Hee Sun Moon

Application of phosphate-solubilizing bacteria for enhancing bioavailability and phytoextraction of cadmium (Cd) from polluted soil

In this study, phosphate-solubilizing bacteria (PSB), Bacillus megaterium, were used to enhance Cd bioavailability and phytoextractability of Cd from contaminated soils. This strain showed a potential for directly solubilizing phosphorous from soils more than 10 folds greater than the control without inoculation. The results of pot experiments revealed that inoculation with B. megaterium significantly increased the extent of Cd accumulation in Brassica juncea and Abutilon theophrasti by two folds relative to the uninoculated control. The maximum Cd concentrations due to inoculation were 1.6 and 1.8 mg Cd g(-1) plant for B. juncea and A. theophrasti after 10 wk, respectively. The total biomass of A. theophrasti was not significantly promoted by the inoculation treatment, yet the total biomass of B. juncea increased from 0.087 to 0.448 g. It is also worth to mention that B. juncea predominantly accumulates Cd in its stems (39%) whereas A. theophrasti accumulates it in its leaves (68%) after 10 wk. The change of the Cd speciation indicated that inoculation of B. megaterium as PSB increased the bioavailabilty of Cd and consequently enhanced its uptake by plants. The present study may provide a new insight for improving phytoremediation using PSB in the Cd-contaminated soils.

Prediction of Cd and Pb toxicity to Vibrio fischeri using biotic ligand-based models in soil.

Biotic ligand-based models to predict site-specific toxicity of Cd and Pb contaminated soil were developed by using a Vibrio fischeri toxicity test. Firstly, competition effect by cations (i.e., Ca, Mg, K) commonly found in soil solution was incorporated into the models. For this purpose, biotic ligand-based model parameters including conditional binding constants of cations and metal ions to binding sites (i.e., biotic ligands) and the fractions of binding sites occupied by the metal ions were determined. Data from aqueous phase toxicity test showed that the difference between model-predicted EC(50) values of Cd and Pb and experimentally determined EC(50) values ranged within a factor of two, suggesting that the developed model parameters were reliable. Secondly, the use of soil solution to predict soil toxicity of Cd and Pb was experimentally verified with freshly spiked and field-aged soils. The results showed linear relationships in both soils, meaning that toxicity of soil solution can be representative of toxicity of soil. Finally, applicability of the developed models in Cd- or Pb-spiked soils was investigated by comparing predicted toxic effects (i.e., % bioluminescence inhibition at given cations and metal activities in soil solution) and experimentally obtained toxic effects determined by Microtox(®) solid phase toxicity test. Our data demonstrate that toxicity of Cd- or Pb-contaminated soil can be predicted by using the developed biotic ligand-based model with the chemical analysis data of soil solution as input data.

Distribution of the Microbial Community Structure in Sulfur-Based Autotrophic Denitrification Columns

Substrate-dependent evolution of a bacterial community capable of transforming nitrate was examined in sulfur-based autotrophic denitrification columns. The 16S rRNA genes and denaturing gradient gel electrophoresis (DGGE) analysis revealed that the initial bacterial consortium was well adapted to column operation time and distribution of nitrate concentration. In the lower part of a 200-day operated column where nitrate was introduced, a bacterial strain designated OTU DE-1 was abundant, occupying 92% of the community. The species was identified as Thiobacillus denitrificans with a similarity of 97% by BLAST search. A heterotroph designated as OTU DE-2 showing a similarity of 94% to Cenibacterium arsenoxidans was then enriched in the middle part of the column occupying 82% of the community, indicating the presence of organic electron donors. Interestingly, OTU DE-5 with a similarity of 98% to Chlorobium limicola, which is commonly present in hydrogen sulfide-rich environments, was found in the upper part...

Use of reporter-gene based bacteria to quantify phenanthrene biodegradation and toxicity in soil

A phenanthrene-degrading bacterium, Sphingomonas paucimobilis EPA505 was used to construct two fluorescence-based reporter strains. Strain D harboring gfp gene was constructed to generate green fluorescence when the strain started to biodegrade phenanthrene. Strain S possessing gef gene was designed to die once phenanthrene biodegradation was initiated and thus to lose green fluorescence when visualized by a live/dead cell staining. Confocal laser scanning microscopic observation followed by image analysis demonstrates that the fluorescence intensity generated by strain D increased and the intensity by strain S decreased linearly at the phenanthrene concentration of up to 200 mg/L. Such quantitative increase and decrease of fluorescence intensity in strain D (i.e., from 1 to 11.90 ± 0.72) and strain S (from 1 to 0.40 ± 0.07) were also evident in the presence of Ottawa sand spiked with the phenanthrene up to 1000 mg/kg. The potential use of the reporter strains in quantitatively determining biodegradable or toxic phenanthrene was discussed.

Enhanced uptake and translocation of arsenic in Cretan brake fern (Pteris cretica L.) through siderophorearsenic complex formation with an aid of rhizospheric bacterial activity.

Siderophores, produced by Pseudomonas aeruginosa, released slightly more Fe (53.6 μmol) than that chelated by ethylenediaminetetraacetic acid (EDTA; i.e. 43.7 μmol) in batch experiment using As-adsorbed ferrihydrite. More importantly, about 1.79 μmol of As was found to be associated with siderophores in the aqueous phase due to siderophore-As complex formation when siderophores were used to release As from ferrihydrite. In contrast, As was not detected in the aqueous phase when EDTA was used, probably due to the readsorption of released As to ferrihydrite. A series of pot experiment was conducted to investigate the effect of siderophores as a microbial iron-chelator on As uptake by Cretan brake fern (Pteris cretica L.) during phtoextraction. Results revealed that P. cretica, a known As hyperaccumulator, grown in the siderophore-amended soil showed about 3.7 times higher As uptake (5.62 mg-Asg(-1)-plant) than the plant grown in the EDTA-treated soil (1.51 mg-Asg(-1)-plant). In addition, As taken up by roots of P. cretica in the presence of siderophores seemed to be favorably translocated to shoots (i.e. stems and leaves). About 79% of the accumulated As was detected in the shoots in the presence of siderophores after ten weeks. Fluorescence microscopic analysis confirmed that As in the roots was delivered to the leaves of P. cretica as a siderophore-As complex.

Microbial succession in response to 1,4-dioxane exposure in activated sludge reactors: Effect of inoculum source and extra carbon addition

Bacterial community succession related to 1,4-dioxane exposure was investigated in two different activated sludge-inoculated reactors (municipal wastewater and dye industrial wastewater sludge), with or without additional carbon source, for 7 weeks. The denaturing gradient gel electrophoresis (DGGE) analysis revealed that microbial succession varied according to the inoculum sludge sources and the presence or absence of the extra carbon source. In the reactor inoculated with the municipal sludge, bacterial species belonging to α - and γ -Proteobacteria and Nitrospira class were dominant over time. On the other hand, bacterial species showing significant homology to β -Proteobacteria (e.g., Methylibium petroleiphilum PM1) and Actinobacteria class, who have been reported to have 1,4-dioxane degradation potential, were found in the industrial sludge-inoculated reactors. The appearance of these bacteria demonstrates that the microbial community structure of the inoculum and the presence of an extra carbon source affect the microbial succession in the system exposed to 1,4-dioxane.

Increased ecological risk due to the hyperaccumulation of As in Pteris cretica during the phytoremediation of an As-contaminated site.

Ecological risk due to the hyperaccumulation of As in Pteris cretica during phytoremediation was evaluated at an abandoned As-contaminated site. Five receptor groups representing terrestrial invertebrates, avian insectivores, small mammals, herbivores, and omnivores were selected as potentially affected ecological receptors. Soil and food ingestion were considered as major exposure pathways. Phytoremediation was performed with P.cretica only and with both P.cretica and siderophores to enhance plant uptake of As. Ecological hazard index (EHI) values for the small mammal greatly exceeded 1.0 even after three weeks of growth regardless of siderophore application, probably due to its limited home range. For the mammalian herbivore, which mainly consumes plant foliage, the EHI values were greater than 5.73 after seven weeks without siderophore application, but the value increased sharply to 29.3 at seven weeks when siderophores were applied. This increased risk could be attributed to the facilitated translocation of As from roots to stems and leaves in P.cretica. Our results suggest that, when a phytoremediation strategy is considered for metals remediation, its ecological consequences should be taken into account to prevent the spread of hyperaccumulated heavy metals throughout the food chain of ecological receptors. Uncertainties involved in the ecological risk assessment process were also discussed.

Differential in vitro bioaccessibility of residual As in a field-aged former smelter site and its implication for potential risk

Chemical forms of arsenic (As) present in a former smelter site were determined. A five-step sequential extraction showed that about 94.8 to 99.2% of total As concentration was found to be present as residual form, and interestingly some of the residual As seemed to be still bioaccessible, when determined with an in vitro bioaccessibility test. However, the extents of bioaccessible As greatly varied among the three soils tested. Soil B showed the highest bioaccessibility being 17.18 mg-As/kg (11.9%) followed by 12.71 (2.02%) and 14.03 mg-As/kg (0.64%) in soils C and A, respectively. When the residual As was treated with hydrofluoric acid (i.e., HF) 65.3 to 80.9 mg-As/kg was extracted and only 4.17 to 7.25% of the HF-extractable As was found to be bioaccessible. In contrast, when the residual As was treated with hydroperchloric acid (i.e., HClO4) only 5.64 to 8.01 mg-As/kg was recovered but 64.5 to 92.5% of the HClO4-extractable As was bioaccessible. The results suggest the presence of differential bioaccessibility of residual As, which apparently depends on the solid phase that As is associated with (i.e., organic matter or clay minerals). Of the As present as residual fraction, the As mainly bound to silicate mineral showed extremely low bioaccessibility and the As associated with refractory organic matter was highly bioaccessible.

Effect of the redox dynamics on microbial-mediated As transformation coupled with Fe and S in flow-through sediment columns

Arsenic (As) biogeochemistry coupled with iron (Fe) and sulfur (S) was studied using columns packed with As(V)-contaminated sediments under two phases: a reduction phase followed by an oxidation phase. During the reduction phase, four identical columns inoculated with G. sulfurreducens were stimulated with 3mM acetate for 60days. The As(III) in the effluent rapidly increased then gradually decreased. The Fe(II) and sulfate concentration indicated ferrous sulfide precipitation inside the column after day 14 and X-ray absorption near edge structure spectra showed that As(III) was enriched at the column outlet. The genera Desulfosporosinus and Anaeromyxobacter as well as the Geobacter inoculum played a primary role in As reduction. During the oxidation phase, dissolved oxygen was consumed by heterotrophic aerobes belonging to the phylum Cloroflexi in the column with acetate, resulting in more As in the effluent. When only nitrate was injected, sulfur-oxidizing bacteria such as Thiobacillus thioparus instantly oxidized the sulfide formed during the first phase, resulting in less As(V) in the aqueous phase compared to the column with dissolved oxygen alone. This study showed that redox gradients and dynamics linked to Fe and S biogeochemistry have an important role in controlling As mobility in subsurface environments.

The Current Status of Strong Acids Production, Consumption, and Spill Cases in Korea

We reviewed literature focusing on the amounts of domestic production, distribution, and consumption of strong acids and their spill cases. In particular, we investigated the chemistry and toxicity of four strong acids classified as “accident preparedness substances,” including hydrochloric, nitric, sulfuric, and hydrofluoric acid. We recommend sulfuric and hydrofluoric acid as the chemicals of priority control based on the amounts used and toxicity. An advanced prevention/ response system needs to be established along with an improved human and social infrastructure to prevent and efficiently respond to chemical accidents. Understanding the behavior and transport of spilled strong acids in the soil and groundwater environments requires a multi-disciplinary approach since they go through a variety of chemical and biogeochemical reactions with complex geomedia. However, no such research has been done in this area in Korea to the best of our knowledge. We expect the results of this study to contribute as basic data to future research.