Seasonal oscillation of microbial iron and sulfate reduction in saltmarsh sediments (Sapelo Island, GA, USA)

Seasonal variations in anaerobic respiration pathways were investigated at three saltmarsh sites using chemical data, sulfate reduction rate measurements, enumerations of culturable populations of anaerobic iron-reducing bacteria (FeRB), and quantification of in situ 16S rRNA hybridization signals targeted for sulfate-reducing bacteria (SRB). Bacterial sulfate reduction in the sediments followed seasonal changes in temperature and primary production of the saltmarsh, with activity levels lowest in winter and highest in summer. In contrast, a dramatic decrease in the FeRB population size was observed during summer at all sites. The collapse of FeRB populations during summer was ascribed to high rates of sulfide production by SRB, resulting in abiotic reduction of bioavailable Fe(III) (hydr)oxides. To test this hypothesis, sediment slurry incubations at 10, 20 and 30 °C were carried out. Increases in temperature and labile organic carbon availability (acetate or lactate additions) increased rates of sulfate reduction while decreasing the abundance of culturable anaerobic FeRB. These trends were not reversed by the addition of amorphous Fe(III) (hydr)oxides to the slurries. However, when sulfate reduction was inhibited by molybdate, no decline in FeRB growth was observed with increasing temperature. Addition of dissolved sulfide adversely impacted propagation of FeRB whether molybdate was added or not. Both field and laboratory data therefore support a sulfide-mediated limitation of microbial iron respiration by SRB. When total sediment respiration rates reach their highest levels during summer, SRB force a decline in the FeRB populations. As sulfate reduction activity slows down after the summer, the FeRB are able to recover.     

Rates and regulation of microbial iron reduction in sediments of the Baltic-North Sea transition

The rates and pathways of anaerobic carbon mineralization processes were investigated at seven stations, ranging from 10 to 56 m water depth, in the Kattegat and Belt Sea, Denmark. Organic carbon mineralization coupled to microbial Mn and Fe reduction was quantified using anaerobic sediment incubation at two stations that were widely separated geographically within the study area. Fe reduction accounted for 75% of the anaerobic carbon oxidation at the station in the northern Kattegat, which is the highest percentage so far reported from subtidal marine sediment. By contrast, sulfate reduction was the dominant anaerobic respiration pathway (95%) at the station in the Great Belt. Dominance of Fe reduction was related to a relatively high sediment Fe content in combination with active reworking of the sediment by infauna. The relative contribution of Fe reduction to anaerobic carbon oxidation at both stations correlated with the concentration of poorly crystalline Fe(III), confirming that the concentration of poorly crystalline Fe(III) exerts a strong control on rates of Fe reduction in marine sediments. The dependence of microbial Fe reduction on concentrations of poorly crystalline Fe(III) was used to quantify the importance of Fe reduction at sites where anaerobic incubations were not applied. This study showed that Fe reduction is an important process in anaerobic carbon oxidation in a wider area of the seafloor in the northern and eastern Kattegat (contribution 60 – 75%). By contrast, Fe reduction is of little significance (6 – 25%) in the more coarse-grained sediments of the shallower western and southern Kattegat, where a low Fe content was an important limiting factor, and in fine-grained sediments of the Belt Sea (4 – 28%), where seasonal oxygen depletion limits the intensity of bioturbation and thereby the availability of Fe(III). A large fraction of the total deposition of organic matter in the Kattegat and Belt Sea occurs in the northern Kattegat, and we estimate 33% of benthic carbon oxidation in the whole area is conveyed by Fe reduction.     

Ferric iron reduction by Thiobacillus ferrooxidans at extremely low pH-values

When ferrous iron and sulfur were supplied, cells of T. ferrooxidans in a well-aerated medium started growth by oxidizing ferrous iron. After ferrous iron depletion a lagphase followed before sulfur oxidation started. During sulfur oxidation at pH-values below 1.3 (±0,2) the ferrous iron concentration increased again, although the oxygen saturation of the medium amounted to more than 95%. The number of viable cells did not increase. Thus resting cells of T. ferrooxidans, which are oxidizing sulfur to maintain their proton balance, reduce ferric to ferrous iron. The ferrous iron-oxidizing system seemed to be inhibited at pH-values below 1.3. At a pH-value of 1.8 the ferrous iron was reoxidized at once. A scheme for the linkage of iron- and sulfur metabolism is discussed.     

Sulfide-induced dissimilatory nitrate reduction to ammonia in anaerobic freshwater sediments

Abstract: Different reduced sulfur compounds (H₂S, FeS, S₂O₃²⁻) were tested as electron donors for dissimilatory nitrate reduction in nitrate-amended sediment slurries. Only in the free sulfide-enriched slurries was nitrate appreciably reduced to ammonia (     ), with concomitant oxidation of sulfide to S⁰ (     ). The initial concentration of free sulfide appears as a factor determining the type of nitrate reduction. At extremely low concentrations of free S²⁻ (metal sulfides) nitrate was reduced via denitrification whereas at higher S²⁻ concentrations, dissimilatory nitrate reduction to ammonia (DNRA) and incomplete denitrification to gaseous nitrogen oxides took place. Sulfide inhibition of NO- and N₂O- reductases is proposed as being responsible for the driving part of the electron flow from S²⁻ to NH₄⁺.     

Chemistry for an essential biological process: the reduction of ferric iron

In biological systems, the predominant form of iron is the trivalent Fe(III) form, which is potentially not readily bioavailable because of its hydrolysis and polymerization to insoluble forms. It is also the easiest of the two predominant forms of iron to chelate selectively. In a short overview of iron chemistry, we point out some of the pitfalls using standard redox potentials, comment on the interaction of ferric complexes with hydrogen peroxide to give hydroxyl radicals and address the release of iron from ferrisiderophores. In biological systems there are two classes of ferric reductases, the soluble flavin reductases found in prokaryotes, and the membrane-bound cytochrome b-like reductases found in eukaryotes. Finally the role of dissimilatory ferric reduction in microbial respiration and biomineralization is discussed.     

Selenate reduction by a Pseudomonas species: a new mode of anaerobic respiration

The high levels of selenium (selenate, selenite) in agricultural drainage water in the San Joaquin Valley of California, which have led to environmental problems, might be lowered if the selenate/selenite could be reduced to elemental insoluble selenium. Two organisms have been newly isolated which do this in anaerobic coculture. One, a strictly anaerobic, Gram-positive rod, reduces selenite to elemental selenium. The other, a Pseudomonas species, was shown to respire selenate to selenite. Cells grown anaerobically in Minimal Medium on acetate plus selenate oxidized 14C-acetate to 14CO2 with concomitant reduction of selenate to selenite and small amounts of elemental selenium.     

Glyphosate inhibition of ferric reductase activity in iron deficient sunflower roots

Iron (Fe) deficiency is increasingly being observed in cropping systems with frequent glyphosate applications. A likely reason for this is that glyphosate interferes with root uptake of Fe by inhibiting ferric reductase in roots required for Fe acquisition by dicot and nongrass species. This study investigated the role of drift rates of glyphosate (0.32, 0.95 or 1.89 mm glyphosate corresponding to 1, 3 and 6% of the recommended herbicidal dose, respectively) on ferric reductase activity of sunflower (Helianthus annuus) roots grown under Fe deficiency conditions. Application of 1.89 mm glyphosate resulted in almost 50% inhibition of ferric reductase within 6 h and complete inhibition 24 h after the treatment. Even at lower rates of glyphosate (e.g. 0.32 mm and 0.95 mm), ferric reductase was inhibited. Soluble sugar concentration and the NAD(P)H oxidizing capacity of apical roots were not decreased by the glyphosate applications. To our knowledge, this is the first study reporting the effects of glyphosate on ferric reductase activity. The nature of the inhibitory effect of glyphosate on ferric reductase could not be identified. Impaired ferric reductase could be a major reason for the increasingly observed Fe deficiency in cropping systems associated with widespread glyphosate usage.     

Inhibition effect of ferric iron on the kinetics of ferrous iron

Ferric iron acted as a non-competitive inhibitor for the biological oxidation of ferrous iron and decreased the inhibitory effects of high concentrations of ferrous iron as well as the auto-inhibitive effect the bacterial cells. A previously developed kinetic model for this reaction was modified to incorporate the inhibition effects of ferric iron. © Rapid Science Ltd. 1998     

Potential rates and pathways of microbial nitrate reduction in coastal sediments

Nitrate reduction plays a key role in the biogeochemical dynamics and microbial ecology of coastal sediments. Potential rates of nitrate reduction were measured on undisturbed sediment slices from two eutrophic coastal environments using flow-through reactors (FTR). Maximum potential nitrate reduction rates ranged over an order of magnitude, with values of up to 933 nmol cm(-3) h(-1), whereas affinity constants for NO(3) (-) fell mostly between 200 and 600 microM. Homogenized sediment slurries systematically yielded higher rates of nitrate reduction than the FTR experiments. Dentrification was the major nitrate removal pathway in the sediments, although excess ammonium production indicated a contribution of dissimilatory nitrate reduction to ammonium under nitrate-limiting conditions.     

Rates and controls of anaerobic microbial respiration across spatial and temporal gradients in saltmarsh sediments

This study was undertaken to determine the rates and controls ofanaerobic respiration reactions coupled to organic matter mineralization as afunction of space and time along a transect from a bioturbated creekbank to themidmarsh in Georgia saltmarsh sediments. Sulfate reduction rates (SRR) weremeasured at 3 sites during 5 sampling periods throughout the growth season. Thesites differed according to hydrologic regime and the abundance of dominantplants and macrofauna. SRR and pore water / solid phase geochemistry showedevidence of enhanced sediment oxidation at sites exposed to intensebioturbation. Iron(III) reduction rates (FeRR) were directly determined insaltmarsh sediments for the first time, and in agreement with measured SRR,higher rates were observed at the bioturbated, unvegetated creekbank (BUC) andbioturbated, vegetated levee (BVL) sites in comparison to a vegetated mid-marsh(MM) site. An unexpected result was the fact that SRR varied nearly as muchbetween sites (2–3 x) as it did with temperature or season (3–4 x).The BVL site, vegetated by the tall form of Spartinaalterniflora, always exhibited the highest SRR and carbon oxidationrates (> 4000 nmol cm^–3 d^–1) with high activity levels extending deep ( 50 cm)into the sediment, while the MM site, dominated by the short form ofSpartina, always exhibited the lowest SRR which werelocalized to the top 15 cm of sediment. SRR and FeRR at BUC wereintermediate between those measured at the BVL and MM. Acetate was the mostabundant microbial fermentation product (concentrations up to > 1mM) in marsh porewaters, and its distribution reflectedrespirationactivity. Chemical exchange, caused by bioturbation, appeared to be the primarycontrol explaining trends in rates of sulfate and Fe(III) reduction withmacrophytes and carbon source acting as secondary controls.     

Zero valent iron as an electron-donor for methanogenesis and sulfate reduction in anaerobic sludge

Zero valent iron (ZVI) is a reactive media commonly utilized in permeable reactive barriers (PRBs). Sulfate reducing bacteria are being considered for the immobilization of heavy metals in PRBs. The purpose of this study was to evaluate the potential of ZVI as an electron donor for sulfate reduction in natural mixed anaerobic cultures. The ability of methanogens to utilize ZVI as an electron-donor was also explored since these microorganisms often compete with sulfate reducers for common substrates. Four grades of ZVI of different particle sizes (1.120, 0.149, 0.044, and 0.010 mm diameter) were compared as electron donor in batch bioassays inoculated with anaerobic bioreactor sludge. Methanogenesis was evaluated in mineral media lacking sulfate. Sulfate reduction was evaluated in mineral media containing sulfate and the specific methanogenic inhibitor, 2-bromoethane sulfonate. ZVI contributed to significant increases in methane production and sulfate reduction-compared to endogenous substrate controls. The rates of methane formation or sulfate reduction were positively correlated with the surface area of ZVI. The highest rates of 0.310 mmol CH4 formed/mol Fe0.day and 0.804 mmol SO4(2-) reduced/mol Fe0.day were obtained with the finest grade of ZVI (0.01 mm). The results demonstrate that ZVI is readily utilized as a slow-release electron donor for methanogenesis and sulfate reduction in anaerobic sludge; and therefore, has a promising potential in bioremediation applications.     

Inhibition of bacterial perchlorate reduction by zero-valent iron

Perchlorate was reduced by a mixed bacterial culture over a pH range of 7.0–8.9. Similar rates of perchlorate reduction were observed between pH 7.0 and 8.5, whereas significantly slower reduction occurred at pH 8.9. Addition of iron metal, Fe(0), to the mixed bacterial culture resulted in slower rates of perchlorate reduction. Negligible perchlorate reduction was observed under abiotic conditions with Fe(0) alone in a reduced anaerobic medium. The inhibition of perchlorate reduction observed in the presence of Fe(0) is in contrast to previous studies that have shown faster rates of contaminant reduction when bacteria and Fe(0) were combined compared to bacteria alone. The addition of Fe(0) resulted in a rise in pH, as well as precipitation of Fe minerals that appeared to encapsulate the bacterial cells. In experiments where pH was kept constant, the addition of Fe(0) still resulted in slower rates of perchlorate reduction suggesting that encapsulation of bacteria by Fe precipitates contributed to the inhibition of the bacterial activity independent of the effect of pH on bacteria. These results provide the first evidence linking accumulation of iron precipitates at the cell surface to inhibition of environmental contaminant degradation. Fe(0) was not a suitable amendment to stimulate perchlorate-degrading bacteria and the bacterial inhibition caused by precipitation of reduced Fe species may be important in other combined anaerobic bacterial–Fe(0) systems. Furthermore, the inhibition of bacterial activity by iron precipitation may have significant implications for the design of in situ bioremediation technologies for treatment of perchlorate plumes.     

Major substrates for microbial sulfate reduction in the sediments of Ise Bay, Japan

To clarify the anaerobic microbial interactions in the process of carbon mineralization in marine eutrophic environments, the microbial sulfate reduction and methane production rates were examined in coastal marine sediments of Ise Bay, Japan, in autumn 1990. Sulfate reduction rates (51–210 nmol ml⁻¹ day⁻¹ at 24°C) were much higher than the methane production ones (<1.78 nmol ml⁻¹ day⁻¹) in the surface sediments (top 2 cm) at the six stations surveyed (water depth: 10.7–23.3 m). Substrates for sulfate-reducing bacteria (SRB) were estimated after the addition of a specific inhibitor for SRB (20 mmol l⁻¹ molybdate) into the sediment slurry, from the substrate accumulation rates. In the presence of the inhibitor, sulfate reduction was completely stopped and volatile fatty acids (mainly acetate) were accumulated, although hydrogen was not. Methane production occurred markedly accompanied by consumption of the accumulated acetate from the third day after the addition of molybdate. The maximum rate of methane production was 1.2–1.9 μmol ml⁻¹ day⁻¹, which was similar to those in highly polluted freshwater sediments such as the Tama River, Tokyo, Japan. These results show that acetate is a common major substrate for sulfate reduction and methane production, and SRB competitively inhibit potential acetoclastic methanogenesis in coastal sediments. Methanogens may potentially inhabit the sediments at low levels of population density and activity.     

An inorganic geochemical argument for coupled anaerobic oxidation of methane and iron reduction in marine sediments

Here, we present results from sediments collected in the Argentine Basin, a non‐steady state depositional marine system characterized by abundant oxidized iron within methane‐rich layers due to sediment reworking followed by rapid deposition. Our comprehensive inorganic data set shows that iron reduction in these sulfate and sulfide‐depleted sediments is best explained by a microbially mediated process—implicating anaerobic oxidation of methane coupled to iron reduction (Fe‐AOM) as the most likely major mechanism. Although important in many modern marine environments, iron‐driven AOM may not consume similar amounts of methane compared with sulfate‐dependent AOM. Nevertheless, it may have broad impact on the deep biosphere and dominate both iron and methane cycling in sulfate‐lean marine settings. Fe‐AOM might have been particularly relevant in the Archean ocean, >2.5 billion years ago, known for its production and accumulation of iron oxides (in iron formations) in a biosphere likely replete with methane but low in sulfate. Methane at that time was a critical greenhouse gas capable of sustaining a habitable climate under relatively low solar luminosity, and relationships to iron cycling may have impacted if not dominated methane loss from the biosphere.     

A genetically related response to iron deficiency stress in muskmelon

A mutant muskmelon (Cucumis melo L.) with characteristic Fe-deficiency chlorosis symptoms was compared to related cultivars in its ability to obtain Fe via the widely known Fe-stress response mechanisms of dicotyledonous plants. The three cultivars (fefe, the Fe-inefficient mutant; Mainstream and Edisto, both Fe efficient plants) were grown in nutrient solution in either 0 or 3.5 mg L^-1 Fe as FeCl₃. None of the three cultivars released reductants or phytosiderophores, but both Edisto and Mainstream produced massive amounts of H+ ions to reduce and maintain the pH of nutrient solutions below pH 4.0. The roots of these two Fe-efficient cultivars were also capable of reducing Fe³⁺ to Fe²⁺. These responses maintained green plants, resulted in high leaf Fe in both Edisto and Mainstream, and produced Mn toxicity in Mainstream. The lack of Fe-deficiency stress response in fefe not only affected leaf Fe concentration and chlorosis, but also resulted in reduced uptake of Mn. The importance of reduced Fe (Fe²⁺) to the Fe-efficient cultivars was confirmed by growing the cultivars with BPDS (4, 7-diphenyl-1, 10-phenanthroline disulfonic acid, a ferrous chelator) and EDDHA [ethylene-diamine di (0-hydroxphenylacetic acid)] (a ferric chelator), and observing increased chlorosis and reduced Fe uptake in BPDS grown plants. The Fe-deficiency response observed in these cultivars points out the diversity of responses to Fe deficiency stress in plants. The fefe mutant has a limited ability to absorb Fe and Mn and perhaps could be used to better understand Mn uptake in plants.     

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.     

Biogeochemical cycling of sulfur and iron in sediments of a south-east Asian mangrove,Phuket Island,Thailand

Benthic sulfate reduction and sediment pools of sulfur and iron were examined during January 1992 at 3 stations in the Ao Nam Bor mangrove, Phuket, Thailand. Patterns of sulfate reduction rates (0–53 cm) reflected differences in physical and biological conditions at the 3 stations, and highest rates were found at the vegetated site within the mangrove (Rhizophora apiculata) forest. Due to extended oxidation of mangrove sediments, a large portion of the added³⁵S-label was recovered in the chromium reducible pools (FeS₂ and S⁰) (41–91% of the reduced sulfur). Pyrite was the most important inorganic sulfur component, attaining pool sizes 50–100 times higher than acid volatile pools (FeS). HCl-extractable (0.5 M HCl) iron pools, including Fe(II)_HCl and Fe(III)_HCl, were generally low and Fe(III)_HCl was only present in the upper surface layers (0–5 cm). Maximum concentrations of dissolved Fe²⁺ (35–285 M) occurred just about the depth where dissolved H₂S accumulated. Furthermore Fe²⁺ and H₂S coexisted only where concentrations of both were low. There was an accumulation of organic sulfur in the deep sediment at 2 stations in the inner part of the mangrove. The reoxidation of reduced sulfides was rapid, and storage of sulfur was minor in the upper sediment layers, where factors like bioturbation, the presence of roots, or tidal mixing enhance oxidation processes.Author of correspondence.