Microbial Iron(II) Oxidation in Littoral Freshwater Lake Sediment: The Potential for Competition between Phototrophic vs. Nitrate-Reducing Iron(II)-Oxidizers

Microbial mixotrophic denitrification using iron(II) as an assisted electron donor

Mixotrophic denitrification processes have a great potential in nitrogen removal in biological wastewater treatment processes. However, so far, few studies have focused on the mixotrophic denitrification system using Fe(II) as an exclusively assisted electron donors and the underlying mechanisms in such a process remain unclear. Furthermore, the mechanisms by which microorganisms cover carbon, nitrogen, phosphorus and iron in an iron-assisted mixotrophic system remain unrevealed. In this work, we explore the feasibility of using Fe(II) as an assisted electron donor for enhancing simultaneous nitrogen and phosphorus removal via long-term reactor operation and batch tests. The results show that Fe(II) could provide electrons for efficient nitrate reduction and that biological reactions played a predominant role in these systems. In these systems Thermomonas, a strain of nitrate-reduction Fe(II)-oxidation bacterium, was enriched and accounted for a maximum abundance of 60.2%. These findings indicate a great potential of the Fe(II)-assisted mixotrophic denitrification system for practical use as an efficient simultaneous nitrogen and phosphorus removal process.

Continuous cultivation of the lithoautotrophic nitrate-reducing Fe(II)-oxidizing culture KS in a chemostat bioreactor

Laboratory-based studies on microbial Fe(II) oxidation are commonly performed for 5-10 days in small volumes with high substrate concentrations, resulting in geochemical gradients and volumetric effects caused by sampling. We used a chemostat to enable uninterrupted supply of medium and investigated autotrophic nitrate-reducing Fe(II)-oxidizing culture KS for 24 days. We analysed Fe- and N-speciation, cell-mineral associations, and the identity of minerals. Results were compared to batch systems (50 and 700 mL-static/shaken). The Fe(II) oxidation rate was highest in the chemostat with 7.57 mM Fe(II) d^-1 , while the extent of oxidation was similar to the other experimental setups (average oxidation of 92% of all Fe(II)). Short-range ordered Fe(III) phases, presumably ferrihydrite, precipitated and later goethite was detected in the chemostat. The 1 mM solid phase Fe(II) remained in the chemostat, up to 15 μM of reactive nitrite was measured, and 42% of visualized cells were partially or completely mineral-encrusted, likely caused by abiotic oxidation of Fe(II) by nitrite. Despite (partial) encrustation, cells were still viable. Our results show that even with similar oxidation rates as in batch cultures, cultivating Fe(II)-oxidizing microorganisms under continuous conditions reveals the importance of reactive nitrogen intermediates on Fe(II) oxidation, mineral formation and cell-mineral interactions.

Distinct arsenic uptake feature in rice reveals the importance of N fertilization strategies

The environmental behavior of arsenic (As) is commonly affected by the biogeochemical processes of iron (Fe) and nitrogen (N). In this study, field experiments were conducted to explore As uptake in rice and As translation and distribution in As-contaminated iron-rich paddy soils after applying different forms of N fertilizers, including urea (CO(NH2)2), ammonium bicarbonate (NH4HCO3), nitrate of potash (KNO3), and ammonium bicarbonate + nitrate of potash (NH4HCO3 + KNO3). The results indicated that applying nitrate N fertilizer inhibited the reduction and dissolution of As-bearing iron minerals and promoted microbial-mediated As(III) oxidation in flooded soil, thus reducing the soil As bioavailability. The concentrations of total As and inorganic As ratio (iAs/TAs) in rice grain decreased by 32.4 % and 15.4 %, respectively. However, the application of ammonium nitrogen promoted the reductive dissolution of As-bearing iron minerals and stimulated microbial As(V) reduction in flooded soil, leading to the release of As from soil to porewater. The total As concentration and inorganic As uptake ratio in rice grain increased by 20.1 % and 6.2 %, respectively, when urea was applied, and by 29.6 % and 10.5 %, respectively, when ammonium bicarbonate was applied. However, the simultaneous application of NH4+ and NO3- had no significant effect on As concentration in rice grain and its transformation in paddy soils. Ammonium nitrogen enhanced the organic As concentration in rice grain because the increased As(III) promoted As methylation in soil. In contrast, nitrate decreased the organic As uptake by rice grain because the decreased As(III) diminished As methylation in soil. The results provide reasonable N fertilization strategies for regulating the As biogeochemical process and reducing the risk of As contamination in rice.

Microbial biofilms as living photoconductors due to ultrafast electron transfer in cytochrome OmcS nanowires

Light-induced microbial electron transfer has potential for efficient production of value-added chemicals, biofuels and biodegradable materials owing to diversified metabolic pathways. However, most microbes lack photoactive proteins and require synthetic photosensitizers that suffer from photocorrosion, photodegradation, cytotoxicity, and generation of photoexcited radicals that are harmful to cells, thus severely limiting the catalytic performance. Therefore, there is a pressing need for biocompatible photoconductive materials for efficient electronic interface between microbes and electrodes. Here we show that living biofilms of Geobacter sulfurreducens use nanowires of cytochrome OmcS as intrinsic photoconductors. Photoconductive atomic force microscopy shows up to 100-fold increase in photocurrent in purified individual nanowires. Photocurrents respond rapidly (<100 ms) to the excitation and persist reversibly for hours. Femtosecond transient absorption spectroscopy and quantum dynamics simulations reveal ultrafast (~200 fs) electron transfer between nanowire hemes upon photoexcitation, enhancing carrier density and mobility. Our work reveals a new class of natural photoconductors for whole-cell catalysis.

Despite enormous potential of solar-driven biocatalysis, most living systems lack photoactive proteins and require toxic and expensive synthetic materials limiting the performance. Here, a class of natural photoconductors is demonstrated through sub-picosecond heme-to-heme electron transfer in bacteria-produced protein nanowires.

Iron Flocs and the Three Domains: Microbial Interactions in Freshwater Iron Mats

Freshwater iron mats are dynamic geochemical environments with broad ecological diversity, primarily formed by the iron-oxidizing bacteria. The community features functional groups involved in biogeochemical cycles for iron, sulfur, carbon, and nitrogen. Despite this complexity, iron mat communities provide an excellent model system for exploring microbial ecological interactions and ecological theories in situ Syntrophies and competition between the functional groups in iron mats, how they connect cycles, and the maintenance of these communities by taxons outside bacteria (the eukaryota, archaea, and viruses) have been largely unstudied. Here, we review what is currently known about freshwater iron mat communities, the taxa that reside there, and the interactions between these organisms, and we propose ways in which future studies may uncover exciting new discoveries. For example, the archaea in these mats may play a greater role than previously thought as they are diverse and widespread in iron mats based on 16S rRNA genes and include methanogenic taxa. Studies with a holistic view of the iron mat community members focusing on their diverse interactions will expand our understanding of community functions, such as those involved in pollution removal. To begin addressing questions regarding the fundamental interactions and to identify the conditions in which they occur, more laboratory culturing techniques and coculture studies, more network and keystone species analyses, and the expansion of studies to more freshwater iron mat systems are necessary. Increasingly accessible bioinformatic, geochemical, and culturing tools now open avenues to address the questions that we pose herein.

Extracellular electron transfer through visible light induced excited-state outer membrane C-type cytochromes of Geobacter sulfurreducens

Dissimilatory metal-reducing bacteria (DMRB) have a variety of c-type cytochromes (OM c-cyts) intercalated in their outer membrane, and this structure serves as the physiological basis for DMRB to carry out the extracellular electron transfer processes. Using Geobacter sulfurreducens as a model DMRB, we demonstrated that visible-light illumination could alter the electronic state of OM c-cyts from the ground state to the excited state in vivo. The existence of excited-state OM c-cyts in vivo was confirmed by spectroscopy. More importantly, excited-state OM c-cyts had a more negative potential compared to their ground-state counterparts, conferring DMRB with an extra pathway to transfer electrons to semi-conductive electron acceptors. To demonstrate this, using a TiO₂-coated electrode as an electron acceptor, we showed that G. sulfurreducens could directly utilise the conduction band of TiO₂ as an electron acceptor under visible-light illumination (λ > 420 nm) without causing TiO₂ charge separation. When G. sulfurreducens was subject to visible-light illumination, the rate of extracellular electron transfer (EET) to TiO₂ accelerated by over 8-fold compared to that observed under dark conditions. Results of additional electrochemical tests provided complementary evidence to support that G. sulfurreducens utilised excited-state OM c-cyts to enhance EET to TiO₂.

Role of organic carbon, nitrate and ferrous iron on the partitioning between denitrification and DNRA in constructed stormwater urban wetlands

Denitrification (DNF) and dissimilatory nitrate reduction to ammonium (DNRA) are two competing nitrate reduction pathways that remove or recycle nitrogen, respectively. However, factors controlling the partitioning between these two pathways are manifold and our understanding of these factors is critical for the management of N loads in constructed wetlands. An important factor that controls DNRA in an aquatic ecosystem is the electron donor, commonly organic carbon (OC) or alternatively ferrous iron and sulfide. In this study, we investigated the role of natural organic carbon (NOC) and acetate at different OC/NO₃^- ratios and ferrous iron on the partitioning between DNF and DNRA using the ¹⁵N-tracer method in slurries from four constructed stormwater urban wetlands in Melbourne, Australia. The carbon and nitrate experiments revealed that DNF dominated at all OC/NO₃^- ratios. The higher DNF and DNRA rates observed after the addition of NOC indicates that nitrate reduction was enhanced more by NOC than acetate. Moreover, addition of NOC in slurries stimulated DNRA more than DNF. Interestingly, slurries amended with Fe²⁺ showed that Fe²⁺ had significant control on the balance between DNF and DNRA. From two out of four wetlands, a significant increase in DNRA rates (p < .05) at the cost of DNF in the presence of available Fe²⁺ suggests DNRA is coupled to Fe²⁺ oxidation. Rates of DNRA increased 1.5-3.5 times in the Fe²⁺ treatment compared to the control. Overall, our study provides direct evidence that DNRA is linked to Fe²⁺ oxidation in some wetland sediments and highlights the role of Fe²⁺ in controlling the partitioning between removal (DNF) and recycling (DNRA) of bioavailable N in stormwater urban constructed wetlands. In our study we also measured anammox and found that it was always <0.05% of total nitrate reduction in these sediments.

Oxidation of Fe(II)-Organic Matter Complexes in the Presence of the Mixotrophic Nitrate-Reducing Fe(II)-Oxidizing Bacterium Acidovorax sp. BoFeN1

Fe(II)-organic matter (Fe(II)-OM) complexes are abundant in the environment and may play a key role for the behavior of Fe and pollutants. Mixotrophic nitrate-reducing Fe(II)-oxidizing bacteria (NRFeOx) reduce nitrate coupled to the oxidation of organic compounds and Fe(II). Fe(II) oxidation may occur enzymatically or abiotically by reaction with nitrite that forms during heterotrophic denitrification. However, it is unknown whether Fe(II)-OM complexes can be oxidized by NRFeOx. We used cell-suspension experiments with the mixotrophic nitrate-reducing Fe(II)-oxidizing bacterium Acidovorax sp. strain BoFeN1 to reveal the role of nonorganically bound Fe(II) (aqueous Fe(II)) and nitrite for the rates and extent of oxidation of Fe(II)-OM complexes (Fe(II)-citrate, Fe(II)-EDTA, Fe(II)-humic acid, and Fe(II)-fulvic acid). We found that Fe(II)-OM complexation inhibited microbial nitrate-reducing Fe(II) oxidation; large colloidal and negatively charged complexes showed lower oxidation rates than aqueous Fe(II). Accumulation of nitrite and fast abiotic oxidation of Fe(II)-OM complexes only happened in the presence of aqueous Fe(II) that probably interacted with (nitrite-reducing) enzymes in the periplasm causing nitrite accumulation in the periplasm and outside of the cells, whereas Fe(II)-OM complexes probably could not enter the periplasm and cause nitrite accumulation. These results suggest that Fe(II) oxidation by mixotrophic nitrate reducers in the environment depends on Fe(II) speciation, and that aqueous Fe(II) potentially plays a critical role in regulating microbial denitrification processes.

Insights into Carbon Metabolism Provided by Fluorescence In Situ Hybridization-Secondary Ion Mass Spectrometry Imaging of an Autotrophic, Nitrate-Reducing, Fe(II)-Oxidizing Enrichment Culture

The enrichment culture KS is one of the few existing autotrophic, nitrate-reducing, Fe(II)-oxidizing cultures that can be continuously transferred without an organic carbon source. We used a combination of catalyzed amplification reporter deposition fluorescence in situ hybridization (CARD-FISH) and nanoscale secondary ion mass spectrometry (NanoSIMS) to analyze community dynamics, single-cell activities, and interactions among the two most abundant microbial community members (i.e., Gallionellaceae sp. and Bradyrhizobium spp.) under autotrophic and heterotrophic growth conditions. CARD-FISH cell counts showed the dominance of the Fe(II) oxidizer Gallionellaceae sp. under autotrophic conditions as well as of Bradyrhizobium spp. under heterotrophic conditions. We used NanoSIMS to monitor the fate of ¹³C-labeled bicarbonate and acetate as well as ¹⁵N-labeled ammonium at the single-cell level for both taxa. Under autotrophic conditions, only the Gallionellaceae sp. was actively incorporating ¹³C-labeled bicarbonate and ¹⁵N-labeled ammonium. Interestingly, both Bradyrhizobium spp. and Gallionellaceae sp. became enriched in [¹³C]acetate and [¹⁵N]ammonium under heterotrophic conditions. Our experiments demonstrated that Gallionellaceae sp. was capable of assimilating [¹³C]acetate while Bradyrhizobium spp. were not able to fix CO₂, although a metagenomics survey of culture KS recently revealed that Gallionellaceae sp. lacks genes for acetate uptake and that the Bradyrhizobium sp. carries the genetic potential to fix CO₂ The study furthermore extends our understanding of the microbial reactions that interlink the nitrogen and Fe cycles in the environment.IMPORTANCE Microbial mechanisms by which Fe(II) is oxidized with nitrate as the terminal electron acceptor are generally referred to as "nitrate-dependent Fe(II) oxidation" (NDFO). NDFO has been demonstrated in laboratory cultures (such as the one studied in this work) and in a variety of marine and freshwater sediments. Recently, the importance of NDFO for the transport of sediment-derived Fe in aquatic ecosystems has been emphasized in a series of studies discussing the impact of NDFO for sedimentary nutrient cycling and redox dynamics in marine and freshwater environments. In this article, we report results from an isotope labeling study performed with the autotrophic, nitrate-reducing, Fe(II)-oxidizing enrichment culture KS, which was first described by Straub et al. (1) about 20 years ago. Our current study builds on the recently published metagenome of culture KS (2).

Photoferrotrophy: Remains of an Ancient Photosynthesis in Modern Environments

Photoferrotrophy, the process by which inorganic carbon is fixed into organic matter using light as an energy source and reduced iron [Fe(II)] as an electron donor, has been proposed as one of the oldest photoautotrophic metabolisms on Earth. Under the iron-rich (ferruginous) but sulfide poor conditions dominating the Archean ocean, this type of metabolism could have accounted for most of the primary production in the photic zone. Here we review the current knowledge of biogeochemical, microbial and phylogenetic aspects of photoferrotrophy, and evaluate the ecological significance of this process in ancient and modern environments. From the ferruginous conditions that prevailed during most of the Archean, the ancient ocean evolved toward euxinic (anoxic and sulfide rich) conditions and, finally, much after the advent of oxygenic photosynthesis, to a predominantly oxic environment. Under these new conditions photoferrotrophs lost importance as primary producers, and now photoferrotrophy remains as a vestige of a formerly relevant photosynthetic process. Apart from the geological record and other biogeochemical markers, modern environments resembling the redox conditions of these ancient oceans can offer insights into the past significance of photoferrotrophy and help to explain how this metabolism operated as an important source of organic carbon for the early biosphere. Iron-rich meromictic (permanently stratified) lakes can be considered as modern analogs of the ancient Archean ocean, as they present anoxic ferruginous water columns where light can still be available at the chemocline, thus offering suitable niches for photoferrotrophs. A few bacterial strains of purple bacteria as well as of green sulfur bacteria have been shown to possess photoferrotrophic capacities, and hence, could thrive in these modern Archean ocean analogs. Studies addressing the occurrence and the biogeochemical significance of photoferrotrophy in ferruginous environments have been conducted so far in lakes Matano, Pavin, La Cruz, and the Kabuno Bay of Lake Kivu. To date, only in the latter two lakes a biogeochemical role of photoferrotrophs has been confirmed. In this review we critically summarize the current knowledge on iron-driven photosynthesis, as a remains of ancient Earth biogeochemistry.

Evidence for the Existence of Autotrophic Nitrate-Reducing Fe(II)-Oxidizing Bacteria in Marine Coastal Sediment

Nitrate-reducing Fe(II)-oxidizing microorganisms were described for the first time ca. 20 years ago. Most pure cultures of nitrate-reducing Fe(II) oxidizers can oxidize Fe(II) only under mixotrophic conditions, i.e., when an organic cosubstrate is provided. A small number of nitrate-reducing Fe(II)-oxidizing cultures have been proposed to grow autotrophically, but unambiguous evidence for autotrophy has not always been provided. Thus, it is still unclear whether or to what extent Fe(II) oxidation coupled to nitrate reduction is an enzymatically catalyzed and energy-yielding autotrophic process or whether Fe(II) is abiotically oxidized by nitrite from heterotrophic nitrate reduction. The aim of the present study was to find evidence for the existence of autotrophic nitrate-reducing Fe(II) oxidizers in coastal marine sediments. Microcosm incubations showed that with increasing incubation times, the stoichiometric ratio of reduced nitrate/oxidized Fe(II) [NO₃⁻_reduced/Fe(II)_oxidized] decreased, indicating a decreasing contribution of heterotrophic denitrification and/or an increasing contribution of autotrophic nitrate-reducing Fe(II) oxidation over time. After incubations of sediment slurries for >10 weeks, nitrate-reducing activity ceased, although nitrate was still present. This suggests that heterotrophic nitrate reduction had ceased due to the depletion of readily available organic carbon. However, after the addition of Fe(II) to these batch incubation mixtures, the nitrate-reducing activity resumed, and Fe(II) was oxidized, indicating the activity of autotrophic nitrate-reducing Fe(II) oxidizers. The concurrent reduction of ¹⁴C-labeled bicarbonate concentrations unambiguously proved that autotrophic C fixation occurred during Fe(II) oxidation and nitrate reduction. Our results clearly demonstrated that autotrophic nitrate-reducing Fe(II)-oxidizing bacteria were present in the investigated coastal marine sediments.

IMPORTANCE Twenty years after the discovery of nitrate-reducing Fe(II) oxidizers, it is still controversially discussed whether autotrophic nitrate-reducing Fe(II)-oxidizing microorganisms exist and to what extent Fe(II) oxidation in this reduction/oxidation process is enzymatically catalyzed or which role abiotic side reactions of Fe(II) with reactive N species play. Most pure cultures of nitrate-reducing Fe(II) oxidizers are mixotrophic; i.e., they need an organic cosubstrate to maintain their activity over several cultural transfers. For the few existing autotrophic isolates and enrichment cultures, either the mechanism of nitrate-reducing Fe(II) oxidation is not known or evidence for their autotrophic lifestyle is controversial. In the present study, we provide evidence for the existence of autotrophic nitrate-reducing Fe(II) oxidizers in coastal marine sediments. The evidence is based on stoichiometries of nitrate reduction and Fe(II) oxidation determined in microcosm incubations and the incorporation of carbon from CO₂ under conditions that favor the activity of nitrate-reducing Fe(II) oxidizers.

The role of microaerophilic Fe-oxidizing micro-organisms in producing banded iron formations

Despite the historical and economic significance of banded iron formations (BIFs), we have yet to resolve the formation mechanisms. On modern Earth, neutrophilic microaerophilic Fe-oxidizing micro-organisms (FeOM) produce copious amounts of Fe oxyhydroxides, leading us to wonder whether similar organisms played a role in producing BIFs. To evaluate this, we review the current knowledge of modern microaerophilic FeOM in the context of BIF paleoenvironmental studies. In modern environments wherever Fe(II) and O2 co-exist, microaerophilic FeOM proliferate. These organisms grow in a variety of environments, including the marine water column redoxcline, which is where BIF precursor minerals likely formed. FeOM can grow across a range of O2 concentrations, measured as low as 2 μm to date, although lower concentrations have not been tested. While some extant FeOM can tolerate high O2 concentrations, many FeOM appear to prefer and thrive at low O2 concentrations (~3-25 μm). These are similar to the estimated dissolved O2 concentrations in the few hundred million years prior to the 'Great Oxidation Event' (GOE). We compare biotic and abiotic Fe oxidation kinetics in the presence of varying levels of O2 and show that microaerophilic FeOM contribute substantially to Fe oxidation, at rates fast enough to account for BIF deposition. Based on this synthesis, we propose that microaerophilic FeOM were capable of playing a significant role in depositing the largest, most well-known BIFs associated with the GOE, as well as afterward when global O2 levels increased.

Submicron-Scale Heterogeneities in Nickel Sorption of Various Cell-Mineral Aggregates Formed by Fe(II)-Oxidizing Bacteria

Fe(II)-oxidizing bacteria form biogenic cell-mineral aggregates (CMAs) composed of microbial cells, extracellular organic compounds, and ferric iron minerals. CMAs are capable of immobilizing large quantities of heavy metals, such as nickel, via sorption processes. CMAs play an important role for the fate of heavy metals in the environment, particularly in systems characterized by elevated concentrations of dissolved metals, such as mine drainage or contaminated sediments. We applied scanning transmission (soft) X-ray microscopy (STXM) spectrotomography for detailed 3D chemical mapping of nickel sorbed to CMAs on the submicron scale. We analyzed different CMAs produced by phototrophic or nitrate-reducing microbial Fe(II) oxidation and, in addition, a twisted stalk structure obtained from an environmental biofilm. Nickel showed a heterogeneous distribution and was found to be preferentially sorbed to biogenically precipitated iron minerals such as Fe(III)-(oxyhydr)oxides and, to a minor extent, associated with organic compounds. Some distinct nickel accumulations were identified on the surfaces of CMAs. Additional information obtained from scatter plots and angular distance maps, showing variations in the nickel-iron and nickel-organic carbon ratios, also revealed a general correlation between nickel and iron. Although a high correlation between nickel and iron was observed in 2D maps, 3D maps revealed this to be partly due to projection artifacts. In summary, by combining different approaches for data analysis, we unambiguously showed the heterogeneous sorption behavior of nickel to CMAs.

Vertical distribution of Fe and Fe(III)-reducing bacteria in the sediments of Lake Donghu, China

In lake sediments, iron (Fe) is the most versatile element, and the redox cycling of Fe has a wide influence on the biogeochemical cycling of organic and inorganic substances. The aim of the present study was to analyze the vertical distribution of Fe and Fe(III)-reducing bacteria (FeRB) in the surface sediment (30 cm) of Lake Donghu, China. At the 3 sites we surveyed, FeRB and Fe(II)-oxidizing bacteria (FeOB) coexisted in anoxic sediments. Geobacter-related FeRB accounted for 5%-31% of the total Bacteria, while Gallionella-related FeOB accounted for only 0.1%-1.3%. A significant correlation between the relative abundance of poorly crystalline Fe and Geobacter spp. suggested that poorly crystalline Fe favored microbial Fe(III) reduction. Poorly crystalline Fe and Geobacter spp. were significantly associated with solid-phase Fe(II) and total inorganic phosphorus levels. Pore water Fe(II) concentrations negatively correlated with NO3(-) at all sites. We concluded that Geobacter spp. were abundant in the sediments of Lake Donghu, and the redox of Fe might participate in the cycling of nitrogen and phosphorus in sediments. These observations provided insight into the roles of microbial Fe cycling in lake sediments.

Redox cycling of Fe(II) and Fe(III) in magnetite by Fe-metabolizing bacteria

Microorganisms are a primary control on the redox-induced cycling of iron in the environment. Despite the ability of bacteria to grow using both Fe(II) and Fe(III) bound in solid-phase iron minerals, it is currently unknown whether changing environmental conditions enable the sharing of electrons in mixed-valent iron oxides between bacteria with different metabolisms. We show through magnetic and spectroscopic measurements that the phototrophic Fe(II)-oxidizing bacterium Rhodopseudomonas palustris TIE-1 oxidizes magnetite (Fe3O4) nanoparticles using light energy. This process is reversible in co-cultures by the anaerobic Fe(III)-reducing bacterium Geobacter sulfurreducens. These results demonstrate that Fe ions bound in the highly crystalline mineral magnetite are bioavailable as electron sinks and electron sources under varying environmental conditions, effectively rendering magnetite a naturally occurring battery.

Electron uptake by iron-oxidizing phototrophic bacteria

Oxidation-reduction reactions underlie energy generation in nearly all life forms. Although most organisms use soluble oxidants and reductants, some microbes can access solid-phase materials as electron-acceptors or -donors via extracellular electron transfer. Many studies have focused on the reduction of solid-phase oxidants. Far less is known about electron uptake via microbial extracellular electron transfer, and almost nothing is known about the associated mechanisms. Here we show that the iron-oxidizing photoautotroph Rhodopseudomonas palustris TIE-1 accepts electrons from a poised electrode, with carbon dioxide as the sole carbon source/electron acceptor. Both electron uptake and ruBisCo form I expression are stimulated by light. Electron uptake also occurs in the dark, uncoupled from photosynthesis. Notably, the pioABC operon, which encodes a protein system essential for photoautotrophic growth by ferrous iron oxidation, influences electron uptake. These data reveal a previously unknown metabolic versatility of photoferrotrophs to use extracellular electron transfer for electron uptake.

Potential role of nitrite for abiotic Fe(II) oxidation and cell encrustation during nitrate reduction by denitrifying bacteria

Microorganisms have been observed to oxidize Fe(II) at neutral pH under anoxic and microoxic conditions. While most of the mixotrophic nitrate-reducing Fe(II)-oxidizing bacteria become encrusted with Fe(III)-rich minerals, photoautotrophic and microaerophilic Fe(II) oxidizers avoid cell encrustation. The Fe(II) oxidation mechanisms and the reasons for encrustation remain largely unresolved. Here we used cultivation-based methods and electron microscopy to compare two previously described nitrate-reducing Fe(II) oxidizers ( Acidovorax sp. strain BoFeN1 and Pseudogulbenkiania sp. strain 2002) and two heterotrophic nitrate reducers (Paracoccus denitrificans ATCC 19367 and P. denitrificans Pd 1222). All four strains oxidized ∼8 mM Fe(II) within 5 days in the presence of 5 mM acetate and accumulated nitrite (maximum concentrations of 0.8 to 1.0 mM) in the culture media. Iron(III) minerals, mainly goethite, formed and precipitated extracellularly in close proximity to the cell surface. Interestingly, mineral formation was also observed within the periplasm and cytoplasm; intracellular mineralization is expected to be physiologically disadvantageous, yet acetate consumption continued to be observed even at an advanced stage of Fe(II) oxidation. Extracellular polymeric substances (EPS) were detected by lectin staining with fluorescence microscopy, particularly in the presence of Fe(II), suggesting that EPS production is a response to Fe(II) toxicity or a strategy to decrease encrustation. Based on the data presented here, we propose a nitrite-driven, indirect mechanism of cell encrustation whereby nitrite forms during heterotrophic denitrification and abiotically oxidizes Fe(II). This work adds to the known assemblage of Fe(II)-oxidizing bacteria in nature and complicates our ability to delineate microbial Fe(II) oxidation in ancient microbes preserved as fossils in the geological record.

Chemolithotrophic nitrate-dependent Fe(II)-oxidizing nature of actinobacterial subdivision lineage TM3

Anaerobic nitrate-dependent Fe(II) oxidation is widespread in various environments and is known to be performed by both heterotrophic and autotrophic microorganisms. Although Fe(II) oxidation is predominantly biological under acidic conditions, to date most of the studies on nitrate-dependent Fe(II) oxidation were from environments of circumneutral pH. The present study was conducted in Lake Grosse Fuchskuhle, a moderately acidic ecosystem receiving humic acids from an adjacent bog, with the objective of identifying, characterizing and enumerating the microorganisms responsible for this process. The incubations of sediment under chemolithotrophic nitrate-dependent Fe(II)-oxidizing conditions have shown the enrichment of TM3 group of uncultured Actinobacteria. A time-course experiment done on these Actinobacteria showed a consumption of Fe(II) and nitrate in accordance with the expected stoichiometry (1:0.2) required for nitrate-dependent Fe(II) oxidation. Quantifications done by most probable number showed the presence of 1 × 10(4) autotrophic and 1 × 10(7) heterotrophic nitrate-dependent Fe(II) oxidizers per gram fresh weight of sediment. The analysis of microbial community by 16S rRNA gene amplicon pyrosequencing showed that these actinobacterial sequences correspond to ~0.6% of bacterial 16S rRNA gene sequences. Stable isotope probing using (13)CO2 was performed with the lake sediment and showed labeling of these Actinobacteria. This indicated that they might be important autotrophs in this environment. Although these Actinobacteria are not dominant members of the sediment microbial community, they could be of functional significance due to their contribution to the regeneration of Fe(III), which has a critical role as an electron acceptor for anaerobic microorganisms mineralizing sediment organic matter. To the best of our knowledge this is the first study to show the autotrophic nitrate-dependent Fe(II)-oxidizing nature of TM3 group of uncultured Actinobacteria.

Nitrate-dependent ferrous iron oxidation by anaerobic ammonium oxidation (anammox) bacteria

We examined nitrate-dependent Fe(2+) oxidation mediated by anaerobic ammonium oxidation (anammox) bacteria. Enrichment cultures of "Candidatus Brocadia sinica" anaerobically oxidized Fe(2+) and reduced NO3(-) to nitrogen gas at rates of 3.7 ± 0.2 and 1.3 ± 0.1 (mean ± standard deviation [SD]) nmol mg protein(-1) min(-1), respectively (37°C and pH 7.3). This nitrate reduction rate is an order of magnitude lower than the anammox activity of "Ca. Brocadia sinica" (10 to 75 nmol NH4(+) mg protein(-1) min(-1)). A (15)N tracer experiment demonstrated that coupling of nitrate-dependent Fe(2+) oxidation and the anammox reaction was responsible for producing nitrogen gas from NO3(-) by "Ca. Brocadia sinica." The activities of nitrate-dependent Fe(2+) oxidation were dependent on temperature and pH, and the highest activities were seen at temperatures of 30 to 45°C and pHs ranging from 5.9 to 9.8. The mean half-saturation constant for NO3(-) ± SD of "Ca. Brocadia sinica" was determined to be 51 ± 21 μM. Nitrate-dependent Fe(2+) oxidation was further demonstrated by another anammox bacterium, "Candidatus Scalindua sp.," whose rates of Fe(2+) oxidation and NO3(-) reduction were 4.7 ± 0.59 and 1.45 ± 0.05 nmol mg protein(-1) min(-1), respectively (20°C and pH 7.3). Co-occurrence of nitrate-dependent Fe(2+) oxidation and the anammox reaction decreased the molar ratios of consumed NO2(-) to consumed NH4(+) (ΔNO2(-)/ΔNH4(+)) and produced NO3(-) to consumed NH4(+) (ΔNO3(-)/ΔNH4(+)). These reactions are preferable to the application of anammox processes for wastewater treatment.