Humic substances (HS) are redox-active natural organic compounds and serve as electron shuttles between microorganisms and iron(III) minerals. Here we demonstrate that electron shuttling is possible ...only at concentrations of dissolved HS of at least 5−10 mg C/L. Although such concentrations can be found in many rivers, lakes, and even in some aquifers there are also many marine and freshwater systems with DOC <5 mg C/L where consequently electron shuttling is not expected to happen. We found that in the case of HS concentrations which do not limit electron shuttling, Geobacter sulfurreducens transfers electrons to HS at least 27 times faster than to Fe(III)hydroxide. Microbially reduced HS transfer electrons to ferrihydrite at least 7 times faster than cells thereby first demonstrating that microbial mineral reduction via HS significantly accelerates Fe(III) mineral reduction and second that electron transfer from reduced HS to Fe(III) minerals represents the rate-limiting step in microbial Fe(III) mineral reduction via HS. Microbial reduction of HS transfers as many electrons to HS as chemical reduction with H2 indicating that all redox-active functional groups that can be reduced at a redox potential of −418 mV (Eh 0 of H2/H+ redox couple at pH 7) can also be reduced by microorganisms.
Many iron (Fe) redox processes that were previously assumed to be purely abiotic, such as photochemical Fe reactions, are now known to also be microbially mediated. Owing to this overlap, discerning ...whether biotic or abiotic processes control Fe redox chemistry is a major challenge for geomicrobiologists and biogeochemists alike. Therefore, to understand the network of reactions within the biogeochemical Fe cycle, it is necessary to determine which abiotic or microbially mediated reactions are dominant under various environmental conditions. In this Review, we discuss the major microbially mediated and abiotic reactions in the biogeochemical Fe cycle and provide an integrated overview of biotic and chemically mediated redox transformations.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
The exergonic reaction of FeS with H₂S to form FeS₂ (pyrite) and H₂ was postulated to have operated as an early form of energy metabolism on primordial Earth. Since the Archean, sedimentary pyrite ...formation has played a major role in the global iron and sulfur cycles, with direct impact on the redox chemistry of the atmosphere. However, the mechanism of sedimentary pyrite formation is still being debated. We present microbial enrichment cultures which grew with FeS, H₂S, and CO₂ as their sole substrates to produce FeS₂ and CH₄. Cultures grew over periods of 3 to 8 mo to cell densities of up to 2 to 9 × 10⁶ cells per mL−1. Transformation of FeS with H₂S to FeS₂ was followed by 57Fe Mössbauer spectroscopy and showed a clear biological temperature profile with maximum activity at 28 °C and decreasing activities toward 4 °C and 60 °C. CH₄ was formed concomitantly with FeS₂ and exhibited the same temperature dependence. Addition of either penicillin or 2-bromoethanesulfonate inhibited both FeS₂ and CH₄ production, indicating a coupling of overall pyrite formation to methanogenesis. This hypothesiswas supported by a 16S rRNA gene-based phylogenetic analysis, which identified at least one archaeal and five bacterial species. The archaeon was closely related to the hydrogenotrophic methanogen Methanospirillum stamsii, while the bacteria were most closely related to sulfate-reducing Deltaproteobacteria, aswell as uncultured Firmicutes and Actinobacteria. Our results show that pyrite formation can be mediated at ambient temperature through a microbially catalyzed redox process, which may serve as a model for a postulated primordial iron−sulfur world.
Natural organic matter can change As speciation via redox reactions and complexation influencing its mobility and toxicity. Here we show that binary and ternary colloids and dissolved complexes of ...As(V), Fe and organic matter (OM) form at environmentally relevant conditions and analyzed these colloids/complexes using ATR-FTIR- and Mössbauer-spectroscopy. Dissolved Fe−OM complexes and ferrihydrite-OM colloids were formed by reacting OM with ferrihydrite (Fe(OH)3). Mössbauer-spectroscopy showed that 95% of the Fe in the Fe−OM fraction were present as ferrihydrite−OM colloids while the remaining 5% were in the dissolved fraction. In As(V) plus Fe−OM systems (containing both dissolved and colloidal Fe−OM), 3.5−8 μg As(V)/mg OC was bound to the Fe−OM complexes/colloids compared to <0.015 μg As(V)/mg OC in As-OM systems (without Fe). Upon filtration of As−Fe−OM complexes/colloids with a 3 kDa filter, ∼6% As was found in the dissolved fraction and ∼94% As in colloidal Fe−OM. This suggests that As(V) is associated with Fe−OM mainly via ferrihydrite-OM colloids but to a small extent also in dissolved Fe−OM complexes via Fe-bridging. Since As-contaminated soils and aquifers contain Fe(III) minerals and OM, colloids of As with OM-loaded ferrihydrite and complexes of As with dissolved Fe−OM have to be considered when studying As transport.
Microbial reduction of Fe(III) minerals at neutral pH is faced by the problem of electron transfer from the cells to the solid-phase electron acceptor and is thought to require either direct ...cell-mineral contact, the presence of Fe(III)-chelators or the presence of electron shuttles, e.g. dissolved or solid-phase humic substances (HS). In this study we investigated to which extent the ratio of Pahokee Peat Humic Acids (HA) to ferrihydrite in the presence and absence of phosphate influences rates of Fe(III) reduction by Shewanella oneidensis MR-1 and the identity of the minerals formed. We found that phosphate generally decreased reduction rates by sorption to the ferrihydrite and surface site blocking. In the presence of low ferrihydrite concentrations (5mM), the addition of HA helped to overcome this inhibiting effect by functioning as electron shuttle between cells and the ferrihydrite. In contrast, at high ferrihydrite concentrations (30mM), the addition of HA did not lead to an increase but rather to a decrease in reduction rates. Confocal laser scanning microscopy images and ferrihydrite sedimentation behaviour suggest that the extent of ferrihydrite surface coating by HA influences the aggregation of the ferrihydrite particles and thereby their accessibility for Fe(III)-reducing bacteria. We further conclude that in presence of dissolved HA, iron reduction is stimulated through electron shuttling while in the presence of only sorbed HA, no stimulation by electron shuttling takes place. In presence of phosphate the stimulation effect did not occur until a minimum concentration of 10mg/l of dissolved HA was reached followed by increasing Fe(III) reduction rates up to dissolved HA concentrations of approximately 240mg/l above which the electron shuttling effect ceased. Not only Fe(III) reduction rates but also the mineral products changed in the presence of HA. Sequential extraction, XRD and 57Fe-Mössbauer spectroscopy showed that crystallinity and grain size of the magnetite produced by Fe(III) reduction in the presence of HA is lower than the magnetite produced in the absence of HA. In summary, this study shows that both the concentration of HA and Fe(III) minerals strongly influence microbial Fe(III) reduction rates and the mineralogy of the reduction products. Thus, deviations in iron (hydr)oxide reactivity with changes in aggregation state, such as HA induced ferrihydrite aggregation, need to be considered within natural environments.
Nitrous oxide (N2O) contributes 8% to global greenhouse gas emissions. Agricultural sources represent about 60% of anthropogenic N2O emissions. Most agricultural N2O emissions are due to increased ...fertilizer application. A considerable fraction of nitrogen fertilizers are converted to N2O by microbiological processes (that is, nitrification and denitrification). Soil amended with biochar (charcoal created by pyrolysis of biomass) has been demonstrated to increase crop yield, improve soil quality and affect greenhouse gas emissions, for example, reduce N2O emissions. Despite several studies on variations in the general microbial community structure due to soil biochar amendment, hitherto the specific role of the nitrogen cycling microbial community in mitigating soil N2O emissions has not been subject of systematic investigation. We performed a microcosm study with a water-saturated soil amended with different amounts (0%, 2% and 10% (w/w)) of high-temperature biochar. By quantifying the abundance and activity of functional marker genes of microbial nitrogen fixation (nifH), nitrification (amoA) and denitrification (nirK, nirS and nosZ) using quantitative PCR we found that biochar addition enhanced microbial nitrous oxide reduction and increased the abundance of microorganisms capable of N2-fixation. Soil biochar amendment increased the relative gene and transcript copy numbers of the nosZ-encoded bacterial N2O reductase, suggesting a mechanistic link to the observed reduction in N2O emissions. Our findings contribute to a better understanding of the impact of biochar on the nitrogen cycling microbial community and the consequences of soil biochar amendment for microbial nitrogen transformation processes and N2O emissions from soil.
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.
It has been shown that reactive soil minerals, specifically iron(III) (oxyhydr)oxides, can trap organic carbon in soils overlying intact permafrost, and may limit carbon mobilization and degradation ...as it is observed in other environments. However, the use of iron(III)-bearing minerals as terminal electron acceptors in permafrost environments, and thus their stability and capacity to prevent carbon mobilization during permafrost thaw, is poorly understood. We have followed the dynamic interactions between iron and carbon using a space-for-time approach across a thaw gradient in Abisko (Sweden), where wetlands are expanding rapidly due to permafrost thaw. We show through bulk (selective extractions, EXAFS) and nanoscale analysis (correlative SEM and nanoSIMS) that organic carbon is bound to reactive Fe primarily in the transition between organic and mineral horizons in palsa underlain by intact permafrost (41.8 ± 10.8 mg carbon per g soil, 9.9 to 14.8% of total soil organic carbon). During permafrost thaw, water-logging and O
limitation lead to reducing conditions and an increase in abundance of Fe(III)-reducing bacteria which favor mineral dissolution and drive mobilization of both iron and carbon along the thaw gradient. By providing a terminal electron acceptor, this rusty carbon sink is effectively destroyed along the thaw gradient and cannot prevent carbon release with thaw.
The redox state and speciation of the metalloid arsenic (As) determine its environmental fate and toxicity. Knowledge about biogeochemical processes influencing arsenic redox state is therefore ...necessary to understand and predict its environmental behavior. Here we quantified arsenic redox changes by pH-neutral goethite α-FeIIIOOH mineral suspensions amended with Fe(II) using wet-chemical and synchrotron X-ray absorption (XANES) analysis. Goethite itself did not oxidize As(III) and, in contrast to thermodynamic predictions, Fe(II)-goethite systems did not reduce As(V). However, we observed rapid oxidation of As(III) to As(V) in Fe(II)-goethite systems. Mössbauer spectroscopy showed initial formation of 57Fe-goethite after 57Fe(II) addition plus a so far unidentified additional Fe(II) phase. No other Fe(III) phase could be detected by Mössbauer, EXAFS, SEM, XRD, or HR-TEM. This suggests that reactive Fe(III) species form as an intermediate Fe(III) phase upon Fe(II) addition and electron transfer into bulk goethite but before crystallization of the newly formed Fe(III) as goethite. In summary this study indicates that in the simultaneous presence of Fe(III) oxyhydroxides and Fe(II), as commonly observed in environments inhabited by iron-reducing microorganisms, As(III) oxidation can occur. This potentially explains the presence of As(V) in reduced groundwater aquifers.
Slow release of nitrate by charred organic matter used as a soil amendment (i.e. biochar) was recently suggested as potential mechanism of nutrient delivery to plants which may explain some agronomic ...benefits of biochar. So far, isolated soil-aged and composted biochar particles were shown to release considerable amounts of nitrate only in extended (>1 h) extractions ("slow release"). In this study, we quantified nitrate and ammonium release by biochar-amended soil and compost during up to 167 h of repeated extractions in up to six consecutive steps to determine the effect of biochar on the overall mineral nitrogen retention. We used composts produced from mixed manures amended with three contrasting biochars prior to aerobic composting and a loamy soil that was amended with biochar three years prior to analysis and compared both to non-biochar amended controls. Composts were extracted with 2 M KCl at 22°C and 65°C, after sterilization, after treatment with H2O2, after removing biochar particles or without any modification. Soils were extracted with 2 M KCl at 22°C. Ammonium was continuously released during the extractions, independent of biochar amendment and is probably the result of abiotic ammonification. For the pure compost, nitrate extraction was complete after 1 h, while from biochar-amended composts, up to 30% of total nitrate extracted was only released during subsequent extraction steps. The loamy soil released 70% of its total nitrate amount in subsequent extractions, the biochar-amended soil 58%. However, biochar amendment doubled the amount of total extractable nitrate. Thus, biochar nitrate capture can be a relevant contribution to the overall nitrate retention in agroecosystems. Our results also indicate that the total nitrate amount in biochar amended soils and composts may frequently be underestimated. Furthermore, biochars could prevent nitrate loss from agroecosystems and may be developed into slow-release fertilizers to reduce global N fertilizer demands.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
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