we compare the kinetics of abiotic Fe(II) oxidation to oxidation in the presence of the bacterium Leptothrix cholodnii Appels isolated from a wetland sediment. Rates of Fe(II) oxidation were determined in batch experiments at 20°C, pH 7 and oxygen concentrations between 3 and 120 μmol/l. The reaction progress in experiments with and without cells exhibited two distinct phases. During the initial phase, the oxygen dependency of microbial Fe(II) oxidation followed a Michaelis-Menten rate expression (K_M = 24.5 ± 10 μmol O₂/l, v_max = 1.8 ± 0.2 μmol Fe(II)/(l min) for 10⁸ cells/ml). In contrast, abiotic rates increased linearly with increasing oxygen concentrations. At similar oxygen concentrations, initial Fe(II) oxidation rates were faster in the experiments with bacteria. During the second phase, the accumulated iron oxides catalyzed further oxidative iron precipitation in both abiotic and microbial reaction systems. That is, abiotic oxidation also dominated the reaction progress in the presence of bacteria. In fact, in some experiments with bacteria, iron oxidation during the second phase proceeded slower than in the absence of bacteria, possibly due to an inhibitory effect of extracellular polymeric substances on the growth of Fe(III) oxides. Thus, our results suggest that the competitive advantage of microbial iron oxidation in low oxygen environments may be limited by the autocatalytic nature of abiotic Fe(III) oxide precipitation, unless the accumulation of Fe(III) oxides is prevented, for example, through a close coupling of Fe(II) oxidation and Fe(III) reduction.     

Influence of Nutrient Concentrations on MPN Quantification and Enrichment of Nitrate-Reducing Fe(II)-Oxidizing and Fe(III)-Reducing Bacteria from Littoral Freshwater Lake Sediments

The application of culture-dependent studies to quantify Fe-metabolizing microorganisms from the environment is a necessity, as there are so far no universal functional marker genes for application in culture-independent studies. Media composition can vary between studies, therefore, we determined the effects of three different growth media on the quantification (MPNs) and identity (via cloning and sequencing of dominant DGGE bands) of nitrate-reducing Fe(II)-oxidizers and lactate- or acetate-oxidizing Fe(III)-reducers from a lacustrine sediment: low sulphate freshwater medium (FWM), sterile filtered bicarbonate-buffered lake water (BLW) and a mixture of both (MIX). We consistently found fewer cells in the BLW than in the FWM and the MIX. The DGGE banding patterns of the microbial communities enriched in different media types clustered together according to the e^? donor and acceptor couples and not according to the medium used. Thus, although the medium composition significantly influenced the quantification and thereby conclusions on the abundance and potential significance of the targeted group within the ecosystem, biodiversity assessments through enrichment cultures were less influenced by the medium, but instead were affected by the type and concentration of the e^? donor/acceptor.     

Reactions of Fe(II)-ATP and Fe(II)-citrate complexes with t-butyl hydroperoxide and cumyl hydroperoxide

Rate constants for the reactions of cumyl hydroperoxide and t-butyl hydroperoxide with ferrous complexes of ATP and citrate were measured at pH 7.4. These ligands are potential chelators of iron(II) in the low-molecular weight iron pool that may catalyze oxidative degradation of biomolecules. The second-order rate constants for the reduction of cumyl hydroperoxide and t-butyl hydroperoxide by ferrous ATP are 3.1 x 10(3) and 1.3 x 10(3) M-1.s-1, respectively, at 25 degrees C and 0.11 M ionic strength. Rates of reduction by ferrous citrate are similar. Activation enthalpies for these reactions average 10 kcal/mol.     

Physiological and Genomic Characterization of a Nitrate-Reducing Fe(II)-Oxidizing Bacterium Isolated from Paddy Soil

In this study, a neutrophilic, heterotrophic bacterium (strain Paddy-2) that is capable of ferrous iron [Fe(II)] oxidation coupled with nitrate (NO₃^?) reduction (NRFO) under anoxic conditions was isolated from paddy soil. The molecular identification by 16S rRNA gene sequencing identified the strain as Cupriavidus metallidurans. Strain Paddy-2 reduced 97.7% of NO₃^?and oxidized 89.7% of Fe(II) over 6?days with initial NaNO₃ and FeCl₂ concentrations of 9.37?mM and 4.72?mM, respectively. Acetate (5?mM) was also supplied as a carbon source and an alternative electron donor. A poorly crystalline Fe(III) mineral was the main component observed after 15?days of growth in culture, whereas lepidocrocite was detected in the X-ray diffraction spectrum after 3?months of culture. The homologous genes in electron transfer during Fe(II) oxidation (cyc1, cymA, FoxY, FoxZ, and mtoD) were also identified in the genomes of strain Paddy-2 and other reported NRFO bacteria. These genes encoding c-Cyts may play a role in electron transfer during the process of NRFO. These results provide evidence for the potential of NO₃^? to affect Fe(II) oxidation and biomineralization in bacterium from anoxic paddy soil.     

Direct and Fe(II)-Mediated Reduction of Technetium by Fe(III)-Reducing Bacteria

The dissimilatory Fe(III)-reducing bacterium Geobacter sulfurreducens reduced and precipitated Tc(VII) by two mechanisms. Washed cell suspensions coupled the oxidation of hydrogen to enzymatic reduction of Tc(VII) to Tc(IV), leading to the precipitation of TcO₂ at the periphery of the cell. An indirect, Fe(II)-mediated mechanism was also identified. Acetate, although not utilized efficiently as an electron donor for direct cell-mediated reduction of technetium, supported the reduction of Fe(III), and the Fe(II) formed was able to transfer electrons abiotically to Tc(VII). Tc(VII) reduction was comparatively inefficient via this indirect mechanism when soluble Fe(III) citrate was supplied to the cultures but was enhanced in the presence of solid Fe(III) oxide. The rate of Tc(VII) reduction was optimal, however, when Fe(III) oxide reduction was stimulated by the addition of the humic analog and electron shuttle anthaquinone-2,6-disulfonate, leading to the rapid formation of the Fe(II)-bearing mineral magnetite. Under these conditions, Tc(VII) was reduced and precipitated abiotically on the nanocrystals of biogenic magnetite as TcO₂ and was removed from solution to concentrations below the limit of detection by scintillation counting. Cultures of Fe(III)-reducing bacteria enriched from radionuclide-contaminated sediment using Fe(III) oxide as an electron acceptor in the presence of 25 μM Tc(VII) contained a single Geobacter sp. detected by 16S ribosomal DNA analysis and were also able to reduce and precipitate the radionuclide via biogenic magnetite. Fe(III) reduction was stimulated in aquifer material, resulting in the formation of Fe(II)-containing minerals that were able to reduce and precipitate Tc(VII). These results suggest that Fe(III)-reducing bacteria may play an important role in immobilizing technetium in sediments via direct and indirect mechanisms.     

Diversity and Biotechnological Potential of Nitrate-Reducing Bacteria from Heavy-Oil Reservoirs (Russia)

Microbiology - Nitrates do not occur in the formation water of oil reservoirs. A number of oil companies use nitrate injection technology to suppress corrosion of steel equipment and to decrease...     

Self-assembly Is Prerequisite for Catalysis of Fe(II) Oxidation by Catalytically Active Subunits of Ferritin

Fe(III) storage by ferritin is an essential process of the iron homeostasis machinery. It begins by translocation of Fe(II) from outside the hollow spherical shape structure of the protein, which is formed as the result of self-assembly of 24 subunits, to a di-iron binding site, the ferroxidase center, buried in the middle of each active subunit. The pathway of Fe(II) to the ferroxidase center has remained elusive, and the importance of self-assembly for the functioning of the ferroxidase center has not been investigated. Here we report spectroscopic and metal ion binding studies with a mutant of ferritin from Pyrococcus furiosus (PfFtn) in which self-assembly was abolished by a single amino acid substitution. We show that in this mutant metal ion binding to the ferroxidase center and Fe(II) oxidation at this site was obliterated. However, metal ion binding to a conserved third site (site C), which is located in the inner surface of each subunit in the vicinity of the ferroxidase center and is believed to be the path for Fe(II) to the ferroxidase center, was not disrupted. These results are the basis of a new model for Fe(II) translocation to the ferroxidase center: self-assembly creates channels that guide the Fe(II) ions toward the ferroxidase center directly through the protein shell and not via the internal cavity and site C. The results may be of significance for understanding the molecular basis of ferritin-related disorders such as neuroferritinopathy in which the 24-meric structure with 432 symmetry is distorted.     

Effect of Oxidation Rate and Fe(II) State on Microbial Nitrate-Dependent Fe(III) Mineral Formation

A nitrate-dependent Fe(II)-oxidizing bacterium was isolated and used to evaluate whether Fe(II) chemical form or oxidation rate had an effect on the mineralogy of biogenic Fe(III) (hydr)oxides resulting from nitrate-dependent Fe(II) oxidation. The isolate (designated FW33AN) had 99% 16S rRNA sequence similarity to Klebsiella oxytoca. FW33AN produced Fe(III) (hydr)oxides by oxidation of soluble Fe(II) [Fe(II)_sol] or FeS under nitrate-reducing conditions. Based on X-ray diffraction (XRD) analysis, Fe(III) (hydr)oxide produced by oxidation of FeS was shown to be amorphous, while oxidation of Fe(II)_sol yielded goethite. The rate of Fe(II) oxidation was then manipulated by incubating various cell concentrations of FW33AN with Fe(II)_sol and nitrate. Characterization of products revealed that as Fe(II) oxidation rates slowed, a stronger goethite signal was observed by XRD and a larger proportion of Fe(III) was in the crystalline fraction. Since the mineralogy of Fe(III) (hydr)oxides may control the extent of subsequent Fe(III) reduction, the variables we identify here may have an effect on the biogeochemical cycling of Fe in anoxic ecosystems.     

Anoxic and Oxic Oxidation of Rocks Containing Fe(II)Mg-Silicates and Fe(II)-Monosulfides as Source of Fe(III)-Minerals and Hydrogen. Geobiotropy.

In this article, anoxic and oxic hydrolyses of rocks containing Fe (II) Mg-silicates and Fe (II)-monosulfides are analyzed at 25 °C and 250–350 °C. A table of the products is drawn. It is shown that magnetite and hydrogen can be produced during low-temperature (25 °C) anoxic hydrolysis/oxidation of ferrous silicates and during high-temperature (250 °C) anoxic hydrolysis/oxidation of ferrous monosulfides. The high-T (350 °C) anoxic hydrolysis of ferrous silicates leads mainly to ferric oxides/hydroxides such as the hydroxide ferric trihydroxide, the oxide hydroxide goethite/lepidocrocite and the oxide hematite, and to Fe(III)-phyllosilicates. Magnetite is not a primary product. While the low-T (25 °C) anoxic hydrolysis of ferrous monosulfides leads to pyrite. Thermodynamic functions are calculated for elementary reactions of hydrolysis and carbonation of olivine and pyroxene and E-pH diagrams are analyzed. It is shown that the hydrolysis of the iron endmember is endothermic and can proceed within the exothermic hydrolysis of the magnesium endmember and also within the exothermic reactions of carbonations. The distinction between three products of the iron hydrolysis, magnetite, goethite and hematite is determined with E-pH diagrams. The hydrolysis/oxidation of the sulfides mackinawite/troilite/pyrrhotite is highly endothermic but can proceed within the heat produced by the exothermic hydrolyses and carbonations of ferromagnesian silicates and also by other sources such as magma, hydrothermal sources, impacts. These theoretical results are confirmed by the products observed in several related laboratory experiments. The case of radiolyzed water is studied. It is shown that magnetite and ferric oxides/hydroxides such as ferric trihydroxide, goethite/lepidocrocite and hematite are formed in oxic hydrolysis of ferromagnesian silicates at 25 °C and 350 °C. Oxic oxidation of ferrous monosulfides at 25 °C leads mainly to pyrite and ferric oxides/hydroxides such as ferric trihydroxide, goethite/lepidocrocite and hematite and also to sulfates, and at 250 °C mainly to magnetite instead of pyrite, associated to the same ferric oxides/hydroxides and sulfates. Some examples of geological terrains, such as Mawrth Vallis on Mars, the Tagish Lake meteorite and hydrothermal venting fields, where hydrolysis/oxidation of ferromagnesian silicates and iron(II)-monosulfides may occur, are discussed. Considering the evolution of rocks during their interaction with water, in the absence of oxygen and in radiolyzed water, with hydrothermal release of H₂ and the plausible associated formation of components of life, geobiotropic signatures are proposed. They are mainly Fe(III)-phyllosilicates, magnetite, ferric trihydroxide, goethite/lepidocrocite, hematite, but not pyrite.     

Cytochrome Involvement in Mn(II) Oxidation by Two Marine Bacteria

Two marine, Mn(II)-oxidizing bacterial cultures, BIII 45 and BIII 82, were examined spectrophotometrically at ambient temperature for their cytochrome complements. Membrane preparations from an ethylenediaminetetracetate-lysozyme treatment of 48-h cultures of both strains contained type b, c, and o cytochromes. No evidence for a type a cytochrome was noted. “Periplasmic” fractions of both strains also contained small amounts of cytochrome, including cytochrome o, but “intracellular” fractions did not. Type c cytochrome in membrane preparations of culture BIII 45 was consistently reduced by Mn(II) when the membranes were suspended in the periplasmic fraction of the culture. In the case of culture BIII 82, type c cytochrome in membrane preparations was consistently reduced by Mn(II) when the membranes were suspended in either periplasmic or intracellular fractions of the strain. Although, based on previous inhibitor studies, type b cytochrome was also expected to be reduced by Mn(II), no spectrophotometric evidence for its reduction was found, probably because not enough of it was reduced under the steady-state conditions of the experiments.     

Lipoxygenase6-Dependent Oxylipin Synthesis in Roots Is Required for Abiotic and Biotic Stress Resistance of Arabidopsis

Jasmonates are oxylipin signals that play important roles in the development of fertile flowers and in defense against pathogens and herbivores in leaves. The aim of this work was to understand the synthesis and function of jasmonates in roots. Grafting experiments with a jasmonate-deficient mutant demonstrated that roots produce jasmonates independently of leaves, despite low expression of biosynthetic enzymes. Levels of 12-oxo-phytodienoic acid, jasmonic acid, and its isoleucine derivative increased in roots upon osmotic and drought stress. Wounding resulted in a decrease of preformed 12-oxo-phytodienoic acid concomitant with an increase of jasmonic acid and jasmonoyl-isoleucine. 13-Lipoxygenases catalyze the first step of lipid oxidation leading to jasmonate production. Analysis of 13-lipoxygenase-deficient mutant lines showed that only one of the four 13-lipoxygenases, LOX6, is responsible and essential for stress-induced jasmonate accumulation in roots. In addition, LOX6 was required for production of basal 12-oxo-phytodienoic acid in leaves and roots. Loss-of-function mutants of LOX6 were more attractive to a detritivorous crustacean and more sensitive to drought, indicating that LOX6-derived oxylipins are important for the responses to abiotic and biotic factors.Oxylipins are ubiquitous signaling molecules that are derived from polyunsaturated fatty acids by enzymatic and nonenzymatic processes. In plants, the biosynthesis and function of oxylipins of the jasmonate family in aboveground tissues has been investigated in detail. Jasmonates comprise 12-oxo-phytodienoic acid (OPDA), jasmonic acid (JA), and derivatives of JA. In leaves, jasmonates accumulate in response to abiotic factors such as wounding, drought, osmotic stress, darkness, and ozone and during interactions with organisms such as herbivores, pathogens, and mutualistic organisms (Wasternack, 2007). The relevance of jasmonates in wound response, ozone tolerance, and the defense against herbivores and necrotrophic pathogens in leaves has been well investigated using mutants in JA biosynthesis and signaling (Browse, 2009a). In addition, jasmonates play an important role in flower development, and Arabidopsis (Arabidopsis thaliana) mutants in the JA pathway are male sterile (Browse, 2009b). The first step in jasmonate biosynthesis is catalyzed by 13-lipoxygenases (LOXs). The resulting 13(S)-hydroperoxyoctadecatrienoic acid (13-HPOTE) is converted by allene oxide synthase (AOS) and allene oxide cyclase to OPDA (Wasternack, 2007). These enzymatic steps are located in plastids. OPDA is transported to peroxisomes and converted to JA. JA can be further metabolized to different derivatives that take place mainly in the cytosol. The conjugation of JA with Ile is an important step because jasmonoyl-Ile (JA-Ile) has been identified as a biologically active jasmonate (Staswick and Tiryaki, 2004). OPDA is also biologically active without conversion to JA derivatives. In contrast to all other jasmonates, the OPDA structure contains an electrophilic α,β-unsaturated carbonyl group that renders OPDA more reactive than JA. Therefore, OPDA is classified as a reactive electrophile species with unique signaling properties different from other jasmonates (Farmer and Davoine, 2007).Of the six lipoxygenase genes present in Arabidopsis, four genes encode 13-LOX. For the respective enzymes LOX2, LOX3, LOX4, and LOX6, it was shown that linolenic acid is the preferred substrate and that 13-HPOTE is formed in vitro (Bannenberg et al., 2009). All four enzymes are proposed to be located in plastids. LOX2 is highly expressed in leaves; expression is up-regulated by jasmonates and stress treatments such as wounding and osmotic stress (Bell and Mullet, 1993; Seltmann et al., 2010a). LOX2 was shown to contribute the majority of jasmonate synthesis upon wounding and osmotic stress and during senescence in leaves (Bell et al., 1995; Glauser et al., 2009). LOX2 is also responsible for the accumulation of arabidopsides (Glauser et al., 2009), which are galactolipids containing esterified OPDA in plastids by direct oxidation of galactolipids (Zoeller et al., 2012). LOX3 and LOX4 are required for the development of fertile flowers (Caldelari et al., 2011). LOX6 shows overall low expression (Bannenberg et al., 2009). Recently, it was reported that LOX6 contributes to the fast accumulation of JA and JA-Ile in wounded leaves and is required for the fast increase of JA and JA-Ile in distal leaves after wounding (Chauvin et al., 2013).In contrast to leaves and flowers, little is known on jasmonate biosynthesis and function in roots. Expression of the plastid-localized enzymes of jasmonate synthesis LOX2, AOS, and allene oxide cyclase2 is very low in roots (Zimmermann et al., 2004). By contrast, enzymes such as 9-LOX and α-dioxygenase1 are strongly expressed in roots. These enzymes are involved in the biosynthesis of oxylipins different from jasmonates, and 9-LOX products have been shown to regulate lateral root development because mutants in LOX1 and LOX5 produce more lateral roots (Vellosillo et al., 2007). However, jasmonate function in roots is still obscure. Here, we analyzed jasmonate accumulation in roots upon different stress treatments and show that mutants defective in LOX6 are impaired in stress-induced jasmonate synthesis and are more susceptible to drought and detritivore feeding.     

Adhesion of Dissimilatory Fe(III)-Reducing Bacteria to Fe(III) Minerals

The metabolism of dissimilatory iron-reducing bacteria (DIRB) may provide a means of remediating contaminated subsurface soils. The factors controlling the rate and extent of bacterial F(III) mineral reduction are poorly understood. Recent research suggests that molecular-scale interactions between DIRB cells and Fe(III) mineral particles play an important role in this process. One of these interactions, cell adhesion to Fe(III) mineral particles, appears to be a complex process that is, at least in part, mediated by a variety of surface proteins. This study examined the hypothesis that the flagellum serves as an adhesin to different Fe(III) minerals that range in their surface area and degree of crystallinity. Deflagellated cells of the DIRB Shewanella algae BrY showed a reduced ability to adhere to hydrous ferric oxide (HFO) relative to flagellated cells. Flagellated cells were also more hydrophobic than deflagellated cells. This was significant because hydrophobic interactions have been previously shown to dominate S. algae cell adhesion to Fe(III) minerals. Pre-incubating HFO, goethite, or hematite with purified flagella inhibited the adhesion of S. algae BrY cells to these minerals. Transposon mutagenesis was used to generate a flagellum-deficient mutant designated S. algae strain NF. There was a significant difference in the rate and extent of S. algae NF adhesion to HFO, goethite, and hematite relative to that of S. algae BrY. Amiloride, a specific inhibitor of Na + -driven flagellar motors, inhibited S. algae BrY motility but did not affect the adhesion of S. algae BrY to HFO. S.algae NF reduced HFO at the same rate as S. algae BrY. Collectively, the results of this study support the hypothesis that the flagellum of S. algae functions as a specific Fe(III) mineral adhesin. However, these results suggest that flagellum-mediated adhesion is not requisite for Fe(III) mineral reduction.     

Isolation of a Novel Peroxisomal Catalase Gene from Sugarcane,Which Is Responsive to Biotic and Abiotic Stresses

Catalase is an iron porphyrin enzyme, which serves as an efficient scavenger of reactive oxygen species (ROS) to avoid oxidative damage. In sugarcane, the enzymatic activity of catalase in a variety (Yacheng05–179) resistant to the smut pathogen Sporisorium scitamineum was always higher than that of the susceptible variety (Liucheng03–182), suggesting that catalase activity may have a positive correlation with smut resistance in sugarcane. To understand the function of catalase at the molecular level, a cDNA sequence of ScCAT1 (GenBank Accession No. {"type":"entrez-nucleotide","attrs":{"text":"KF664183","term_id":"676657290","term_text":"KF664183"}}KF664183), was isolated from sugarcane infected by S. scitamineum. ScCAT1 was predicted to encode 492 amino acid residues, and its deduced amino acid sequence shared a high degree of homology with other plant catalases. Enhanced growth of ScCAT1 in recombinant Escherichia coli Rosetta cells under the stresses of CuCl₂, CdCl₂ and NaCl indicated its high tolerance. Q-PCR results showed that ScCAT1 was expressed at relatively high levels in the bud, whereas expression was moderate in stem epidermis and stem pith. Different kinds of stresses, including S. scitamineum challenge, plant hormones (SA, MeJA and ABA) treatments, oxidative (H₂O₂) stress, heavy metal (CuCl₂) and hyper-osmotic (PEG and NaCl) stresses, triggered a significant induction of ScCAT1. The ScCAT1 protein appeared to localize in plasma membrane and cytoplasm. Furthermore, histochemical assays using DAB and trypan blue staining, as well as conductivity measurement, indicated that ScCAT1 may confer the sugarcane immunity. In conclusion, the positive response of ScCAT1 to biotic and abiotic stresses suggests that ScCAT1 is involved in protection of sugarcane against reactive oxidant-related environmental stimuli.     

Abiotic and Biotic Factors Affect Microhabitat Selection by the Firebrat, Thermobia domestica (Packard) (Thysanura: Lepismatidae)

Arthropod microhabitat selection involves a hierarchical assessment of abiotic and biotic factors. In choice experiments, we tested firebrat, Thermobia domestica, microhabitat preferences. Firebrats preferred elevated (35°C) over ambient (20°C) temperature, black over white shelter, and small (1 cm) over large (15.5 cm) entrance holes. Food availability did not alter shelter selection by firebrats. Medium juveniles, large juveniles and adults, in homo- and heterogeneous populations, preferred high (4.5 and 6.0 mm) over low (1.5 and 3.0 mm) shelter heights. Small juveniles, however, selected shelters with conspecifics, not by size. Females held at 35°C, but not 20 or 25°C, laid large numbers of eggs. Thus, abiotic characteristics of a shelter, coupled with the presence of conspecifics, affect microhabitat selection by firebrats. These findings may improve entrapment and management systems of firebrats.

Phototrophic Fe(II) Oxidation Promotes Organic Carbon Acquisition by Rhodobacter capsulatus SB1003

Anoxygenic phototrophic Fe(II) oxidation is usually considered to be a lithoautotrophic metabolism that contributes to primary production in Fe-based ecosystems. In this study, we employed Rhodobacter capsulatus SB1003 as a model organism to test the hypothesis that phototrophic Fe(II) oxidation can be coupled to organic carbon acquisition. R. capsulatus SB1003 oxidized Fe(II) under anoxic conditions in a light-dependent manner, but it failed to grow lithoautotrophically on soluble Fe(II). When the strain was provided with Fe(II)-citrate, however, growth was observed that was dependent upon microbially catalyzed Fe(II) oxidation, resulting in the formation of Fe(III)-citrate. Subsequent photochemical breakdown of Fe(III)-citrate yielded acetoacetic acid that supported growth in the light but not the dark. The deletion of genes (RRC00247 and RRC00248) that encode homologs of atoA and atoD, required for acetoacetic acid utilization, severely impaired the ability of R. capsulatus SB1003 to grow on Fe(II)-citrate. The growth yield achieved by R. capsulatus SB1003 in the presence of citrate cannot be explained by lithoautotrophic growth on Fe(II) enabled by indirect effects of the ligand [such as altering the thermodynamics of Fe(II) oxidation or preventing cell encrustation]. Together, these results demonstrate that R. capsulatus SB1003 grows photoheterotrophically on Fe(II)-citrate. Nitrilotriacetic acid also supported light-dependent growth on Fe(II), suggesting that Fe(II) oxidation may be a general mechanism whereby some Fe(II)-oxidizing bacteria mine otherwise inaccessible organic carbon sources.