Biogeochemical Cycling Of Iron Research Articles

Distribution of Acidophilic Microorganisms in Natural and Man-made Acidic Environments.

Acidophilic microorganisms can thrive in both natural and man-made environments. Natural acidic environments comprise hydrothermal sites on land or in the deep sea, cave systems, acid sulfate soils and acidic fens, as well as naturally exposed ore deposits (gossans). Man-made acidic environments are mostly mine sites including mine waste dumps and tailings, acid mine drainage and biomining operations. The biogeochemical cycles of sulfur and iron, rather than those of carbon and nitrogen, assume centre stage in these environments. Ferrous iron and reduced sulfur compounds originating from geothermal activity or mineral weathering provide energy sources for acidophilic, chemolithotrophic iron- and sulfur-oxidizing bacteria and archaea (including species that are autotrophic, heterotrophic or mixotrophic) and, in contrast to most other types of environments, these are often numerically dominant in acidic sites. Anaerobic growth of acidophiles can occur via the reduction of ferric iron, elemental sulfur or sulfate. While the activities of acidophiles can be harmful to the environment, as in the case of acid mine drainage, they can also be used for the extraction and recovery of metals, as in the case of biomining. Considering the important roles of acidophiles in biogeochemical cycles, pollution and biotechnology, there is a strong need to understanding of their physiology, biochemistry and ecology.

Impact of methane occurrence on iron speciation in the sediments of the Gdansk Basin (Southern Baltic Sea)

Due to changing climate conditions, such inland seas as the Baltic are expected to become more eutrophicated and less saline (causing lower availability of sulphates). This may promote methanogenesis as the main process of organic matter (OM) degradation in marine sediments. Presence of methane, in turn, may affect biogeochemical cycling of many elements, including iron. Thus, in the present study we attempted to investigate the influence of CH4 on the Fe forms in marine sediments. Sediment cores were collected from three physico-chemically different stations within the Southern Baltic, taking into account such parameters as water depth, OM content, bottom zone oxygenation and distance from the Vistula River (main source of anthropogenic material). At two sampling stations methane was present in shallow sediments (P1, MET2) and at one station (W6) any traces of this gas were not determined. Iron species (FeCARB, FeOX1, FeOX2, FeMAG, FePRS, FeT, FeFeS2, FeFeS) in sediments were investigated using the sequential extraction. Pore water was analysed to obtain vertical profiles of hydrogen sulphide, sulphate and dissolved inorganic carbon (DIC). Additionally, such parameters as water and OM content, CH4 and Eh in sediments were determined. Results showed that methanogenesis affects the biogeochemical iron cycling in sediments of the Southern Baltic, leading to the increase of carbonates containing ferrous iron (as a result of DIC production during intensive OSR, AOM and methanogenesis) and decrease of ferric iron compounds used in the AOM. Moreover, in the areas of lower salinity, pyrite formation is limited by an insufficient amount of hydrogen sulphide, leading to the situation when a significant part of Fe is accumulated in the form of monosulphides. In turn, in the areas of higher salinity, where oxygen deficiencies occur more often, more hydrogen sulphide is present in pore water. Pyrite formation is then limited by iron, not by sulphur.

Distinct iron cycling in a Southern Ocean eddy

Mesoscale eddies are ubiquitous in the iron-limited Southern Ocean, controlling ocean-atmosphere exchange processes, however their influence on phytoplankton productivity remains unknown. Here we probed the biogeochemical cycling of iron (Fe) in a cold-core eddy. In-eddy surface dissolved Fe (dFe) concentrations and phytoplankton productivity were exceedingly low relative to external waters. In-eddy phytoplankton Fe-to-carbon uptake ratios were elevated 2–6 fold, indicating upregulated intracellular Fe acquisition resulting in a dFe residence time of ~1 day. Heavy dFe isotope values were measured for in-eddy surface waters highlighting extensive trafficking of dFe by cells. Below the euphotic zone, dFe isotope values were lighter and coincident with peaks in recycled nutrients and cell abundance, indicating enhanced microbially-mediated Fe recycling. Our measurements show that the isolated nature of Southern Ocean eddies can produce distinctly different Fe biogeochemistry compared to surrounding waters with cells upregulating iron uptake and using recycling processes to sustain themselves.

Biogeochemical cycling of iron oxides in the rhizosphere of plants grown on ferruginous duricrust (canga)

Goethite-cemented duricrusts, also known as canga, commonly occur as a capping rock protecting underlying iron ore deposits. The processes that govern canga formation are still unclear but include recurrent partial dissolution and recrystallisation of goethite through biogeochemical cycling of iron, hypothesised to be catalysed by plants and bacteria. In the present study, the effect of plant exudates on mobilisation of iron in canga was examined using model plants grown on crushed canga in RHIZOtest devices, which separate roots from substrate by a semi-permeable membrane. Moderate plant-induced acidification of the canga was detected, however the primary driver of mineral dissolution was the synergistic effect of reductive and ligand-promoted dissolution, identified by an increase in organic acids concentration and the presence of low concentrations of free ferrous iron. Whilst organic acids exudation lasted, iron cations were stabilised in solution; once the organic acids were degraded by microorganisms, the free cations precipitated as iron oxy-hydroxides. Mineralogical analysis and high-resolution microscopy confirmed our hypothesis that plants that grow in this iron-rich substrate contribute to iron dissolution indirectly (e.g., during phosphate solubilisation), and that the resulting surplus iron not taken up by the plants is redeposited, promoting the cementation of the residual minerals. Understanding the contribution of plants to the iron cycling in canga is crucial when formulating post-mining rehabilitation strategies for iron ore sites.

Dual nitrogen-oxygen isotopic analysis and kinetic model for enzymatic nitrate reduction coupled with Fe(II) oxidation by Pseudogulbenkiania sp. strain 2002

While microbially mediated coupling process of nitrate reduction and Fe(II) oxidation (NRFO) plays important role in widespread nitrogen and iron biogeochemical cycles in the natural environments, the underlying mechanism of NRFO is still not clearly understood. Here, N and O isotope fractionation during microbial nitrate reduction by Pseudogulbenkiania sp. strain 2002 with or without Fe(II) were investigated. The nitrate reduction rate decreased substantially in the presence of Fe(II), and cells became encrusted with Fe(III) minerals. However, the N isotope enrichment factors (ε) with or without Fe(II) were identical within uncertainty (24.1 ± 2.4‰ and 25.1 ± 2.4‰), implying that Fe(II) oxidation had little influence on the extent of N isotope fractionation and that cell encrustation was unlikely to cause additional mass transfer limitation. Isotopic composition ratios of 18O to 15N (18ε: 15ε) were 0.73 ± 0.13 with and 0.50 ± 0.08 without Fe(II), close to reported values for nitrate reduction by periplasmic nitrate reductases, implying that enzymatic nitrate reduction mainly occurs in strain 2002's periplasmic space. A kinetic model based on elementary reactions was established, and rate constants of enzymatic nitrate reducing reactions were obtained in a living cell system. Combining dual N-O isotope fractionation analysis with kinetic modeling is a promising approach for interpreting diverse enzymatic pathways during NRFO processes.

Magnetite magnetofossils record biogeochemical remanent magnetization in hydrogenetic ferromanganese crusts

AbstractRecords of natural remanent magnetization (NRM) and geomagnetic polarity reversals preserved within ferromanganese (Fe-Mn) crusts, together with the application of fine-resolution magnetostratigraphic analysis, have been successfully demonstrated. However, because Fe-bearing precipitates or minerals are thought to be either oxides and/or hydroxides precipitated from ambient oxic seawater or detrital minerals, the magnetic properties of the ferromagnetic minerals and the genetic mechanisms remain controversial; moreover, the origin of the NRM is unclear. Here, we show that nanometer-scale magnetite crystals found in Fe-Mn crusts from the Pacific Ocean and South China Sea are magnetosome fossils based on their narrow size range, chain arrangement, chemical purity, and crystallographic perfection, as indicated by transmission electron microscopy. Furthermore, our new data from rock magnetic and electron paramagnetic resonance analyses, combined with a previously reported micro-magnetostratigraphic sequence, indicate that magnetotactic bacteria and their post-mortem remains contribute to a biogeochemical remanent magnetization of Fe-Mn crusts. In addition, the results provide evidence for a previously unappreciated pathway for the biogeochemical cycling of iron in the deep ocean.

Biogeochemical cycling of iron: Implications for biocementation and slope stabilisation

Microbial biofilms growing in iron-rich seeps surrounding Lake Violão, Carajás, Brazil serve as a superb natural system to study the role of iron cycling in producing secondary iron cements. These seeps flow across iron duricrusts (referred to as canga in Brazil) into hydraulically restricted lakes in northern Brazil. Canga caps all of the iron ore deposits in Brazil, protecting them from being destroyed by erosion in this active weathering environment. Biofilm samples collected from these seeps demonstrated heightened biogeochemical iron cycling, contributing to the relatively rapid, seasonal formation of iron-rich cements. The seeps support iron-oxidising lineages including Sideroxydans, Gallionella, and an Azoarcus species revealed by 16S rRNA gene sequencing. In contrast, a low relative abundance of putative iron reducers; for example, Geobacter species (<5% of total sequences in any sample), corresponds to carbon limitation in this canga-associated ecosystem. This carbon limitation is likely to restrict anoxic niches to within biofilms. Examination of a canga rock sample collected from the edge of Lake Violão revealed an array of well- to poorly-preserved microbial fossils in secondary iron cements. These heterogeneous cements preserved bacterial cell envelopes and possibly extracellular polymeric substances within the microfossil iron-rich cements (termed biocements). Bacterial iron reduction initiates the sequence, and intuitively is the rate-limiting step in this broadly aerobic environment. The organic framework of the active- and paleo-biofilms appears to provide a scaffold for the formation of some cements within canga and likely expedites cement formation. The accelerated development of these resilient iron-rich biocements in the lake edge environment compared with surroundings duricrust-associated environments may provide insights into new approaches to remediate mined land, aiding to stabilise slopes, reduce erosion, restore functional hydrogeology and provide a substrate akin to natural canga for revegetation using endemic canga plant species, which have adapted to grow on iron-rich substrates.

Iron and phosphorus biochemical systems and the Cryogenian-Ediacaran transition, Jacadigo basin, Brazil: Implications for the Neoproterozoic oxygenation event

Termination of the Marinoan Snowball Earth ice age (635 Ma) marked the transition to a greenhouse world. This climate change forever modified the biogeochemical cycling of iron and phosphorus, ended large-scale iron formation deposition, began accumulation of phosphorus on marine shelves, and led to the Ediacaran radiation of eukaryotes. The Jacadigo Basin, Brazil, contains a nearly complete record of this critical transition.Glaciomarine diamictites and iron formation accumulated during two Marinoan ice advances with hydrothermal input of iron delivered via ice-margin upwelling. Biochemically precipitated rhythmites of siderite, sedimentary apatite, and hematite represent microbially mediated, sub-sea ice precipitation. Siderite laminae preserve microbial textures and have a mean ∂13C = −8.80‰, PDB (+/−0.86‰) reflecting degradation of organic matter at the seafloor. These millimeter-scale rhythmites are a sensitive record of sub-ice dynamics because they formed in response to short-term fluctuations of O2 due to seasonal sub-ice photosynthesis. They demonstrate the connection between ice cover, O2, and cycling of iron and nutrient elements such as phosphorus.These biochemical rhythmites suggest that Cryogenian sea ice limited oxygen production prior to the onset of the Neoproterozoic Oxygenation Event. O2 increased enough to concentrate bioavailable phosphorus at the seafloor, which was essential for later diversification of metazoans in the Ediacaran. Such sub-ice Cryogenian biochemical systems may provide Earth-based analogs for life on ice-covered worlds, such as Europa and Enceladus.

Inhibition effect of polyvinyl chloride on ferrihydrite reduction and electrochemical activities of Geobacter metallireducens.

Geobacter metallireducens GS15, a model of dissimilatory iron-reducing bacteria, is the key regulator in biogeochemical iron cycling. How the emerging contaminant microplastics involved in the iron cycling are driven by microbes on the microscale remains unknown. Hence, the influences of two typical microplastics, polybutylene terephthalate-hexane acid (PBAT) and polyvinyl chloride (PVC), were explored on the activity of G. metallireducens GS15 with ferrihydrite or ferric citrate as the respective electron acceptors. The results showed that the iron (II) contents in PBAT- and PVC-treatment groups were 16.79 and 6.81 mM, respectively, at the end of the experiment. Compared with the PBAT-treatment group, scanning electron microscopy and energy dispersive spectrometery revealed that merely a small amount of iron-containing products covered the surface of PVC. Moreover, PBAT and PVC could both retard the electroactivity of G. metallireducens GS15 at the beginning of microbial fuel cell operation. On the basis of the results above, microplastic PVC might exhibit potential inhibition of the iron cycling process driven by G. metallireducens GS15 with ferrihydrite as the terminal electron acceptor. This study extended our understanding of the influence of the microplastics PBAT and PVC on microbially mediated biogeochemical iron cycling. The findings might have an important implication on the biogeochemical elements cycling in the ecosystem with the involvement of emerging contaminant microplastics.

Benzene promotes microbial Fe(III) reduction and flavins secretion

Abstract The microbial reduction of Fe(III) (oxyhydr)oxide is widespread in subsurface and plays a critical role in both the biogeochemical cycle of iron and the fate of contaminants. Monocyclic aromatic compounds are ubiquitous constituents of organic matter in many geologic environments and contaminated subsurface. Benzene is a typical monocyclic aromatic compound and frequently occurs in the subsurface environment. Due to its carcinogenicity and cytotoxicity, benzene may be toxic to the coexisted Fe(III)-reducing bacteria and thereby inhibit the microbial Fe(III) reduction. However, there is limited knowledge about the impact of the coexisting monocyclic aromatic compounds on the microbial Fe(III) reduction. In this study, the reduction of ferrihydrite by the dissimilatory iron-reducing bacterium Shewanella oneidensis MR-1 (MR-1) was investigated in the presence of benzene. Results showed that benzene had a negligible impact on the growth, cell morphology and integrity of MR-1, but it promoted the microbial Fe(III) reduction. The promotion of microbial Fe(III) reduction is maximum at benzene concentration of 3.8 μM. In the presence of 3.8 μM benzene, the produced Fe(II) from microbial Fe(III) reduction in 60 h doubled that in the absence of benzene, and the Fe(II)-O content of mineral surface after reduction experiment increased 4.73%. The promotion of microbial Fe(III) reduction was ascribed to the benzene induced increase of cell membrane permeability, which facilitated extracellular electron transfer and the secretion and release of flavin mononucleotide (FMN) as electron shuttle or cofactor. The impacts of benzene on the FMN secretion and microbial Fe(III) reduction have broad implications for both the cycling of iron and the biogeochemical transformation of redox-sensitive elements and contaminants in the benzene-containing subsurface environments.

The differentiation of iron-reducing bacterial community and iron-reduction activity between riverine and marine sediments in the Yellow River estuary

Rivers are the primary contributors of iron and other elements to the global oceans. Iron-reducing bacteria play an important biogeochemical role in coupling the iron and carbon redox cycles. However, the extent of changes in community structures and iron-reduction activities of iron-reducing bacteria in riverine and coastal marine sediments remains unclear. This study presents information on the spatial patterns and relative abundance of iron-reducing bacteria in sediments of the Yellow River estuary and the adjacent Bohai Sea. High-throughput sequencing of bacterial 16S rRNA found that the highest relative abundances and diversities were from the estuary (Yellow River–Bohai Sea mixing zone). Pseudomonas, Thiobacillus, Geobacter, Rhodoferax, and Clostridium were the most abundant putative iron-reducing bacteria genera in the sediments of the Yellow River. Vibrio, Shewanella, and Thiobacillus were the most abundant in the sediments of the Bohai Sea. The putative iron-reducing bacterial community was positively correlated with the concentrations of total nitrogen and ammonium in coastal marine sediments, and was significantly correlated with the concentration of nitrate in river sediments. The riverine sediments, with a more diverse iron-reducing bacterial community, exhibited increased activity of Fe(III) reduction in enrichment cultures. The estuary-wide high abundance of putative iron-reducing bacteria suggests that the effect of river–sea interaction on bacterial distribution patterns is high. The results of this study will help the understanding of the biogeochemical cycling of iron in riverine and coastal marine environments.

Single-cell determination of iron content in magnetotactic bacteria: implications for the iron biogeochemical cycle.

Magnetotactic bacteria (MTB) are ubiquitous aquatic microorganisms that mineralize dissolved iron into intracellular magnetic crystals. After cell death, these crystals are trapped into sediments that remove iron from the soluble pool. MTB may significantly impact the iron biogeochemical cycle, especially in the ocean where dissolved iron limits nitrogen fixation and primary productivity. A thorough assessment of their impact has been hampered by a lack of methodology to measure the amount of, and variability in, their intracellular iron content. We quantified the iron mass contained in single MTB cells of Magnetospirillum magneticum strain AMB-1 using a time-resolved inductively coupled plasma-mass spectrometry methodology. Bacterial iron content depends on the external iron concentration, and reaches a maximum value of ~10-6 ng of iron per cell. From these results, we calculated the flux of dissolved iron incorporation into environmental MTB populations and conclude that MTB may mineralize a significant fraction of dissolved iron into crystals.

Electron shuttles enhance anaerobic oxidation of methane coupled to iron(III) reduction

Anaerobic oxidation of methane (AOM) has recently been coupled with the reduction of insoluble electron acceptors such as iron minerals. However, effects of electron shuttles (ESs) on this process and the underlying coupling mechanisms remain not well understood. Here, we evaluated AOM-coupled ferrihydrite reduction by a mixed culture in the absence and presence of ESs. The results showed that ESs (AQS, flavin, HA and AQDS) significantly enhanced the rate (up to 7.4 times) of AOM-dependent ferrihydrite reduction compared with the control. The enhancements were linearly related with the electron transfer capacity of ESs. Illumina high-throughput sequencing and DNA-based stable isotope probing revealed that the AOM-coupled iron reduction depended on the syntrophic interaction of Methanobacterium and the partner bacteria. Methanobacterium as the dominant microorganism, did not assimilate methane into its biomasses. However, it played a crucial role in the partial oxidation of methane into an intermediate (i.e. propionate), which was then assimilated by the partner bacteria (e.g. Cellulomonas, Desulfovibrio, Actinotalea, etc.) for ferrihydrite reduction. This work suggests that ESs in natural environments can mitigate the methane emissions by facilitating the AOM process and biogeochemical cycles of iron.

Contribution of Microaerophilic Iron(II)-Oxidizers to Iron(III) Mineral Formation.

Neutrophilic microbial aerobic oxidation of ferrous iron (Fe(II)) is restricted to pH-circumneutral environments characterized by low oxygen where microaerophilic Fe(II)-oxidizing microorganisms successfully compete with abiotic Fe(II) oxidation. However, accumulation of ferric (bio)minerals increases competition by stimulating abiotic surface-catalyzed heterogeneous Fe(II) oxidation. Here, we present an experimental approach that allows quantification of microbial and abiotic contribution to Fe(II) oxidation in the presence or initial absence of ferric (bio)minerals. We found that at 20 μM O2 and the initial absence of Fe(III) minerals, an iron(II)-oxidizing enrichment culture (99.6% similarity to Sideroxydans spp.) contributed 40% to the overall Fe(II) oxidation within approximately 26 h and oxidized up to 3.6 × 10-15 mol Fe(II) cell-1 h-1. Optimum O2 concentrations at which enzymatic Fe(II) oxidation can compete with abiotic Fe(II) oxidation ranged from 5 to 20 μM. Lower O2 levels limited biotic Fe(II) oxidation, while at higher O2 levels abiotic Fe(II) oxidation dominated. The presence of ferric (bio)minerals induced surface-catalytic heterogeneous abiotic Fe(II) oxidation and reduced the microbial contribution to Fe(II) oxidation from 40% to 10% at 10 μM O2. The obtained results will help to better assess the impact of microaerophilic Fe(II) oxidation on the biogeochemical iron cycle in a variety of environmental natural and anthropogenic settings.

Isolation and characterization of a thermophilic sulfur- and iron-reducing thaumarchaeote from a terrestrial acidic hot spring

A deep-branching clade of Thaumarchaeota, conventionally called Terrestrial hot spring creanarchaeotic group (THSCG), is a missing link between thaumarchaeotic ammonia oxidizers and the deeper-branching non-ammonia oxidizers, such as Crenarchaeota and Candidatus Korarchaeota. Here, we report isolation of the first cultivated representative from the THSCG, named as NAS-02. Physiological characterization demonstrated that the isolate was a thermoacidophilic, sulfur- and iron-reducing organoheterotroph, which was supported by gene contents encoded in its complete genome. There was no evidence for ammonia oxidation by the isolate. Members in THSCG are likely thermophiles, and may play roles in degrading cell debris as a scavenger and in biogeochemical cycling of sulfur and iron in the hot environments, as suggested by the physiological characteristics of the isolate and the geographical distribution of the 16S rRNA gene sequences of THSCG in terrestrial hot springs and marine hydrothermal fields. Phylogenetic analysis suggests that the THSCG lineage represented by NAS-02 has gained the ability of sulfur reduction via horizontal gene transfer. Based on the phylogeny and physiology, we propose the name Conexivisphaera calidus gen. nov., sp. nov. to accommodate the isolate.

Preferential molecular fractionation of dissolved organic matter by iron minerals with different oxidation states

The biogeochemical cycles of iron and dissolved organic matter (DOM) are strongly associated, especially at different oxidation sates. However, the role of iron minerals with different oxidation states in DOM preservation has not been clearly characterized at the molecular level. In this study, we employed electrospray ionization coupled with Fourier-transform ion cyclotron resonance mass spectrometry (ESI-FT-ICR-MS) to investigate the molecular fractionation of DOM by adsorption onto four iron minerals with different oxidation states (ferrihydrite, hematite (α-Fe2O3), iron sulfide (FeS), and pyrite (FeS2)). The sorption capacity (normalized to per gram adsorbent) with respect to DOM was in the order of ferrihydrite > iron sulfide > hematite > pyrite. After normalization by specific surface area, the sorption capacity of ferrihydrite plummeted to the second place and was much lower than that of iron sulfide. The sorptive fractionation of DOM was significant for ferrihydrite and iron sulfide but was not obvious for pyrite. Compounds high in unsaturation and nominal oxidation state of carbon or rich in oxygen were preferentially bound to ferrihydrite and iron sulfide, leaving compounds low in unsaturation or poor in oxygenated groups in solution. For DOM containing abundant oxygen functional groups, hematite may result in stronger sorptive fractionation than iron sulfide. These findings provided new insights into the stability of aromatic and aliphatic carbon with various oxygenated groups coupled to redox iron transformation in soils.

Fast Photomineralization of Dissolved Organic Matter in Acid Mine Drainage Impacted Waters.

Acid mine drainage (AMD) formed from pyrite (iron disulfide) weathering contributes to ecosystem degradation in impacted waters. Solar irradiation has been shown to be an important factor in the biogeochemical cycling of iron in AMD-impacted waters, but its impact on dissolved organic matter (DOM) is unknown. With a typical AMD-impacted water (pH 2.7-3) collected from the Perry State Forest watershed in Ohio, we observed highly efficient (>80%) photochemical mineralization of DOM within hours in a solar simulator resembling twice summer sunlight at 40°N. We confirmed that the mineralization was initially induced by •OH formed from FeOH2+ photodissociation and was inhibited 2-fold by dissolved oxygen removal, suggesting the importance of both the photochemical reaction and oxygen involvement. Size exclusion chromatography and Fourier transform ion cyclotron resonance mass spectrometry elucidated that any remaining organic matter was comprised of smaller and highly aliphatic compounds. The quantitative and qualitative changes in DOM are likely to constitute an important component in regional carbon cycling and nutrient release and to influence downstream aquatic ecosystems in AMD-affected watersheds.

Cryptic Cycling of Complexes Containing Fe(III) and Organic Matter by Phototrophic Fe(II)-Oxidizing Bacteria.

Fe-organic matter (Fe-OM) complexes are abundant in the environment and, due to their mobility, reactivity, and bioavailability, play a significant role in the biogeochemical Fe cycle. In photic zones of aquatic environments, Fe-OM complexes can potentially be reduced and oxidized, and thus cycled, by light-dependent processes, including abiotic photoreduction of Fe(III)-OM complexes and microbial oxidation of Fe(II)-OM complexes, by anoxygenic phototrophic bacteria. This could lead to a cryptic iron cycle in which continuous oxidation and rereduction of Fe could result in a low and steady-state Fe(II) concentration despite rapid Fe turnover. However, the coupling of these processes has never been demonstrated experimentally. In this study, we grew a model anoxygenic phototrophic Fe(II) oxidizer, Rhodobacter ferrooxidans SW2, with either citrate, Fe(II)-citrate, or Fe(III)-citrate. We found that strain SW2 was capable of reoxidizing Fe(II)-citrate produced by photochemical reduction of Fe(III)-citrate, which kept the dissolved Fe(II)-citrate concentration at low (<10 μM) and stable concentrations, with a concomitant increase in cell numbers. Cell suspension incubations with strain SW2 showed that it can also oxidize Fe(II)-EDTA, Fe(II)-humic acid, and Fe(II)-fulvic acid complexes. This work demonstrates the potential for active cryptic Fe cycling in the photic zone of anoxic aquatic environments, despite low measurable Fe(II) concentrations which are controlled by the rate of microbial Fe(II) oxidation and the identity of the Fe-OM complexes.IMPORTANCE Iron cycling, including reduction of Fe(III) and oxidation of Fe(II), involves the formation, transformation, and dissolution of minerals and dissolved iron-organic matter compounds. It has been shown previously that Fe can be cycled so rapidly that no measurable changes in Fe(II) and Fe(III) concentrations occur, leading to a so-called cryptic cycle. Cryptic Fe cycles have been shown to be driven either abiotically by a combination of photochemical reduction of Fe(III)-OM complexes and reoxidation of Fe(II) by O2, or microbially by a combination of Fe(III)-reducing and Fe(II)-oxidizing bacteria. Our study demonstrates a new type of light-driven cryptic Fe cycle that is relevant for the photic zone of aquatic habitats involving abiotic photochemical reduction of Fe(III)-OM complexes and microbial phototrophic Fe(II) oxidation. This new type of cryptic Fe cycle has important implications for biogeochemical cycling of iron, carbon, nutrients, and heavy metals and can also influence the composition and activity of microbial communities.

Temporal changes in magnetic signal of burnt soils – A compelling three years pilot study

Wildfires strongly affect soils, including iron biogeochemical cycling and carbon storage. Thus, it is important to reveal the dynamics of iron oxide synthesis and transformations during and after a wildfire. This study investigates the temporal stability of strongly magnetic minerals appearing after a wildfire. Following a designed experimental fire, samples from vegetation ash and mineral soil were taken immediately after and at progressively longer time intervals. The magnetic susceptibility monitoring of samples during three years period demonstrates an initial increase in magnetic signal of ash-rich material taken immediately after the fire followed by a gradual decrease over time. The behavior of samples collected later after the fire showed only a moderate decrease. It is suggested that the magnetic susceptibility rise during the laboratory storage could be due to the increased availability of nutrients and microbial activity immediately after the fire and related intense redox reactions involving iron oxide particles. The decreasing trend in magnetic susceptibility is ascribed to the oxidation of ultrafine magnetite particles with time. All mineral soil samples from the deeper level showed an initial susceptibility increase, assigned to a similar process. Magnetic susceptibility monitoring was also carried out on samples gathered shortly after natural wildfires. The soil samples affected by strong and long wildfires show a decrease in magnetic susceptibility with time. This effect is more pronounced in the surface black-colored ash layer. The ash material from a site disturbed by a strong short wildfire demonstrates behavior similar to the ash from the experimental fire. It is supposed that the temporal evolution of magnetic susceptibility of the mineral alteration products of wildfires is influenced by the grain size of the produced iron oxide particles, their redox reactions mediated by the heterotrophic bacteria, as well as the amplifying role of pyrogenic carbon for intensification of the redox reactions.

Molecular underpinnings for microbial extracellular electron transfer during biogeochemical cycling of earth elements.

Microbial extracellular electron transfer (EET) is electron exchanges between the quinol/quinone pools in microbial cytoplasmic membrane and extracellular substrates. Microorganisms with EET capabilities are widespread in Earth hydrosphere, such as sediments of rivers, lakes and oceans, where they play crucial roles in biogeochemical cycling of key elements, including carbon, nitrogen, sulfur, iron and manganese. Over the past 12 years, significant progress has been made in mechanistic understanding of microbial EET at the molecular level. In this review, we focus on the molecular mechanisms underlying the microbial ability for extracellular redox transformation of iron, direct interspecies electron transfer as well as long distance electron transfer mediated by the cable bacteria in the hydrosphere.