Fuminori Sakamoto

Chemical states of fallout radioactive Cs in the soils deposited at Fukushima Daiichi Nuclear Power Plant accident

The chemical states of radioactive Cs (caused by Fukushima Daiichi Nuclear Power Plant accident) in the contaminated soils have been characterized by the desorption experiments using appropriate reagent solutions and size fractionation of the contaminated soils. More than 65% of radioactive Cs remained in the residual fraction of the soil samples after treatment of 1 mole L−1 NH4Cl solution and 1 mole L−1 CH3COOH solution. Approximately 70% of radioactive Cs in the residual fraction were associated with the size fractions larger than the elutriated one, even though mica-like minerals were present in the elutriated one. These results strongly suggest that radioactive Cs was irreversibly associated with soil components other than mica-like minerals in the contaminated soil.

Association of Actinides with Microorganisms and Clay: Implications for Radionuclide Migration from Waste-Repository Sites

We conducted a series of basic studies on the microbial accumulation of actinides to elucidate their migration behavior around backfill materials used in the geological disposal of radioactive wastes. We explored the interactions of U(VI) and Pu(VI) with Bacillus subtilis, kaolinite clay, and within a mixture of the two, directly analyzing their association with the bacterium in the mixture by transmission electron microscopy (TEM) and scanning electron microscopy (SEM). The accumulation of U by the mixture rose as the numbers of B. subtilis cells increased. Treating the kaolinite with potassium acetate (CH 3 COOK) removed approximately 80% of the associated uranium while only 65% was removed in the presence of B. subtilis. TEM-EDS analysis confirmed that most of the U taken from solution was associated with B. subtilis. XANES analyses revealed that the oxidation state of uranium associated with B. subtilis, kaolinite, and with the mixture containing both was U(VI). The amount of Pu sorbed by B. subtilis increased with time, but did not reach equilibrium in 48 h; in kaolinite alone, equilibrium was attained within 8 h. After 48 h, the oxidation state of Pu in the solutions exposed to B. subtilis and to the mixture had changed to Pu(V), whereas the oxidation state of the Pu associated with both was Pu(IV). In contrast, there was no change in the oxidation state of Pu in the solution nor on kaolinite after exposure to Pu(VI). SEM-EDS analysis indicated that most of the Pu in the mixture was associated with the bacteria. These results suggest that U(VI) and Pu(VI) preferentially are sorbed to bacterial cells in the presence of kaolinite clay, and that the mechanism of accumulation of U and Pu differs. U(VI) is sorbed directly to the bacterial cells, whereas Pu(VI) first is reduced to Pu(V) and then to Pu(IV), and the latter is associated with the cells. These results have important implications on the migrations of radionuclides around the repository sites of geological disposal. Microbial cells compete with clay colloids for radionuclides accumulation, and because of their higher affinity and larger size, the microbes accumulate radionuclides and migrate much slower than do the clay colloids. Additionally, biofilm coatings formed on the fractured rock surfaces also accumulate radionuclides, thereby retarding radionuclide migration.

Application of micro-PIXE technique to uptake study of cesium by Saccharomyces cerevisiae

Abstract We examined the accumulation and distribution of Cs, and the presence of other elements in yeast ( Saccharomyces cerevisiae ) cells by the micro-PIXE (particle induced X-ray emission) system developed at the TIARA facility, JAERI, and by energy dispersive spectroscopy (EDS) coupled to a scanning electron microscope. The effects of Cs on yeast growth were determined by measuring the optical density at 600 nm. Addition of 1 mM Cs did not have any effect on the growth of the yeast. Micro-PIXE analysis of cells grown in the presence of Cs showed that Cs was uniformly distributed in the cells. Using PIXE, Cs, P, K and Fe can be detected, whereas only P and S can be determined by the EDS. Cells exposed to Cs showed an increase in Cs peak intensity, and decrease in P, K and Fe with time. These results suggest that micro-PIXE is a useful technique to detect low concentration of toxic elements in microorganisms as well as to monitor their changes as function of growth.

Response of Saccharomyces cerevisiae to Heavy Element Stress: Lead vs. Uranium

We have examined the responses of Saccharomyces cerevisiae in media containing Pb 2+ ions and compared it to those in media containing UO 2+ 2 ions to elucidate the stress effects of heavy elements. Cultivation of S. cerevisiae in a medium containing 1.0 × 10 −4 M Pb showed nearly the same growth as that in the control medium while growth was inhibited in the medium containing 1.0 × 10 −3 M Pb. Backscattering electron image analyses of thin sections of cells showed that Pb had accumulated inside the cells. These results were different from those for UO 2+ 2 ions obtained in the previous study, where 1.0 × 10 −4 M U inhibited the growth of S. cerevisiae, and UO 2+ 2 ions were sorbed on the cell surface. Two-dimensional (2D) gel electrophoresis analysis of the proteins extracted from the S. cerevisiae exposed to Pb 2+ ions showed that several specific protein spots were expressed after cultivation with Pb, that did not appear in the control medium. The isoelectric point in the gel strip and molecular sizes of the specific proteins expressed by Pb 2+ were different from those by UO 2+ 2 . Our results suggest that Pb 2+ more weakly inhibits the growth of S. cerevisiae than UO 2+ 2 , and produces different stresses in S. cerevisiae than UO 2+ 2 .

Accumulation of Co in Yeast Cells under Metabolically Active Condition —Implication for Role of Yeast in Migration of Radioactive Co—

The accumulation and change in the chemical states of Co have been studied to elucidate the role of the yeast Saccharomyces cerevisiae in the migration of radioactive cobalt in the environment. The yeast was grown in a solution containing Co(II) ions, a carbon source, and essential elements (metabolically active condition). For comparison, an adsorption experiment of Co(II) ions on the yeast cells under resting condition without essential elements was performed. Time courses of Co concentration in the solution, in the cells, and chemical states of the accumulated Co were determined by inductively coupled plasma atomic emission spectrometry (ICP-AES), particle-induced X-ray emission (PIXE), and X-ray absorption fine structure (XAFS) analyses. The time courses of Co concentration in the solution showed that a higher amount of Co was accumulated by the yeast cells under the metabolically active condition than under the resting one. PIXE analyses showed the concurrent accumulation of Fe with Co accumulation under the metabolically active condition, suggesting the intracellular accumulation of Co. XAFS analyses showed that the k3-weighted extended-XAFS functions and the radial structural function of Co accumulated by the yeast cells under the metabolically active condition are similar to those under the resting condition, indicating that the chemical states of the accumulated Co were nearly the same between both conditions. These results indicate that the yeast performs better retardation of the migration of Co under the metabolically active condition than under the resting one.

Modeling of the Interaction of Pu(VI) with the Mixture of Microorganism and Clay

A model analysis of the transformation of Pu(VI) in a mixture of a common soil bacterium, Bacillus subtilis, and kaolinite clay was carried out. A simplified kinetic model, where the adsorptions of Pu(VI) and Pu(V) are assumed to be in instantaneous equilibrium and the two-step reduction of Pu(VI) to Pu(V) and then to Pu(IV) is described by a first-order rate law, was proposed. When we assumed in the model analysis that the reduction rate of Pu(V) to Pu(IV) was higher for B. subtilis than for kaolinite, the estimated fractions of Pu in the solution and in the mixture, and the oxidation states of Pu in the solution and in the mixture were in good agreement with the measured ones. On the contrary, the assumption that the reduction rate of Pu(V) to Pu(IV) for kaolinite was the same as that for B. subtilis gave a wrong prediction of Pu association with the mixture. These results strongly suggest that Pu(V) on the bacterial cell is reduced to Pu(IV) by the bacterially mediated electron transfer process during the accumulation of Pu(VI) in the mixture.

Yeast Genes Involved in Uranium Tolerance and Uranium Accumulation: A Functional Screening Using the Nonessential Gene Deletion Collection

We screened 4908 non-essential gene deletion mutant yeast strains for uranium sensitivity and low accumulation by growth in agar medium containing uranium. All mutant strains grew successfully on agar media containing 0 or 0.2 mM uranium for one week at 30o C. Thirteen strains with single gene deletions showed reduced growth in the agar medium containing 0.5 mM uranium and were identified as uranium-sensitive mutant strains. The phosphate transporter genes of PHO86, PHO84, PHO2, and PHO87 were among the deleted genes in the uranium-sensitive mutant strains, suggesting that genes concerned with phosphate transport contribute to uranium tolerance. Seventeen single-deletion strains showed lower uranium accumulation than the wild-type after exposure to agar medium containing 0.5 mM uranium, and were identified as mutant strains with low uranium accumulation. Among the deleted genes in these strains were cell membrane proteins, phospholipid-binding proteins, and cell wall proteins, suggesting that cell surface proteins contribute to uranium accumulation.

Application of the micro-PIXE technique for analyzing arsenic in biomat and lower plants of lichen and mosses around an arsenic mine site, at Gunma, Japan

Abstract Microhabitats of bacteria (biomat) and lower plants, such as lichen and mosses, are known to accumulate hazardous elements. Since the concentration of hazardous elements in the environment is quite low, we have applied the in-air μ-PIXE (particle induced X-ray emission) system developed in the TIARA facility of JAERI, which has low concentration detection limit of ppm, to measure As, one of the hazardous elements, distributions in biomat, lichen and mosses observed around an abandoned As mine site in Gunma, Japan to elucidate the applicability of these biomat and lower plants as bio-indicators of As. Spatial distributions of As, Fe, Si and S in all biomat, lichen and moss collected within 3 m from the mine entrance indicate that As is localized, and is associated with silicate and Fe-containing compounds. In addition, the intensity ratio of peak area for As to Fe in μ-PIXE spectrum of the moss collected from the concrete wall at 3 m downstream of the mine water discharge position is different from those of the lower plants on the rock near the closed entrance, but is the same as that of biomat formed at the mine water discharge position. This indicates that As trapped by the moss on the concrete wall probably has the same origin as the biomat. It is concluded that application of μ-PIXE analysis to the measurement of As in the lower plants and biomat gives not only the distribution of the hazardous element of As, but also the information of the origin.

Direct accumulation pathway of radioactive cesium to fruit-bodies of edible mushroom from contaminated wood logs.

This paper presents the accumulation process of radioactive Cs in edible mushrooms. We here first report the direct accumulation pathway of radioactive Cs from contaminated wood logs to the fruit-bodies of shiitake mushrooms through the basal portion of the stipe. In this pathway, radioactive Cs is not transported through the hyphae. This pathway results in a high accumulation of radioactive Cs in the fruit-body, more by the excess accumulation of radioactive Cs from the wood logs than that through the hyphae. We grew the fruit-bodies of Shiitake mushroom from radioactive-Cs-contaminated wood logs. The spatial distributions of radioactive Cs and Prussian blue as a tracer of interstitial water in the cross section of the wood log measured after the harvest of the fruit-body from the inoculated sawdust spawn area indicated that some fraction of the radioactive Cs and Prussian blue were transported directly to the basal portion of the stipe during the growth of the fruit-bodies.

Study of optimum condition for synthesis of [γ-32P]ATP with high specific radioactivity

The synthesis of [γ-32P]ATP with high specific radioactivity was investigated by the enzymatic method. We utilized a part of the glycolytic pathway which consists of three reaction steps. First, we examined the maximum radioactivity of H332PO4 without the decrease of labeling yield. Next, we examined the minimum amount of reagent without the decrease of labeling yield, because some reagents contain the phosphorus species as impurity. In this paper, we propose the optimum condition for synthesis of [γ-32P]ATP with high specific radioactivity. As our calculation, we will be able to obtain about 220 TBq/mmol specific radioactivity using 1.5 GBq of H332PO4.