Rhodobacter Capsulatus SB1003 Research Articles

Enhanced photo-fermentative hydrogen production by constructing Rhodobacter capsulatus-ZnO/ZnS hybrid system

This study incorporated ZnO/ZnS nanoparticles with Rhodobacter capsulatus SB1003, forming a hybrid system to promote photo-fermentative hydrogen production. The results indicate that the material’s photocatalytic activity and concentration significantly affected hydrogen yield. The addition of ZnO/ZnS exhibited a more significant auxiliary effect than ZnO and achieved an approximately 30% increase in hydrogen production compared to the control group. ZnO/ZnS enhanced the production of extracellular polymers, thereby strengthening the synergy between the nanomaterials and the bacteria. The photogenerated electrons from ZnO/ZnS were utilized by the photosynthetic bacteria. Furthermore, the activity of nitrogenase was enhanced, resulting in improved hydrogen production performance. This study provides insights into hydrogen production by photosynthetic bacteria with the assistance of inorganic semiconductor nanomaterials.

Nitrous oxide reduction by two partial denitrifying bacteria requires denitrification intermediates that cannot be respired.

Denitrification is a form of anaerobic respiration wherein nitrate (NO3-) is sequentially reduced via nitrite (NO2-), nitric oxide, and nitrous oxide (N2O) to dinitrogen gas (N2) by four reductase enzymes. Partial denitrifying bacteria possess only one or some of these four reductases and use them as independent respiratory modules. However, it is unclear if partial denitrifiers sense and respond to denitrification intermediates outside of their reductase repertoire. Here, we tested the denitrifying capabilities of two purple nonsulfur bacteria, Rhodopseudomonas palustris CGA0092 and Rhodobacter capsulatus SB1003. Each had denitrifying capabilities that matched their genome annotation; CGA0092 reduced NO2- to N2, and SB1003 reduced N2O to N2. For each bacterium, N2O reduction could be used both for electron balance during growth on electron-rich organic compounds in light and for energy transformation via respiration in darkness. However, N2O reduction required supplementation with a denitrification intermediate, including those for which there was no associated denitrification enzyme. For CGA0092, NO3- served as a stable, non-catalyzable molecule that was sufficient to activate N2O reduction. Using a β-galactosidase reporter, we found that NO3- acted, at least in part, by stimulating N2O reductase gene expression. In SB1003, NO2- but not NO3- activated N2O reduction, but NO2- was slowly removed, likely by a promiscuous enzyme activity. Our findings reveal that partial denitrifiers can still be subject to regulation by denitrification intermediates that they cannot use.IMPORTANCEDenitrification is a form of microbial respiration wherein nitrate is converted via several nitrogen oxide intermediates into harmless dinitrogen gas. Partial denitrifying bacteria, which individually have some but not all denitrifying enzymes, can achieve complete denitrification as a community by cross-feeding nitrogen oxide intermediates. However, the last intermediate, nitrous oxide (N2O), is a potent greenhouse gas that often escapes, motivating efforts to understand and improve the efficiency of denitrification. Here, we found that at least some partial denitrifying N2O reducers can sense and respond to nitrogen oxide intermediates that they cannot otherwise use. The regulatory effects of nitrogen oxides on partial denitrifiers are thus an important consideration in understanding and applying denitrifying bacterial communities to combat greenhouse gas emissions.

Anoxygenic phototrophic arsenite oxidation by a Rhodobacter strain.

Microbially mediated arsenic redox transformations are key for arsenic speciation and mobility in rice paddies. Whereas anaerobic anoxygenic photosynthesis coupled to arsenite (As(III)) oxidation has been widely examined in arsenic-replete ecosystems, it remains unknown whether this light-dependent process exists in paddy soils. Here, we isolated a phototrophic purple bacteria, Rhodobacter strain CZR27, from an arsenic-contaminated paddy soil and demonstrated its capacity to oxidize As(III) to arsenate (As(V)) using malate as a carbon source photosynthetically. Genome sequencing revealed an As(III)-oxidizing gene cluster (aioXSRBA) encoding an As(III) oxidase. Functional analyses showed that As(III) oxidation under anoxic phototrophic conditions correlated with transcription of the large subunit of the As(III) oxidase aioA gene. Furthermore, the non-As(III) oxidizer Rhodobacter capsulatus SB1003 heterologously expressing aioBA from strain CZR27 was able to oxidize As(III), indicating that aioBA was responsible for the observed As(III) oxidation in strain CZR27. Our study provides evidence for the presence of anaerobic photosynthesis-coupled As(III) oxidation in paddy soils, highlighting the importance of light-dependent, microbe-mediated arsenic redox changes in paddy arsenic biogeochemistry.

Ammonium Ion Promoted Biodegradable Plastic Precursor D-3-Hydroxybutyrate Production from Bicarbonate and Acetone with Dual Biocatalysts System

Abstract Dual biocatalytic synthesis of D-3-hydroxybutyrate, a precursor of biodegradable polyester, poly-(3-hydoroxybutyrate), from CO2 and acetone via acetoacetate as an intermediate with enzyme extract including acetone carboxylase (AC) and 3-hydroxybutyrate dehydrogenase obtained from Rhodobacter capsulatus SB1003 cultured in acetone-bicarbonate medium in the presence of ATP and NADH is accomplished. The catalytic activity of AC was enhanced in the presence of ammonium chloride, and the conversion yield of acetone to D-3-hydroxybutyrate by the dual biocatalytic system was estimated to be up to 95% after 24 h incubation.

Expression of biocatalysts and their use in monomer synthesis for biodegradable polymers from acetone and CO2

One-pot β-hydroxybutyrate synthesis from CO2 and acetone in an aqueous solution with acetone carboxylase and β-hydroxybutyrate dehydrogenase extracted from Rhodobacter capsulatus SB1003 by culturing in an acetone–bicarbonate medium was developed.

Visible-light driven 3-hydroxybutyrate synthesis from CO2 and acetone with the hybrid system of photocatalytic NADH regeneration and multi-biocatalysts.

Poly-3-hydroxybutyrate (PHB)-derived plastics are polymer materials with excellent biodegradability, being insoluble in water and relatively resistant to hydrolysis. There is a need for a method capable of synthesizing 3-hydroxybutyrate, a monomer of PHB, from a renewable material. In this work, visible-light driven 3-hydroxybutyrate from CO2 and acetone with the system consisting of triethanolamine, water-soluble zinc porphyrin, pentamethylcyclopentadienyl coordinated rhodium complex, NAD+ and a cell extract containing acetone carboxylase and 3-hydroxybutyrate dehydrogenase from Rhodobacter capsulatus SB1003 cultured in acetone-bicarbonate medium is established. In particular, the conversion yield for acetone to 3-hydroxybutyrate was improved up to 81% in this system after 7 h irradiation.

Time-dependent enhancement of fluorescence from Rhodobacter capsulatus SB1003 and its critical dependence on concentration temperature and static magnetic field.

Continuous exposure of 395 nm light increases the fluorescence emission intensity of photosynthetic purple non-sulphur bacteria, Rhodobacter capsulatus (SB1003). We show that such an increase in fluorescence emission of extracellular pigment complexes (PC) from these photosynthetic bacteria depends on the concentration of the pigment and temperature and can also be modulated by the static magnetic field. The time-dependent enhanced emission disappears either at or below 300K or below a threshold sample concentration (0.1mg/ml). The enhanced emission reappears at this condition (T < 278K) if a static magnetic field (395mT) is introduced during fluorescence measurement. The time dependence of emission is expressed in terms of a first-order rate constant, k = dF/(Fdt). The sign of k shifts from positive to negative as PC concentration is lowered than a threshold value, implying onset of fluorescence decay (k < 0) rather than amplification (k > 0). At PC concentration higher than a threshold, k becomes negative if the temperature is lowered. But, surprisingly, at low temperature, a static magnetic field reverts the k value to positive. We explain the logical nature of k-switching and photo-dynamics of the aforesaid microbial fluorescence emission by aggregation of protoporphyrin rings present in the PC. While the simultaneous presence of decay in fluorescence and susceptibility to static magnetic field suggests the dominance of triplet states at low temperatures, the process is reversed by SMF-induced removal of spin degeneracy. At higher temperatures, the optical excitability and lack of magnetic response suggest the dominance of singlet states. We propose that the restructuring of the singlet-triplet distribution by intersystem crossing may be the basis of this logical behaviour. In context with microbial function, time-dependent enhancement of fluorescence also implies relay of red photons to the neighbouring microbes not directly exposed to the incident radiation, thus serving as an indirect photosynthetic regulator.

Biosynthesis of cycloartenol by expression of plant and bacterial oxidosqualene cyclases in engineered Rhodobacter capsulatus

Cyclic triterpenes are a large group of secondary metabolites produced by plants, fungi and bacteria. They have diverse biological functions, and offer potential health benefits for humans. Although various terpenes from the mono-, sesqui- and diterpene classes are easy to produce in engineered bacteria, heterologous synthesis of cyclic triterpenes is more challenging. We have recently shown that the triterpene cycloartenol can be produced in Rhodobacter capsulatus SB1003 but initial titers were low with 0.34mgL−1. To assess, if this phototrophic α-proteobacterium can be engineered for enhanced triterpene production, we followed two alternative strategies by comparing the performance of the R. capsulatus SB1003 wildtype strain with two recombinant strains carrying either a mevalonate pathway implemented from Paracoccus zeaxanthinifaciens or a deletion in the intrinsic carotenoid biosynthesis gene crtE. These strains are thus engineered for an enhanced isoprenoid biosynthesis or a suppressed precursor conversion by the competing carotenoid pathway. Moreover, three different cycloartenol synthase (CAS) genes from Arabidopsis thaliana or the myxobacterial strains Stigmatella aurantiaca Sga15 and DW4/3-1 were tested for heterologous cycloartenol synthesis. We found that the heterologous expression of mevalonate pathway enzymes had little impact on cycloartenol levels irrespective of the chosen CAS. In contrast, the use of the newly constructed carotenoid-deficient crtE deletion strain showed threefold increased cycloartenol product titers. We conclude that R. capsulatus is a promising alternative host for the functional expression of triterpene biosynthetic enzymes from plants and microbes. Apparently, product titers can also be improved by suppression of competing precursor consumption.

Differences in possible TCA cycle replenishing pathways in purple non-sulfur bacteria possessing glyoxylate pathway

Pathways replenishing tricarboxylic acid cycle were divided into four major groups based on metabolite serving as source for oxaloacetic acid or other tricarboxylic acid cycle component synthesis. Using this metabolic map, the analysis of genetic potential for functioning of tricarboxylic acid cycle replenishment pathways was carried out for seven strains of purple non-sulfur bacterium Rhodopseudomonas palustris. The results varied from strain to strain. Published microarray data for phototrophic acetate cultures of Rps. palustris CGA009 were analyzed to validate activity of the putative pathways. All the results were compared with the results for another purple non-sulfur bacterium, Rhodobacter capsulatus SB1003 and species-specific differences were clarified.

The photosynthetic bacteria Rhodobacter capsulatus and Synechocystis sp. PCC 6803 as new hosts for cyclic plant triterpene biosynthesis.

Cyclic triterpenes constitute one of the most diverse groups of plant natural products. Besides the intriguing biochemistry of their biosynthetic pathways, plant triterpenes exhibit versatile bioactivities, including antimicrobial effects against plant and human pathogens. While prokaryotes have been extensively used for the heterologous production of other classes of terpenes, the synthesis of cyclic triterpenes, which inherently includes the two-step catalytic formation of the universal linear precursor 2,3-oxidosqualene, is still a major challenge. We thus explored the suitability of the metabolically versatile photosynthetic α-proteobacterium Rhodobacter capsulatus SB1003 and cyanobacterium Synechocystis sp. PCC 6803 as alternative hosts for biosynthesis of cyclic plant triterpenes. Therefore, 2,3-oxidosqualene production was implemented and subsequently combined with different cyclization reactions catalyzed by the representative oxidosqualene cyclases CAS1 (cycloartenol synthase), LUP1 (lupeol synthase), THAS1 (thalianol synthase) and MRN1 (marneral synthase) derived from model plant Arabidopsis thaliana. While successful accumulation of 2,3-oxidosqualene could be detected by LC-MS analysis in both hosts, cyclase expression resulted in differential production profiles. CAS1 catalyzed conversion to only cycloartenol, but expression of LUP1 yielded lupeol and a triterpenoid matching an oxidation product of lupeol, in both hosts. In contrast, THAS1 expression did not lead to cyclic product formation in either host, whereas MRN1-dependent production of marnerol and hydroxymarnerol was observed in Synechocystis but not in R. capsulatus. Our findings thus indicate that 2,3-oxidosqualene cyclization in heterologous phototrophic bacteria is basically feasible but efficient conversion depends on both the respective cyclase enzyme and individual host properties. Therefore, photosynthetic α-proteo- and cyanobacteria are promising alternative candidates for providing new bacterial access to the broad class of triterpenes for biotechnological applications.

Purification and characterization of S-adenosyl-L-methionine:uroporphyrinogen Ⅲ methyltransferase from Rhodobacter capsulatus SB1003

Biosynthesis of vitamin B₁₂ (VB₁₂) requires the methylation at positions C-2 and C-7 of the precursor uroporphyrinogen Ⅲ (urogen Ⅲ) to precorrin-2 by S-adenosyl-L-methionine uroporphyrinogen Ⅲ methyltransferase (SUMT), which is a potential bottleneck step. Most of SUMTs are inhibited by urogen Ⅲ and by-product S-adenosyl-L-homocysteine (SAH). In order to mine an SUMT that lacks such an inhibitory property to drive greater flux through the VB₁₂ biosynthetic pathway, we cloned two SUMT genes (RCcobA1, RCcobA2) from Rhodobacter capsulatus SB1003 and expressed them in Escherichia coli BL21 (DE3). Thereafter, the two enzymes were purified and their specific activity of 27.3 U/mg, 68.9 U/mg were determined respectively. The latter was 2.4 times higher than PDcobA (27.9 U/mg) from Pseudomonas denitrifican. Additionally, RCcobA2 could tolerate over 70 μmol/L urogen Ⅲ, which has never been reported before. Hence, RCcobA2 can be used as an efficient enzyme to regulate the VB₁₂ metabolic pathway and enhance VB₁₂ production in industrial strains.

Enhancing photo-fermentative hydrogen production performance of Rhodobacter capsulatus by disrupting methylmalonate-semialdehyde dehydrogenase gene

Abstract The effects of mmsA disruption in Rhodobacter capsulatus SB1003 on the pigment content, poly-β-hydroxybutyrate (PHB) content and photo-fermentative hydrogen production were studied. A transposon mutant assigned ZY29 with mutation on mmsA and an mmsA partially deleted mutant assigned ZYDM9 ( mmsA − ) were obtained. The mutants showed reduced pigment and PHB content, and improved photo-fermentative hydrogen production performance. The hydrogen yield and maximum hydrogen production rate of ZYDM9 were 4675 ± 76 mL/L and 92.4 ± 2.6 mL/(Lh), which increased by 22.8% and 20% compared with that of the WT, respectively. In the meantime, its pigment and PHB content were 20.7 ± 0.1 nmol/mg and 54.5 ± 2.6 mg/g-dcw, which reduced by 34.2% and 43.4% compared with that of WT, respectively. The optimum parameters for hydrogen production were initial pH 7.0, carbon nitrogen ratio (g/g) 3.0:0.5, and light intensity 800 ± 35 W/m 2 .

Coexpression of Mo- and Fe-nitrogenase in Rhodobacter capsulatus enhanced its photosynthetic hydrogen production

The effects of mopAB disruption in Rhodobacter capsulatus SB1003 on the nitrogenase expression level, activity and photo-fermentative hydrogen production were studied. The results showed that the disruption of mopAB derepressed the expression of Fe nitrogenase and showed no negative effect on the expression of Mo–Fe nitrogenase. The nitrogenase activity of YMP1 (mopAB−) was 4.22 ± 0.30 nmol acetylene/(min·mg-dcw), which was higher than that of 2.97 ± 0.20 nmol acetylene/(min·mg-dcw) obtained from the wild type strain SB1003. The hydrogen yields of YMP1 were enhanced by 26.7%, 39.7% and 35.3% compared with those obtained from SB1003 under the light intensity of 2500 lux, 5000 lux and 8500 lux respectively. The maximum hydrogen production rates of YMP1 were enhanced by 14.5%, 15.1% and 24.1% compared with those of SB1003 under the same condition. The results suggest that expressing nitrogenase related genes could be an alternative way for improving the hydrogen production performance of purple non-sulfur bacteria.

Enhanced photosynthetic hydrogen production performance of Rhodobacter capsulatus by deactivating CBB cycle and cytochrome c oxidase

The effect of redox balance on photosynthetic bacteria growth and hydrogen generation is significant since it affects the distribution of electron flow. In this study, cbbP encoding the key enzymes in CO2 fixation cycle, and ccoNOQP encoding cyt cbb3 oxidase in cytochrome respiratory chain, were knocked out from the genome of Rhodobacter capsulatus SB1003 to obtain the mutants Wb301 (ΔccoNOQP), Wbp02 (ΔcbbP) and W3p03 (ΔcbbP, ΔccoNOQP). The transcriptional and translational level of nifH gene showed that the nifH expression levels in Wb301 and W3p03 were enhanced by 93.5% and 62.5%, and the activities of nitrogenase of Wb301 and W3p03 were enhanced by 30% and 10%, compared with those of wild type strain, respectively. In 60 mL reactors, the hydrogen yields of Wb301 and W3p03 increased by 44.8% and 36.0%, and the maximum hydrogen evolution rates increased by 15.8% and 41.7% compared with that of the wild type, respectively. The mechanisms of CBB and cytochrome c oxidase deactivation on the growth and hydrogen production performance were discussed.

Iron and copper act synergistically to delay anaerobic growth of bacteria.

Transition metals are known to cause toxic effects through their interaction with oxygen, but toxicity under anoxic conditions is poorly understood. Here we investigated the effects of iron (Fe) and copper (Cu) on the anaerobic growth and gene expression of the purple phototrophic bacterium Rhodopseudomonas palustris TIE-1. We found that Fe(II) and Cu(II) act synergistically to delay anaerobic growth at environmentally relevant metal concentrations. Cu(I) and Cu(II) had similar effects both alone and in the presence of ascorbate, a Cu(II) reductant, indicating that reduction of Cu(II) to Cu(I) by Fe(II) is not sufficient to explain the growth inhibition. Addition of Cu(II) increased the toxicity of Co(II) and Ni(II); in contrast, Ni(II) toxicity was diminished in the presence of Fe(II). The synergistic anaerobic toxicity of Fe(II) and Cu(II) was also observed for Escherichia coli MG1655, Shewanella oneidensis MR-1, and Rhodobacter capsulatus SB1003. Gene expression analyses for R. palustris identified three regulatory genes that respond to Cu(II) and not to Fe(II): homologs of cueR and cusR, two known proteobacterial copper homeostasis regulators, and csoR, a copper regulator recently identified in Mycobacterium tuberculosis. Two P-type ATPase efflux pumps, along with an F(o)F(1) ATP synthase, were also upregulated by Cu(II) but not by Fe(II). An Escherichia coli mutant deficient in copA, cus, and cueO showed a smaller synergistic effect, indicating that iron might interfere with one or more of the copper homeostasis systems. Our results suggest that interactive effects of transition metals on microbial physiology may be widespread under anoxic conditions, although the molecular mechanisms remain to be more fully elucidated.

Functional assignment of gene AAC16202.1 from Rhodobacter capsulatus SB1003: New insights into the bacterial SDR sorbitol dehydrogenases family

Short-chain dehydrogenases/reductases (SDR) constitute one of the largest enzyme superfamilies with over 60,000 non-redundant sequences in the database, many of which need a correct functional assignment. Among them, the gene AAC16202.1 (NCBI) from Rhodobacter capsulatus SB1003 has been assigned in Uniprot both as a sorbitol dehydrogenase (#D5AUY1) and, as an N-acetyl-d-mannosamine dehydrogenase (#O66112), both enzymes being of biotechnological interest. When the gene was overexpressed in Escherichia coli Rosetta (DE3)pLys, the purified enzyme was not active toward N-acetyl-d-mannosamine, whereas it was active toward d-sorbitol and d-fructose. However, the relative activities toward xylitol and l-iditol (0.45 and 6.9%, respectively) were low compared with that toward d-sorbitol. Thus, the enzyme could be considered sorbitol dehydrogenase (SDH) with very low activity toward xylitol, which could increase its biotechnological interest for determining sorbitol without the unspecific cross-determination of added xylitol in food and pharma compositions. The tetrameric enzyme (120 kDa) showed similar catalytic efficiency (2.2 × 10(3) M(-1) s(-1)) to other sorbitol dehydrogenases for d-sorbitol, with an optimum pH of 9.0 and an optimum temperature of 37 °C. The enzyme was also more thermostable than other reported SDH, ammonium sulfate being the best stabilizer in this respect, increasing the melting temperature (T(m)) up to 52.9 °C. The enzyme can also be considered as a new member of the Zn(2+) independent SDH family since no effect on activity was detected in the presence of divalent cations or chelating agents. Finally, its in silico analysis enabled the specific conserved sequence blocks that are the fingerprints of bacterial sorbitol dehydrogenases and mainly located at C-terminal of the protein, to be determined for the first time. This knowledge will facilitate future data curation of present databases and a better functional assignment of newly described sequences.

Functional Characterization of the FoxE Iron Oxidoreductase from the Photoferrotroph Rhodobacter ferrooxidans SW2

Photoferrotrophy is presumed to be an ancient type of photosynthetic metabolism in which bacteria use the reducing power of ferrous iron to drive carbon fixation. In this work the putative iron oxidoreductase of the photoferrotroph Rhodobacter ferrooxidans SW2 was cloned, purified, and characterized for the first time. This protein, FoxE, was characterized using spectroscopic, thermodynamic, and kinetic techniques. It is a c-type cytochrome that forms a trimer or tetramer in solution; the two hemes of each monomer are hexacoordinated by histidine and methionine. The hemes have positive reduction potentials that allow downhill electron transfer from many geochemically relevant ferrous iron forms to the photosynthetic reaction center. The reduction potentials of the hemes are different and are cross-assigned to fast and slow kinetic phases of ferrous iron oxidation in vitro. Lower reactivity was observed at high pH and may contribute to prevent ferric iron precipitation inside or at the surface of the cell. These results help fill in the molecular details of a metabolic process that likely contributed to the deposition of precambrian banded iron formations, globally important sedimentary rocks that are found on every continent today.

Improved photo – Hydrogen production by transposon mutant of Rhodobacter capsulatus with reduced pigment

The light shielding effect of photosynthetic bacteria in photo fermentation process, which is caused by high content of pigment, hinders their hydrogen production rates under intense light irradiation. In order to mitigate this effect and improve hydrogen production efficiency, it is necessary to screen mutants that hold less pigment content. In this study, a mini-Tn5 transposon encoded plasmid pRL27 was employed for transposon mutagenesis of Rhodobacter capsulatus SB1003. A mutant named MC1417 showed significant lower light absorbance from 330 nm to 900 nm compared to its parental strain by UV–visible spectra, and its bacteriochlorophyll a content was reduced by 38%. The results showed that its photo fermentative hydrogen production was improved by 50.5% on the basis of BChl a content using acetate and butyrate as carbon source under intense light irradiation, indicating that it is effective on improving hydrogen production by repressing the pigment biosynthesis. DNA sequencing and BLAST in NCBI Genebank showed that the mutation occurred within its pucDE gene.

Photomixotrophic growth of Rhodobacter capsulatus SB1003 on ferrous iron.

This study investigates the role iron oxidation plays in the purple non-sulfur bacterium Rhodobacter capsulatus SB1003. This organism is unable to grow photoautotrophically on unchelated ferrous iron [Fe(II)] despite its ability to oxidize chelated Fe(II). This apparent paradox was partly resolved by the discovery that SB1003 can grow photoheterotrophically on the photochemical breakdown products of certain ferric iron-ligand complexes, yet whether it could concomitantly benefit from the oxidation of Fe(II) to fix CO(2) was unknown. Here, we examine carbon fixation by stable isotope labeling of the inorganic carbon pool in cultures growing phototrophically on acetate with and without Fe(II). We show that R. capsulatus SB1003, an organism formally thought incapable of phototrophic growth on Fe(II), can actually harness the reducing power of this substrate and grow photomixotrophically, deriving carbon both from organic sources and from fixation of inorganic carbon. This suggests the possibility of a wider occurrence of photoferrotrophy than previously assumed.

Rhodobacter capsulatus catalyzes light-dependent Fe(II) oxidation under anaerobic conditions as a potential detoxification mechanism.

Diverse bacteria are known to oxidize millimolar concentrations of ferrous iron [Fe(II)] under anaerobic conditions, both phototrophically and chemotrophically. Yet whether they can do this under conditions that are relevant to natural systems is understood less well. In this study, we tested how light, Fe(II) speciation, pH, and salinity affected the rate of Fe(II) oxidation by Rhodobacter capsulatus SB1003. Although R. capsulatus cannot grow photoautotrophically on Fe(II), it oxidizes Fe(II) at rates comparable to those of bacteria that do grow photoautotrophically on Fe(II) as soon as it is exposed to light, provided it has a functional photosystem. Chelation of Fe(II) by diverse organic ligands promotes Fe(II) oxidation, and as the pH increases, so does the oxidation rate, except in the presence of nitrilotriacetate; nonchelated forms of Fe(II) are also more rapidly oxidized at higher pH. Salt concentrations typical of marine environments inhibit Fe(II) oxidation. When growing photoheterotrophically on humic substances, R. capsulatus is highly sensitive to low concentrations of Fe(II); it is inhibited in the presence of concentrations as low as 5 microM. The product of Fe(II) oxidation, ferric iron, does not hamper growth under these conditions. When other parameters, such as pH or the presence of chelators, are adjusted to promote Fe(II) oxidation, the growth inhibition effect of Fe(II) is alleviated. Together, these results suggest that Fe(II) is toxic to R. capsulatus growing under strictly anaerobic conditions and that Fe(II) oxidation alleviates this toxicity.