Evidence for Microbial Fe(III) Reduction in Anoxic, Mining-Impacted Lake Sediments (Lake Coeur d'Alene, Idaho)

Mining-impacted sediments of Lake Coeur d'Alene, Idaho, contain more than 10% metals on a dry weight basis, approximately 80% of which is iron. Since iron (hydr)oxides adsorb toxic, ore-associated elements, such as arsenic, iron (hydr)oxide reduction may in part control the mobility and bioavailability of these elements. Geochemical and microbiological data were collected to examine the ecological role of dissimilatory Fe(III)-reducing bacteria in this habitat. The concentration of mild-acid-extractable Fe(II) increased with sediment depth up to 50 g kg⁻¹, suggesting that iron reduction has occurred recently. The maximum concentrations of dissolved Fe(II) in interstitial water (41 mg liter⁻¹) occurred 10 to 15 cm beneath the sediment-water interface, suggesting that sulfidogenesis may not be the predominant terminal electron-accepting process in this environment and that dissolved Fe(II) arises from biological reductive dissolution of iron (hydr)oxides. The concentration of sedimentary magnetite (Fe₃O₄), a common product of bacterial Fe(III) hydroxide reduction, was as much as 15.5 g kg⁻¹. Most-probable-number enrichment cultures revealed that the mean density of Fe(III)-reducing bacteria was 8.3 × 10⁵ cells g (dry weight) of sediment⁻¹. Two new strains of dissimilatory Fe(III)-reducing bacteria were isolated from surface sediments. Collectively, the results of this study support the hypothesis that dissimilatory reduction of iron has been and continues to be an important biogeochemical process in the environment examined.     

Ferribacterium limneticum, gen. nov., sp. nov., an Fe(III)-reducing microorganism isolated from mining-impacted freshwater lake sediments

A dissimilatory Fe(III)-reducing bacterium was isolated from mining-impacted lake sediments and designated strain CdA-1. The strain was isolated from a 4-month enrichment culture with acetate and Fe(III)-oxyhydroxide. Strain CdA-1 is a motile, obligately anaerobic rod, capable of coupling the oxidation of acetate and other organic acids to the reduction of ferric iron. Fe(III) reduction was not observed using methanol, ethanol, isopropanol, propionate, succinate, fumarate, H₂, citrate, glucose, or phenol as potential electron donors. With acetate as an electron donor, strain CdA-1 also grew by reducing nitrate or fumarate. Growth was not observed with acetate as electron donor and O₂, sulfoxyanions, nitrite, trimethylamine N-oxide, Mn(IV), As(V), or Se(VI) as potential terminal electron acceptors. Comparative 16 S rRNA gene sequence analyses show strain CdA-1 to be most closely related (93.6% sequence similarity) to Rhodocyclus tenuis. However, R. tenuis did not grow heterotrophically by Fe(III) reduction, nor did strain CdA-1 grow photrophically. We propose that strain CdA-1 represents a new genus and species, Ferribacterium limneticum. Strain CdA-1 represents the first dissimilatory Fe(III) reducer in the β subclass of Proteobacteria, as well as the first Fe(III) reducer isolated from mine wastes. Received: 14 July 1998 / Accepted: 14 December 1998     

Biological control of hog waste odor through stimulated microbial Fe(III) reduction

Odor control and disposal of swine waste have inhibited expansion of swine production facilities throughout the United States. Swine waste odor is associated primarily with high concentrations of volatile fatty acids (VFAs). Here, we demonstrate that stimulated Fe(III) reduction in hog manure can rapidly remove the malodorous compounds and enhance methane production by 200%. As part of these studies, we enumerated the indigenous Fe(III)-reducing population in swine waste and identified members of the family Geobacteraceae as the dominant species. These organisms were present at concentrations as high as 2 x 10(5) cells g(-1). Several pure cultures of Fe(III) reducers, including Geobacter metallireducens, Geobacter humireducens, Geobacter sulfurreducens, Geobacter grbiciae, Geothrix fermentans, and Geovibrio ferrireducens, readily degraded some or all of the malodorous VFAs found in swine manure. In contrast, Shewanella algae did not degrade any of these compounds. We isolated an Fe(III) reducer, Geobacter strain NU, from materials collected from primary swine waste lagoons. This organism degraded all of the malodorous VFAs tested and readily grew in swine waste amended with Fe(III). When raw waste amended with Fe(III) was inoculated with strain NU, the VFA content rapidly decreased, corresponding with an almost complete removal of the odor. In contrast, the raw waste without Fe(III) or strain NU showed a marked increase in VFA content and a rapid pH drop. This study showed that Fe(III) supplementation combined with appropriate bioaugmentation provides a simple, cost-effective approach to deodorize and treat swine waste, removing a significant impediment to the expansion of pork production facilities.     

Abundance, distribution, and activity of Fe(II)-oxidizing and Fe(III)-reducing microorganisms in hypersaline sediments of Lake Kasin, southern Russia

The extreme osmotic conditions prevailing in hypersaline environments result in decreasing metabolic diversity with increasing salinity. Various microbial metabolisms have been shown to occur even at high salinity, including photosynthesis as well as sulfate and nitrate reduction. However, information about anaerobic microbial iron metabolism in hypersaline environments is scarce. We studied the phylogenetic diversity, distribution, and metabolic activity of iron(II)-oxidizing and iron(III)-reducing Bacteria and Archaea in pH-neutral, iron-rich salt lake sediments (Lake Kasin, southern Russia; salinity, 348.6 g liter(-1)) using a combination of culture-dependent and -independent techniques. 16S rRNA gene clone libraries for Bacteria and Archaea revealed a microbial community composition typical for hypersaline sediments. Most-probable-number counts confirmed the presence of 4.26 × 10(2) to 8.32 × 10(3) iron(II)-oxidizing Bacteria and 4.16 × 10(2) to 2.13 × 10(3) iron(III)-reducing microorganisms per gram dry sediment. Microbial iron(III) reduction was detected in the presence of 5 M NaCl, extending the natural habitat boundaries for this important microbial process. Quantitative real-time PCR showed that 16S rRNA gene copy numbers of total Bacteria, total Archaea, and species dominating the iron(III)-reducing enrichment cultures (relatives of Halobaculum gomorrense, Desulfosporosinus lacus, and members of the Bacilli) were highest in an iron oxide-rich sediment layer. Combined with the presented geochemical and mineralogical data, our findings suggest the presence of an active microbial iron cycle at salt concentrations close to the solubility limit of NaCl.     

微生物厌氧氨氧化耦合铁还原的研究进展及展望

厌氧氨氧化耦合铁还原[ammonium oxidation coupled to Fe(Ⅲ) reduction, Feammox]作为一种连接氮循环和铁循环之间的氮代谢途径,在自然界中氨氮转化过程中起到了重要作用。系统研究Feammox驱动的氮铁的生物地球化学耦合过程及其受控因素,有助于深入理解地球元素循环的微生物机制,也有助于揭示Feammox在缺氧地质历史时期对古海洋氮库演变和含铁矿物形成过程中的作用。本文从Feammox发展历史、相关微生物、影响因素和潜在地质意义等方面综述了Feammox的研究过程和研究内容,并对Feammox的未来研究方向提出展望。     

Differences in Fe(III) reduction in the hyperthermophilic archaeon, Pyrobaculum islandicum, versus mesophilic Fe(III)-reducing bacteria

The discovery that all hyperthermophiles that have been evaluated have the capacity to reduce Fe(III) has raised the question of whether mechanisms for dissimilatory Fe(III) reduction have been conserved throughout microbial evolution. Many studies have suggested that c-type cytochromes are integral components in electron transport to Fe(III) in mesophilic dissimilatory Fe(III)-reducing microorganisms. However, Pyrobaculum islandicum, the hyperthermophile in which Fe(III) reduction has been most intensively studied, did not contain c-type cytochromes. NADPH was a better electron donor for the Fe(III) reductase activity in P. islandicum than NADH. This is the opposite of what has been observed with mesophiles. Thus, if previous models for dissimilatory Fe(III) reduction by mesophilic bacteria are correct, then it is unlikely that a single strategy for electron transport to Fe(III) is present in all dissimilatory Fe(III)-reducing microorganisms.     

Mechanisms for accessing insoluble Fe(III) oxide during dissimilatory Fe(III) reduction by Geothrix fermentans

Mechanisms for Fe(III) oxide reduction were investigated in Geothrix fermentans, a dissimilatory Fe(III)-reducing microorganism found within the Fe(III) reduction zone of subsurface environments. Culture filtrates of G. fermentans stimulated the reduction of poorly crystalline Fe(III) oxide by washed cell suspensions, suggesting that G. fermentans released one or more extracellular compounds that promoted Fe(III) oxide reduction. In order to determine if G. fermentans released electron-shuttling compounds, poorly crystalline Fe(III) oxide was incorporated into microporous alginate beads, which prevented contact between G. fermentans and the Fe(III) oxide. G. fermentans reduced the Fe(III) within the beads, suggesting that one of the compounds that G. fermentans releases is an electron-shuttling compound that can transfer electrons from the cell to Fe(III) oxide that is not in contact with the organism. Analysis of culture filtrates by thin-layer chromatography suggested that the electron shuttle has characteristics similar to those of a water-soluble quinone. Analysis of filtrates by ion chromatography demonstrated that there was as much as 250 microM dissolved Fe(III) in cultures of G. fermentans growing with Fe(III) oxide as the electron acceptor, suggesting that G. fermentans released one or more compounds capable of chelating and solubilizing Fe(III). Solubilizing Fe(III) is another strategy for alleviating the need for contact between cells and Fe(III) oxide for Fe(III) reduction. This is the first demonstration of a microorganism that, in defined medium without added electron shuttles or chelators, can reduce Fe(III) derived from Fe(III) oxide without directly contacting the Fe(III) oxide. These results are in marked contrast to those with Geobacter metallireducens, which does not produce electron shuttles or Fe(III) chelators. These results demonstrate that phylogenetically distinct Fe(III)-reducing microorganisms may use significantly different strategies for Fe(III) reduction. Thus, it is important to know which Fe(III)-reducing microorganisms predominate in a given environment in order to understand the mechanisms for Fe(III) reduction in the environment of interest.     

Microbial reduction of Fe(III) in acidic sediments: isolation of Acidiphilium cryptum JF-5 capable of coupling the reduction of Fe(III) to the oxidation of glucose.

To evaluate the microbial populations involved in the reduction of Fe(III) in an acidic, iron-rich sediment, the anaerobic flow of supplemental carbon and reductant was evaluated in sediment microcosms at the in situ temperature of 12 degrees C. Supplemental glucose and cellobiose stimulated the formation of Fe(II); 42 and 21% of the reducing equivalents that were theoretically obtained from glucose and cellobiose, respectively, were recovered in Fe(II). Likewise, supplemental H(2) was consumed by acidic sediments and yielded additional amounts of Fe(II) in a ratio of approximately 1:2. In contrast, supplemental lactate did not stimulate the formation of Fe(II). Supplemental acetate was not consumed and inhibited the formation of Fe(II). Most-probable-number estimates demonstrated that glucose-utilizing acidophilic Fe(III)-reducing bacteria approximated to 1% of the total direct counts of 4', 6-diamidino-2-phenylindole-stained bacteria. From the highest growth-positive dilution of the most-probable-number series at pH 2. 3 supplemented with glucose, an isolate, JF-5, that could dissimilate Fe(III) was obtained. JF-5 was an acidophilic, gram-negative, facultative anaerobe that completely oxidized the following substrates via the dissimilation of Fe(III): glucose, fructose, xylose, ethanol, glycerol, malate, glutamate, fumarate, citrate, succinate, and H(2). Growth and the reduction of Fe(III) did not occur in the presence of acetate. Cells of JF-5 grown under Fe(III)-reducing conditions formed blebs, i.e., protrusions that were still in contact with the cytoplasmic membrane. Analysis of the 16S rRNA gene sequence of JF-5 demonstrated that it was closely related to an Australian isolate of Acidiphilium cryptum (99.6% sequence similarity), an organism not previously shown to couple the complete oxidation of sugars to the reduction of Fe(III). These collective results indicate that the in situ reduction of Fe(III) in acidic sediments can be mediated by heterotrophic Acidiphilium species that are capable of coupling the reduction of Fe(III) to the complete oxidation of a large variety of substrates including glucose and H(2).     

The potential significance of microbial Fe(III) reduction during deposition of Precambrian banded iron formations

During deposition of late Archean–early Palaeoproterozoic Precambrian banded iron formations (BIFs) the downward flux of ferric hydroxide (Fe(OH)₃) and phytoplankton biomass should have facilitated microbial Fe(III) reduction. However, quantifying the significance of such a metabolic pathway in the Precambrian is extremely difficult, considering the post‐depositional alteration of the rocks and the lack of ideal modern analogues. Consequently, we have very few constraints on the Fe cycle at that time, namely (i) the concentration of dissolved Fe(II) in the ocean waters; (ii) by what mechanisms Fe(II) was oxidized (chemical, photochemical or biological, the latter using either O₂ or light); (iii) where the ferric hydroxide was precipitated (over the shelf vs. open ocean); (iv) the amount of phytoplankton biomass, which relates to the nutrient status of the surface waters; (v) the relative importance of Fe(III) reduction vs. the other types of metabolic pathways utilized by sea floor microbial communities; and (vi) the proportion of primary vs. diagenetic Fe(II) in BIF. Furthermore, although estimates can be made regarding the quantity of reducing equivalents necessary to account for the diagenetic Fe(II) component in Fe‐rich BIF layers, those same estimates do not offer any insights into the magnitude of Fe(III) actually generated within the water column, and hence, the efficiency of Fe and C recycling prior to burial. Accordingly, in this study, we have attempted to model the ancient Fe cycle, based simply on conservative experimental rates of photosynthetic Fe(II) oxidation in the euphotic zone. We estimate here that under ideal growth conditions, as much as 70% of the biologically formed Fe(III) could have been recycled back into the water column via fermentation and organic carbon oxidation coupled to microbial Fe(III) reduction. By comparing the potential amount of biomass generated phototrophically with the reducing equivalents required for Fe(III) reduction and magnetite formation, we also hypothesize that another anaerobic metabolic pathway might have been utilized in the surface sediment to oxidize the fermentation by‐products. Based on the premise that the deep ocean waters were anoxic, this role could have been fulfilled by methanogens, and maybe even methanotrophs that employed Fe(III) reduction.     

Dissimilatory Fe(III) and Mn(IV) reduction.

The oxidation of organic matter coupled to the reduction of Fe(III) or Mn(IV) is one of the most important biogeochemical reactions in aquatic sediments, soils, and groundwater. This process, which may have been the first globally significant mechanism for the oxidation of organic matter to carbon dioxide, plays an important role in the oxidation of natural and contaminant organic compounds in a variety of environments and contributes to other phenomena of widespread significance such as the release of metals and nutrients into water supplies, the magnetization of sediments, and the corrosion of metal. Until recently, much of the Fe(III) and Mn(IV) reduction in sedimentary environments was considered to be the result of nonenzymatic processes. However, microorganisms which can effectively couple the oxidation of organic compounds to the reduction of Fe(III) or Mn(IV) have recently been discovered. With Fe(III) or Mn(IV) as the sole electron acceptor, these organisms can completely oxidize fatty acids, hydrogen, or a variety of monoaromatic compounds. This metabolism provides energy to support growth. Sugars and amino acids can be completely oxidized by the cooperative activity of fermentative microorganisms and hydrogen- and fatty-acid-oxidizing Fe(III) and Mn(IV) reducers. This provides a microbial mechanism for the oxidation of the complex assemblage of sedimentary organic matter in Fe(III)- or Mn(IV)-reducing environments. The available evidence indicates that this enzymatic reduction of Fe(III) or Mn(IV) accounts for most of the oxidation of organic matter coupled to reduction of Fe(III) and Mn(IV) in sedimentary environments. Little is known about the diversity and ecology of the microorganisms responsible for Fe(III) and Mn(IV) reduction, and only preliminary studies have been conducted on the physiology and biochemistry of this process.     

Fixation of phosphorus in lake sediments using iron(III)chloride: experiences,expectations

After a reduction of the external phosphorus loading to a lake, an internal loading from the sediments may delay the improvement of the water quality. The accepted method to combat internal loading is careful dredging of the upper sediment layers (Cooke et al., 1986), but this method is costly and time consuming. Addition of phosphorus binding agents to the sediments might offer an alternative. In the Netherlands the use of aluminum compounds, the most common phosphorus binding agent, for water quality improvement purposes is not favoured. Therefore a sediment treatment with a solution of iron(III)chloride was tested. Iron was chosen because it is considered to be a natural binder of phosphate. 100 g m^–2 of Fe³⁺ were added to the sediments of the shallow (1.75 m average depth) and eutrophic Lake Groot Vogelenzang (The Netherlands) in October and November 1989. The iron(III)chloride solution was diluted 100 times with lake water and mixed with the surface sediments with a water jet.Following the addition the concentrations of total phosphorus (Fig. 1), chlorophyll-a and suspended solids decreased. This improvement of the water quality lasted for three months. After this time the total phosphorus concentration increased again, but remained at a lower level than in spring and summer of 1989. The phosphorus release rate from the sediments as measured from intact sediment cores decreased from 4 to 1.2 mg P m^–2 d^–1 (n = 5), and the bioavailability of the sediment phosphorus, as measured with bioassays, decreased from 34 to 23% (n = 5) shortly after the treatment. One year after the treatment the release rate was increased to 3 mg P m^–2 d^–1 (n = 5). Before treatment, the lake was thought to have a residence time of over one year. However, the chloride added to the lake disappeared according to a dilution rate of 0.03 d^–1 or a retention time of about 35 days. A high external loading due to rapid flushing with phosphorus-rich water from surrounding lakes possibly prevented a more durable improvement in water quality. Another possibility is that the iron addition has lost its phosphate binding capacity due to reduction or binding with other anions like carbonate or sulphide. Therefore the suitability of the method to reduce internal loading and especially the long term availability of added iron to bind phosphorus needs additional proof.The treatment of the 18 ha area of Lake Groot Vogelenzang took three weeks. The operational costs were about US$ 125000. This is fast and cheap compared to dredging. Application of the technique is limited to those cases where the sediments are not polluted with micro-pollutants and the water depth need not be increased.     

Chemical and biological mobilization of Fe(III) in marsh sediments

Iron reduction in marine sulfitic environments may occur via a mechanism involving direct bacterial reduction with the use of hydrogen as an electron donor, direct bacterial reduction involving carbon turnover, or by indirect reduction where sulfide acts to reduce iron. In the presented experiments, the relative importance of direct and indirect mechanisms of iron reduction, and the contribution of these two mechanisms to overall carbon turnover has been evaluated in two marsh environments. Sediments collected from two Northeastern US salt marshes each having different Fe (III) histories were incubated with the addition of reactive iron (as amorphous oxyhydroxide). These sediments were either incubated alone or in conjunction with sodium molybdate. Production of both inorganic and organic pore water constituents and a calculation of net carbon production were used as measures to compare the relative importance of direct bacterial reduction and indirect bacterial reduction. Results indicate that in the environments tested, the majority of the reduced iron found results from indirect reduction mediated by hydrogen sulfide, a result of dissolution and precipitation phenomena, or is a result of direct bacterial reduction using hydrogen as an electron donor. Direct iron reduction plays a minor role in carbon turnover in these environments.     

Observations on microbial activity in a seasonally anoxic,nutrient-enriched maritime Antarctic lake

Summary Amos Lake is a shallow, permanently cold, maritime Antarctic lake which is almost totally anoxic for much of the 9 month period of winter ice cover each year. The lake catchment is a source of considerable nutrient enrichment for the lake due to the presence, in summer, of large numbers of seals and seabirds. A dense growth of phytoplankton was supported by this enrichment and, in turn, was a source of nutrients for the diverse microflora of the lake. The phytoplankton had a very high assimilation efficiency and an apparent high tolerance of anoxia. Bacterioplankton numbers and activity were closely linked to algal activity and both groups showed a strong seasonality typical of polar systems. Anaerobic bacteria (notably sulphate reducers and methanogens) were not as significant as anticipated, being largely restricted to the deeper lake trough region and possibly subject to substrate limitation later in the anoxic period. The results are discussed in the context of previous findings for other maritime Antarctic lakes.     

Limiting factors for microbial Fe(III) -reduction in a landfill leachate polluted aquifer (Vejen, Denmark)

Abstract Aquifer sediment samples from two locations within the anaerobic leachate plume of a municipal landfill were compared with respect to microbiology (especially Fe(III)-reduction) and geochemistry. The samples close to the landfill were characterized by low contents of Fe(III), whereas samples from the more distant cluster were rich in Fe(III)-oxides. The active microbial population seemed to be less dense in samples more distant from the landfill (measured by ATP and phospholipid fatty acids (PLFA)), but the microbial communities were very similar in the two sample clusters according to the composition of PLFA. Very little, if any, Fe(III)-reduction was observed close to the landfill, but all the more distant samples showed evident microbially mediated Fe(III)-reduction. After amendment with both acetate and Fe(III), all the samples showed a potential for Fe(III)-reduction, and the in situ Fe(III)-reduction seemed to be limited by the lack of Fe(III)-availability. It was suggested, that Fe(III)-reducing populations might be facultative, surviving by use of other electron-acceptors than Fe(III), when Fe(III) is not available for reduction.     

Evidence for the biological turnover of thiols in anoxic marine sediments

The concentrations of methanethiol (MSH) and 3-mercaptopropionate (3-MPA) increased for a period of up to 24 h in fresh slurries of anoxic Biscayne Bay sediments. Other endogenous thiols such as glutathione (GSH) deceased immediately after slurry preparation or were not detectable at all. The maximum concentrations reached by 3-MPA and MSH were sometimes as high as 1 M, but were usually in the 100 to 300 nM range. After the initial increases, the concentrations of these thiols decreased rapidly to nearly constant levels of 20 nM for MSH and < 1nM for 3-MPA. In pre-incubated slurries, which had constant levels of thiols, the addition of microbial inhibitors including tungstate, molybdate, chloroform, and a mixture of chloramphenicol plus tetracycline caused MSH and 3-MPA to accumulate steadily. In the presence of inhibitors, accumulation rates of MSH ranged from 18 to 730 nM · d^-1 and those of 3-MPA ranged from 0 to 185 nM · d^-1. Tungstate and chloroform generally gave the highest accumulation rates, while molybdate gave the lowest, possibly due to its complexation with sulfhydryl compounds. BES (2-Bromoethanesulfonate) was also tested for its effects, but no 3-MPA and only trace amounts (19 nM · d^-1) of MSH accumulated with this treatment. However, additions of BES (10 mM) to sulfidic sediments caused significant (8 M · d^-1) production of 2-mercaptoethanesulfonate (HS-CoM). Formation of HS-CoM was abiotic and was due to sulfide attack on the bromine atom in BES. The accumulations of 3-MPA and MSH in the presence of several different microbial inhibitors, suggests that these thiols may turn over in anoxic sediments. The relatively low concentrations of thiols observed in pore water profiles may be due to continuous microbial removal of these compounds. Much larger amounts of thiols were associated with sediment particles than present in the pore water. Evidence is presented which suggests that bound thiols may be exchangeable with the porewater, and therefore potentially available for microbial consumption.     

Dissimilatory Fe(III) reduction by Clostridium beijerinckii isolated from freshwater sediment using Fe(III) maltol enrichment

A microorganism which reduces Fe(III) during the fermentation of glucose was isolated from freshwater sediment. The Fe(III) was supplied to enrichment cultures as a soluble complex with the bidentate ligand maltol (3-hydroxy-2-methyl-4-pyrone). Advantages that were afforded by the use of Fe(III)(maltol)3 over previously published methods included negation of the requirement for assays of Fe(II) formation. Because Fe(III)(maltol)3 has a characteristic deep red colour, Fe(III) reduction could be quantified spectrophotometrically by monitoring the disappearance of the complex in liquid cultures. Furthermore, Fe(III) reduction on agar plates containing the complex was apparent by zones of decolourisation around the bacterial colonies. 16S rRNA gene sequencing indicated the isolate to be a strain of Clostridium beijerinckii. Growth experiments were performed on the isolate in batch cultures with varying concentrations of Fe(III) citrate and 50 mM glucose. Increasing the level of Fe(III) citrate present was found to alter the fermentation balance, with less acidic products being formed. The presence of Fe(III) led to increases in the growth rate and growth yield, which were both approximately doubled when the supply of the cation reached 25 mM. A NAD(P)H-dependent Fe(III) reductase activity was localised to the bacterial membrane and found not to be sensitive to respiratory inhibitors. Taken together, these data suggest that dissimilatory Fe(III) reduction by the isolate provides a means of utilising the cation as an electron sink, thus facilitating pyridine nucleotide to be recycled during fermentative metabolism.     

Iron and sulfate reduction structure microbial communities in (sub-)Antarctic sediments

Permanently cold marine sediments are heavily influenced by increased input of iron as a result of accelerated glacial melt, weathering, and erosion. The impact of such environmental changes on microbial communities in coastal sediments is poorly understood. We investigated geochemical parameters that shape microbial community compositions in anoxic surface sediments of four geochemically differing sites (Annenkov Trough, Church Trough, Cumberland Bay, Drygalski Trough) around South Georgia, Southern Ocean. Sulfate reduction prevails in Church Trough and iron reduction at the other sites, correlating with differing local microbial communities. Within the order Desulfuromonadales, the family Sva1033, not previously recognized for being capable of dissimilatory iron reduction, was detected at rather high relative abundances (up to 5%) while other members of Desulfuromonadales were less abundant (<0.6%). We propose that Sva1033 is capable of performing dissimilatory iron reduction in sediment incubations based on RNA stable isotope probing. Sulfate reducers, who maintain a high relative abundance of up to 30% of bacterial 16S rRNA genes at the iron reduction sites, were also active during iron reduction in the incubations. Thus, concurrent sulfate reduction is possibly masked by cryptic sulfur cycling, i.e., reoxidation or precipitation of produced sulfide at a small or undetectable pool size. Our results show the importance of iron and sulfate reduction, indicated by ferrous iron and sulfide, as processes that shape microbial communities and provide evidence for one of Sva1033’s metabolic capabilities in permanently cold marine sediments.Subject terms: Microbial ecology, Biogeochemistry     

Self-stacked submersible microbial fuel cell (SSMFC) for improved remote power generation from lake sediments

Electric energy can be harvested from aquatic sediments by utilizing microbial fuel cells (MFCs). A main challenge of this application is the limited voltage output. In this study, an innovative self-stacked submersible MFC (SSMFC) was developed to improve the voltage generation from lake sediments. The SSMFC successfully produced a maximum power density of 294 mW/m(2) and had an open circuit voltage (OCV) of 1.12 V. However, voltage reversal was observed in one cell at high current density. Investigation on the cause for voltage reversal revealed that voltage reversal was occurring only when low external resistance (≤ 400Ω in this study) was applied. In addition, the internal resistance and OCV were the most important parameters for predicting which cell unit had the highest probability to undergo voltage reversal. Use of a capacitor was found to be an effective way to prevent voltage reversal and at the same time store power. These results provide new insight into the development of effective MFC system, capable of extracting energy and promoting bioremediation of organic pollutants from sediments.     

Phylogenetic diversity of archaea and bacteria in the anoxic zone of a meromictic lake (Lake Pavin, France)

The compositions of archaeal and bacterial populations at different depths (60 m [mixolimnion-chemocline interface], 70 m [chemocline-subchemocline interface], 90 m, and 92 m [the water-sediment interface]) in the anoxic zone of the water column in Lake Pavin, a freshwater permanently stratified mountain lake in France, were determined. Phylogenetic trees were constructed from sequences to assess archaeal and bacterial diversity at the four sites.