An Experimental Investigation of the Effect of Bacillus megaterium on Apatite Dissolution

Field observations suggest that some mineral dissolution rates can be enhanced by microbial activity indirectly, without direct contact with the mineral surface. A series of apatite dissolution experiments was performed to better understand this rate enhancement process. Far-from equilibrium abiotic apatite dissolution rates, measured in mixed-flow reactors at 25°C were enhanced by increasing concentration of aqueous organic acids and decreasing aqueous phosphate activity, demonstrating the existence of indirect pathways for microbial rate enhancement. Further apatite dissolution experiments were performed in closed-system reactors in the presence of Bacillus megaterium , a common heterotrophic aerobe. Experiments were designed to allow the bacteria to be either in direct contact or indirect contact with the apatite; in the latter case, the microbes were physically separated from the apatite using dialysis bags. Apatite dissolution in indirect contact with Bacillus megaterium was 50 to 900% faster than abiotic controls. Bacterial rate enhancement was, however, 3 to over 10 times lower when Bacillus megaterium was in direct contract versus indirect contact with the apatite surfaces. These results show that (1) bacteria can accelerate rates without being in physical contact with the dissolving mineral, and (2) microbially mediated dissolution may be less effective when bacteria are in direct contact with mineral surfaces. Supression of mineral dissolution is interpreted to stem from the preferential colonization of reactive sites on the mineral surface.     

Metabolic regulation in Pseudomonas oxalaticus OX1. Diauxic growth on mixtures of oxalate and formate or acetate

Diauxic growth was observed in batch cultures of Pseudomonas oxalaticus when cells were pregrown on acetate and then transferred to mixtures of acetate and oxalate. In the first phase of growth only acetate was utilized. After the exhaustion of acetate from the medium enzymes involved in the metabolism of oxalate were synthesized during a lag phase of 2 h, followed by a second growth phase on oxalate. When the organism was pregrown on oxalate, oxalate utilization from the mixture with acetate completely ceased after a few hours during which acetate became the preferred substrate. Similar observations were made with formate/oxalate mixtures in which formate was the preferred substrate. Until formate was exhausted, it completely suppressed oxalate metabolism, again resulting in diauxic growth. However, when the organism was pregrown on oxalate and then transferred to mixtures of oxalate and formate, both substrates were utilized simultaneously although the initial rate of oxalate utilization from the mixture was strongly reduced as compared to growth on oxalate alone.Since both preferred substrates cross the cytoplasmic membrane by diffusion, whereas oxalate is accumulated by an inducible, active transport system, the effect of acetate and formate on oxalate transport was studied at different external pH values. At pH 5.5 both substrates completely inhibited oxalate transport. However, at pH 7.5, the pH at which the diauxic growth experiments were performed, formate and acetate did not affect oxalate transport. Growth patterns and enzymes profiles suggest that, at higher pH values, formate and acetate possibly affect oxalate utilization via an effect on the internal pool of oxalyl-CoA, the first product of oxalate metabolism.Abbreviations PMS phenazine methosulphate - RuBPCase ribulosebisphosphate carboxylase - DCPIP 2,6-dichlorophenolindophenol - FDH formate dehydrogenase - p.m.f. protonmotive force     

Impact of carbon sources on growth and oxalate synthesis by the phytopathogenic fungus <Emphasis Type="Italic">Sclerotinia sclerotiorum</Emphasis>

The impact of various supplemental carbon sources (oxalate, glyoxylate, glycolate, pyruvate, formate, malate, acetate, and succinate) on growth and oxalate formation (i.e., oxalogenesis) by Sclerotinia sclerotiorum was studied. With isolates D-E7, 105, W-B10, and Arg-L of S. sclerotiorum, growth in an undefined broth medium (0.1% soytone; pH 5) with 25 mM glucose and 25 mM supplemental carbon source was increased by the addition of malate and succinate. Oxalate accumulation occurred in the presence of glucose and a supplemental carbon source, with malate, acetate, and succinate supporting the most oxalate synthesis. With S. sclerotiorum Arg-L, oxalate-to-biomass ratios, an indicator of oxalogenic potential, were dissimilar when the organism was grown in the presence of different carbon sources. The highest oxalate-to-biomass ratios were observed with pyruvate, formate, malate, acetate, and succinate. Time-course studies with acetate-supplemented cultures revealed that acetate and glucose consumption by S. sclerotiorum D-E7 coincided with oxalogenesis and culture acidification. By day 5 of incubation, oxalogenesis was halted when cultures reached a pH of 3 and were devoid of acetate. In succinate-supplemented cultures, oxalogenesis essentially paralleled glucose and succinate utilization over the 9-day incubation period; during this time period, culture pH declined but never fell below 4. Overall, these results indicate that carbon sources can regulate the accumulation of oxalate, a key pathogenicity determinant for S. sclerotiorum.     

Reversible suppression of in vitro biomineralization by activation of protein kinase A

Parathyroid hormone (PTH-(1-34)) potently suppresses apatite deposition in osteoblastic cultures. These inhibitory effects are mediated through signaling events following PTH receptor binding. Using both selective inhibitors and activators of protein kinase A (PKA), this study shows that a transient activation of PKA is sufficient to account for PTH's inhibition of apatite deposition. This inhibition is not a result of reduced cell proliferation, reduced alkaline phosphatase activity, increased collagenase production, or lowering medium pH. Rather, data suggest a functional relationship between matrix assembly and apatite deposition in vitro. Bone sialoprotein (BSP) and apatite co-localize in the extracellular matrix of mineralizing cultures, with matrix deposition of BSP temporally preceding that of apatite. Transient activation of PKA by either PTH-(1-34) or short term cAMP analog treatment blocks the deposition of BSP in the extracellular matrix without a significant reduction in the total amount of BSP synthesized and secreted. This effect is reversible after allowing the cultures to recover in the absence of PKA activators for several days. Thus, a transient activation of PKA may suppress mineral deposition in vitro as a consequence of altering the assembly of an extracellular matrix permissive for apatite formation.     

Limestone Corrosion by Neutrophilic Sulfur-Oxidizing Bacteria: A Coupled Microbe-Mineral System

Subsurface karst aquifers receiving sulfidic water can host complex chemolithotrophic microbial communities that are capable of dissolving limestone, forming new karstic habitat. Neutrophilic sulfur-oxidizing bacteria use reduced sulfur compounds as energy rich substrate, potentially producing sulfuric acid as a geochemically reactive byproduct. The physicochemical relationship between a biofilm forming on a limestone surface and the extent of microbial influence on dissolution rate, however, are unknown. We investigated the rate of Madison limestone dissolution by sulfur-oxidizers both in the field at Lower Kane Cave, WY (LKC), and in the laboratory using continuous flow culture reactors and microbial mat collected from LKC. In the field, a microbial consortium rapidly colonized limestone chips forming a thick biofilm, with deep etching of mineral surfaces underneath. In the laboratory we found that a microbial biofilm oxidizing thiosulfate on the limestone surface accelerated dissolution rate up to 7 times faster than the abiotic baseline rate. In contrast, experiments done with H₂S or a mixture of H₂S and thiosulfate had no effect on dissolution rate. We hypothesize that the laboratory mat community dominated by Thiothrix sp. oxidizes thiosulfate to sulfate and H⁺, while H₂S is partially oxidized to S°. When all sulfur substrate is withheld, the community oxidizes stored intracellular sulfur, briefly accelerating limestone dissolution even in the absence of external supplied substrate. Accelerated corrosion occurs only in the reactive micro-environment under the biofilm, disconnected from the bulk reactor solution. When experiments are repeated where the microbial population is separated from the limestone by a dialysis membrane barrier, measured pH drop is greater, but there is only slight enhancement of rate. This work confirms our working hypothesis that neutrophilic sulfur-oxidizers colonize and rapidly dissolve limestone surfaces, possibly to buffer the production of excess acidity.     

Long-term availability of nutients in forest soil derived from fast- and slow-release fertilizers

The availability of P, K and Mg was studied in boreal forest soil treated 10 years earlier with slow- and fast-release fertilizers. Fast release superphosphate, potassium chloride and magnesium sulphate and slow-release apatite (P) and biotite (K, Mg) were applied alone or together with urea or urea+limestone. The concentrations of total and exchangeable nutrients in the organic horizon and the concentration of exchangeable nutrients in the uppermost mineral horizon were measured. CO₂ production during aerobic laboratory incubation was used to estimate the microbial activity and substrate-induced respiration to determine the microbial biomass C in soil. Biotite caused a moderate but persistent increase in pH in the organic horizon, but this increase was smaller than with lime. The fast-release fertilizers had no effects on the nutrient status of the soil 10 years after the fertilization. However, apatite and biotite still increased the total content of Mg, K and P and the concentrations of exchangeable Mg and soluble P in soil. On the other hand, simultaneous addition of lime and biotite reduced the release of soluble P from apatite. The reduction in soil microbial activity found with urea and the fast-release salts soon after application was no longer evident 10 years later. There was no increase in nitrification in the fertilized soils, not even with the urea+lime treatment. The previous results right after the application and the results presented here do not indicate major leaching of nutrients from the slow-release fertilizers to the deeper soil profiles.     

Experimental Study of Microbial Pyrite Oxidation: A Suggestion for Geologically Useful Biosignatures

The oxidation of pyrite in cultures of Acidithiobacillus ferrooxidans (A.f) was studied. The experiments were performed at an initial pH of 2.5 at 28°C. The concentrations of total dissolved iron in solution and the pH were monitored during the first 36 days. Pyrite surfaces were examined by scanning electron microscopy and energy-dispersive spectrometry (SEM-EDS) after 100 days. The concentrations of total dissolved iron and hydrogen ions increased significantly in the presence of bacteria. SEM examination indicated that the crystal surfaces were subjected to two types of dissolution phenomena. Cracks were observable on the of crystal surfaces under both biotic and abiotic conditions, whereas rounded and polygonal pits appeared additionally on the surfaces under biotic conditions. The co-occurrence of the rounded and polygonal pits on the crystal surfaces and the presence of A.f at the pyrite surface suggests that A.f promotes pyrite oxidation by a contact mechanism. We propose that the rounded and polygonal pits be considered to represent a practical biosignature for tracing the evolution of microbial iron oxidation in the remote past.     

Dissolution and growth of calcium oxalate monohydrate I. Effect of magnesium and pH

The rate of dissolution of calcium oxalate monohydrate and of a calcium oxalate renal stone was measured in 0.9% NaCl solution at different levels of magnesium concentration and pH. The growth of calcium oxalate obtained by chemical reaction between Ca²⁺ and oxalate ions at a concentration similar to that existing in normal urine was also investigated as a function of pH and magnesium concentration. It was found that both magnesium and pH exert a fine kinetic control on the precipitation and growth of calcium oxalate monohydrate. Magnesium had no effect on the dissolution. The possible role of magnesium and pH in calcium oxalate urolithiasis has been discussed in the light of previous reports and of the data presented in this study.     

Calcite and dolomite dissolution rates in the context of microbe–mineral surface interactions

Although microbes have been shown to alter the dissolution rate of carbonate minerals, a mechanistic understanding of the consequences of microbial surface colonization on carbonate dissolution has yet to be achieved. Here we report the use of vertical scanning interferometry (VSI) to study the effect of Shewanella oneidensis MR‐1 surface colonization on the dissolution rates of calcite (CaCO₃) and dolomite (CaMg(CO₃)₂) through qualitative analysis of etch pit development and quantitative measurements of surface‐normal dissolution rates. By quantifying and comparing the significant processes occurring at the microbe–mineral interface, the dominant mechanism of mineral dissolution during surface colonization was determined. MR‐1 attachment under aerobic conditions was found to influence carbonate dissolution through two distinct mechanistic pathways: (1) inhibition of carbonate dissolution through interference with etch pit development and (2) excavation of carbonate material at the cell–mineral interface during irreversible attachment to the mineral surface. The relative importance of these two competing effects was found to vary with the solubility of the carbonate mineral studied. For the faster‐dissolving calcite substrates, inhibition of dissolution by attachment and subsequent extracellular polysaccharide (EPS) production was the dominant effect associated with MR‐1 surface colonization. This interference with etch pit development resulted in a 40–70% decrease in the surface normal dissolution rate relative to cell‐free controls, depending primarily on the concentration of cells in solution. However, in the case of the slower‐dissolving dolomite substrates, carbonate material displaced during the entrenchment of cells on the surface far outweighed the abiotic dissolution rate. Therefore, during the initial stages of surface colonization, dolomite dissolution rates were actually enhanced by MR‐1 attachment. This study demonstrates the dynamic and competitive relationship between microbial surface colonization and mineral dissolution that may be expected to occur in natural environments.     

The pH mediated effects of initial glucose concentration on the transitory occurrence of extracellular metabolites, gas exchange and growth yields of aerobic batch cultures of Klebsiella pneumoniae

The changes in growth kinetics in aerobic batch cultures of Klebsiella pneumoniae were followed by measurements of extracellular metabolites, rates of gas exchange, dissolved oxygen tension, pH, and carbon balance at all stages of growth. When the initial growth-limiting glucose concentration in media without pH control was increased from 1.0 g carbon L(-1) to 2.2 g carbon L(-1), the number of different, mainly acidic, extracellular metabolites of glucose at the end of exponential growth increased, while the proportion of acetate decreased. During the postexponential growth phase, the extracellular metabolites were oxidized, resulting in an increasing complexity of changes in pH, gas exchange, and dissolved oxygen tension with increasing initial substrate concentration. All these parameters showed concomitant stepwise changes. This pattern was independent of the dissolved oxygen tension in the range 30-200 muM. When pH was kept constant, the number, slope, and relative magnitude of the steps in gas exchange and dissolved oxygen tension were pH-dependent, being most complex at low pH. Detailed carbon balances showed that 20% of the initial glucose was converted into extracellular metabolites at the end of exponential growth at neutral pH. In the postexponential phase, pyruvate (2%) was reoxidized first followed by acetate (13%). The observed molar growth yield coefficient (Y(ATP)) was 8.4 if the transitory occurrence of pyruvate and acetate was accounted for, and 6.4 if it was neglected. The corrected observed molar growth yield coefficient (Y'(ATP)) was 9.4 and compared well with the true molar growth yield coefficient (Y(Max) (ATP)), which was found to be 11.0. Specific in situ respiration rates of the exponential growth phase of cultures grown at different controlled pH values compared well with in situ values for energy-limited chemostat grown cells at the same growth rates, suggesting that growth in the batch culture was energy-limited throughout the exponential growth phase. This view was supported by low levels of intracellular glycogen and exopolysaccharides of all cultures, by the value of Y'(ATP) of 9.4, and by a constant specific production rate of the extracellular metabolites throughout exponential growth. It was concluded that even under strictly aerobic conditions, control of pH is as important as control of dissolved oxygen tension during growth of enterobacteriaceae in batch cultures.     

Silicon absorption by wheat (Triticum aestivum L.)

Although silicon (Si) is a quantitatively major inorganic constituent of higher plants the element is not considered generally essential for them. Therefore it is not included in the formulation of any of the solution cultures widely used in plant physiological research. One consequence of this state of affairs is that the absorption and transport of Si have not been investigated nearly as much as those of the elements accorded 'essential' status. In this paper we report experiments showing that Si is rapidly absorbed by wheat (Triticum aestivum L.) plants from solution cultures initially containing Si at 0.5 mM, a concentration realistic in terms of the concentrations of the element in soil solutions. Nearly mature plants (headed out) 'preloaded' with Si absorbed it at virtually the same rate as did plants grown previously in solutions to which Si had not been added. The rate of Si absorption increased by more than an order of magnitude between the 2-leaf and the 7-8 leaf stage, with little change thereafter. This revised version was published online in June 2006 with corrections to the Cover Date.     

Factors controlling the carbon isotope fractionation of tetra- and trichloroethene during reductive dechlorination by Sulfurospirillum ssp. and Desulfitobacterium sp. strain PCE-S

Carbon stable isotope fractionation of tetrachloroethene (PCE) and trichloroethene (TCE) was investigated during reductive dechlorination. Growing cells of Sulfurospirillum multivorans, Sulfurospirillum halorespirans, or Desulfitobacterium sp. strain PCE-S, the respective crude extracts and the abiotic reaction with cyanocobalamin (vitamin B(12)) were used. Fractionation of TCE (alphaC=1.0132-1.0187) by S. multivorans was more than one order of magnitude higher than values previously observed for tetrachloroethene (PCE) (alphaC=1.00042-1.0017). Similar differences in fractionation were observed during reductive dehalogenation by the close relative S. halorespirans with alphaC=1.0046-1.032 and alphaC=1.0187-1.0229 for PCE and TCE respectively. TCE carbon isotope fractionation (alphaC=1.0150) by the purified PCE-reductive dehalogenase from S. multivorans was more than one order of magnitude higher than fractionation of PCE (alphaC=1.0017). Carbon isotope fractionation of TCE by Desulfitobacterium sp. strain PCE-S (alphaC=1.0109-1.0122) as well as during the abiotic reaction with cyanocobalamin (alphaC=1.0154) was in a similar range to previously reported values for fractionation by mixed microbial cultures. In contrast with previous results with PCE, no effects due to rate limitations, uptake or transport of the substrate to the reactive site could be observed during TCE dechlorination. Our results show that prior to a mechanistic interpretation of stable isotope fractionation factors it has to be carefully verified how other factors such as uptake or transport affect the isotope fractionation during degradation experiments with microbial cultures.     

Characterization of Rhizobium sp. Q32 Isolated from Weathered Rocks and its Role in Silicate Mineral Weathering

One mineral-solubilizing strain was isolated from weathered potassic trachyte surfaces and identified as Rhizobium sp. Q32 based on the 16S rRNA gene sequence analysis. The mineral (feldspar and biotite) dissolution potential and the physiological characteristics of the bacterium were investigated. Laboratory mineral dissolution experiments indicated that inoculation with the live bacterium significantly increased feldspar and biotite dissolution by a factor of 1.2–4.7 for Si and 1.2–1.5 for K in comparison with the dead bacterium inoculated controls. In addition, extracellular polysaccharide production by the bacterium increased with time but the bacterium produced small pH changes (6.0–6.5) in the course of mineral dissolution experiment. The bacterium was found to produce siderophores and have the characteristics of acid or alkali and salt tolerance and temperature resistance. The result suggested that feldspar and biotite dissolution may be mainly caused by extracellular polysaccharide and/or siderophores produced by the bacterium.     

Acetoin production in growing Leuconostoc mesenteroides

Leuconostoc mesenteroides NCDO 518, provided with oxygen and pyruvate, preferentially used oxygen as accessory electron acceptor and converted pyruvate to acetoin. With glucose, 5.6 mM, as sole energy source only small amounts of acetoin were formed (0.08–0.21 mM). With glucose, 5.6 mM, and pyruvate, 20 mM, substantial amounts of acetoin were produced in growing, aerated cultures at pH 5 (2.8 mM, equivalent to 0.5 mol [mol glucose fermented]^–1). On exhaustion of glucose, growth ceased but metabolism of pyruvate continued with the formation of acetate and a little acetoin. In aerated cultures at pH 6 the general pattern was similar to that at pH 5 but less acetoin (0.6 mM) was formed during the growth phase and, after the exhaustion of glucose, pyruvate was converted very slowly to acetate only. Leuc. mesenteroides did not grow with pyruvate as sole energy source.     

Microbial Fe(III) oxide reduction potential in Chocolate Pots hot spring,Yellowstone National Park

Chocolate Pots hot springs (CP) is a unique, circumneutral pH, iron‐rich, geothermal feature in Yellowstone National Park. Prior research at CP has focused on photosynthetically driven Fe(II) oxidation as a model for mineralization of microbial mats and deposition of Archean banded iron formations. However, geochemical and stable Fe isotopic data have suggested that dissimilatory microbial iron reduction (DIR) may be active within CP deposits. In this study, the potential for microbial reduction of native CP Fe(III) oxides was investigated, using a combination of cultivation dependent and independent approaches, to assess the potential involvement of DIR in Fe redox cycling and associated stable Fe isotope fractionation in the CP hot springs. Endogenous microbial communities were able to reduce native CP Fe(III) oxides, as documented by most probable number enumerations and enrichment culture studies. Enrichment cultures demonstrated sustained DIR driven by oxidation of acetate, lactate, and H₂. Inhibitor studies and molecular analyses indicate that sulfate reduction did not contribute to observed rates of DIR in the enrichment cultures through abiotic reaction pathways. Enrichment cultures produced isotopically light Fe(II) during DIR relative to the bulk solid‐phase Fe(III) oxides. Pyrosequencing of 16S rRNA genes from enrichment cultures showed dominant sequences closely affiliated with Geobacter metallireducens, a mesophilic Fe(III) oxide reducer. Shotgun metagenomic analysis of enrichment cultures confirmed the presence of a dominant G. metallireducens‐like population and other less dominant populations from the phylum Ignavibacteriae, which appear to be capable of DIR. Gene (protein) searches revealed the presence of heat‐shock proteins that may be involved in increased thermotolerance in the organisms present in the enrichments as well as porin–cytochrome complexes previously shown to be involved in extracellular electron transport. This analysis offers the first detailed insight into how DIR may impact the Fe geochemistry and isotope composition of a Fe‐rich, circumneutral pH geothermal environment.     

Impact of nutritional supplements and monosaccharides on growth, oxalate accumulation, and culture pH by Sclerotinia sclerotiorum

Sclerotinia sclerotiorum D-E7 was studied to determine the impact of nutritional supplements and monosaccharides on growth, oxalate accumulation, and culture pH in broth media (initial pH c. 5). Cultures with 0.1% nutritional supplement (tryptone, yeast extract, or soytone) yielded minimal growth, 2-3 mM oxalate, and a final culture pH of 4.2-4.8. In contrast, cultures with 0.1% nutritional supplement and 25 mM glucose yielded significant growth, minimal oxalate (<1 mM), and a final culture pH of 2.8-3.7. Similar trends were observed when glucose in 0.1% soytone cultures was replaced with 25 mM d-mannose, l-arabinose, or d-xylose. With 1% soytone-25 mM glucose cultures, growth and oxalate accumulation ( approximately 21 mM) occurred with little change in initial pH. This was not the case with 1% soytone-250 mM glucose cultures; increased glucose levels resulted in a decrease in oxalate accumulation ( approximately 7 mM) and in final culture pH (3.4). Time-course studies with these cultures revealed that oxalate accumulation was suppressed during growth when the culture pH dropped to <4. Overall, these results indicate that (1) the decrease in external pH (i.e. acidification) was independent of oxalate accumulation and (2) acidification coupled to glucose-dependent growth regulated oxalate accumulation by Sclerotinia sclerotiorum.     

Dissolution of Fluorapatite by Pseudomonas fluorescens P35 Resulting in Fluorine Release

Chemical weathering of fluorine-bearing minerals is widely accepted as the main mechanism for the release of fluorine (F) to groundwater. Here, we propose a potential mechanism of F release via microbial dissolution of fluorapatite (Ca₅(PO₄)₃F), which has been neglected previously. Batch culture experiments were conducted at 30°C with a phosphate-solubilizing bacteria strain, Pseudomonas fluorescens P35, and rock phosphates as the sole source of phosphate for microbial growth in parallel with abiotic controls. Rock phosphates consisted of 55–91% of fluorapatite and 5–10% of dolomite before microbial dissolution as indicated by X-ray diffraction (XRD). Mineral composition and morphology changed after microbial dissolution characterized by the disappearance of dolomite and the development of etched cavities on rock phosphate surfaces. The pH of media used was approximately 7.4 at the beginning and increased gradually to 7.7 in abiotic controls; with the inoculum, the pH decreased to acidic values of 3.7–3.8 after 27 h. Phosphate, calcium, and fluoride were released from the rock phosphate to the acidified medium. At 42 h, the concentration of F reached 8.1–10.3 mg L^?1. The elevated F concentration was two times higher than the F levels in groundwater in regions diagnosed with fluorosis, and was toxic to the bacteria, as demonstrated by a precipitous decrease in live cells. Geochemical modeling demonstrated that the oxidation of glucose (the carbon source for microbial growth in the medium) to gluconic acid could decrease the pH to 3.7–3.8 and result in the dissolution of fluorapatite and dolomite. Dolomite and fluorapatite remained unsaturated, while concentrations of dissolved phosphorus (P), calcium (Ca), and F increased throughout the time course Fluorite reached saturation [saturation index (SI) 0.22–0.42] after 42 h in rock phosphate–amended biotic systems. However, fluorite was not detected in XRD patterns of the final residue from microcosms. Given that phosphate-solubilizing bacteria are ubiquitous in soil and groundwater ecosystems, they could play an important role in fluorapatite dissolution and the release of F to groundwater.     

Reduction of bromate by biogenic sulfide produced during microbial sulfur disproportionation

Bromate (BrO₃ ⁻) is a carcinogenic contaminant formed during ozonation of waters that contain trace amounts of bromide. Previous research shows that bromate can be microbially reduced to bromide using organic (i.e. acetate, glucose, ethanol) and inorganic (H₂) electron-donating substrates. In this study, the reduction of bromate by a mixed microbial culture was investigated using elemental sulfur (S⁰) as an electron donor. In batch bioassays performed at 30°C, bromate (0.30 mM) was completely converted to bromide after 10 days and no accumulation of intermediates occurred. Bromate was also reduced in cultures supplemented with thiosulfate and hydrogen sulfide as electron donor. Our results demonstrated that S⁰-disproportionating microorganisms were responsible for the reduction of bromate in cultures spiked with S⁰ through an indirect mechanism involving microbial formation of sulfide and subsequent abiotic reduction of bromate by the biogenic sulfide. Confirmation of this mechanism is the fact that bromate was shown to undergo rapid chemical reduction by sulfide (but not S⁰ or thiosulfate) in abiotic experiments. Bromate concentrations above 0.30 mM inhibited sulfide formation by S⁰-disproportionating bacteria, leading to a decrease in the rate of bromate reduction. The results suggest that biological formation of sulfide from by S⁰ disproportionation could support the chemical removal of bromate without having to directly use sulfide as a reagent.     

Effect of biotite,zeolite, heavy clay,bentonite and apatite on the uptake of radiocesium by grass from peat soil

Biotite is a potassium rich mineral, which is used as a fertilizer in organic farming and as a soil amendment in conventional farming. Its ability to reduce ¹³⁴Cs uptake by ryegrass from peat soil was studied in pot experiments and compared with zeolite, heavy clay, bentonite and apatite. In addition, the long-term effect of biotite on ¹³⁷Cs uptake from peat soil was studied in the peat field. In the pot experiments in the first cut of ryegrass, the minerals decreased ¹³⁴Cs uptake by plants in the following order: zeolite > heavy clay > bentonite > biotite > apatite. Apatite did not have any effect on the plant ¹³⁴Cs level. In the later cuts, the uptake of ¹³⁴Cs from biotite-treated soil decreased further while that from soils treated with other minerals remained unchanged or even increased. In general, ¹³⁴Cs uptake by plants decreased with increasing mineral level. The decrease of ¹³⁴Cs uptake became more efficient, especially at the early growth stage, by mixing small amounts of zeolite in biotite. The results of the field experiment indicated the long-term effect of biotite on reducing ¹³⁴Cs uptake by plants. Biotite application rate was 30 t ha-1. The five-year mean of the plant/soil concentration ratio of ¹³⁷Cs was 0.05 for biotite-treated soil, in contrast to 0.14 for the control soil. On the whole, biotite reduced considerably the ¹³⁷Cs level of plants on peat soil and this effect was long-lasting. For an effective reduction of plant radiocesium a great quantity of biotite is needed and therefore it is most suitable for greenhouse cultivation where contaminated slightly decomposed peat is used as a growing medium.