Research Group for Bioactinides ChemistryGroup Leader: Toshihiko Ohnuki
Members: Naofumi Kozai, Fuminori Sakamoto, Takumi Saito, Qianqian Yu, Kazuhiro Shiina,Jun Namekawa, Kenji Tanaka

The research objectives of the group are to elucidate achemical state change of actinides and lanthanides including
nano-particles formation in the biological reaction environments,
and to develop biological remediation system for radiocesium
contaminated land. In the year of 2014, we have studied theeffects of 1) nanoparticle of Ce oxide on microbial activity, 2)
Ce phosphate nanoparticle grown up at the surface of microbial
cells on the sorption by Mn oxide, and 3) minerals on
accumulation of Cs by fungus Saccharomyces cerevisiae.
Metal nanoparticles affect microbial activity
Microbial activity can alter the mobility of radionuclides
through biosorption, biomineralization, or the production ofchelating compounds in the environment [1]. In previous studies,
we demonstrated that microorganisms precipitate nanoparticles
comprising heavy metals such as rare earth elements and uranyl
ions outside the cell and release phosphate from inside the cell.
Although the mechanism of nanoparticle formation bymicroorganisms has been explored in many previous studies, therole of nanoparticles still remain unknown. Specifically, theinfluence of nanoparticles on microbial activity is poorly
understood, even though microbial activity potentially affects
local and global ecosystems.
To understand the effects of nanoparticles on microorganisms,
we examined the effects of CeO 2 nanoparticles (CeNPs) on
yeast focusing on microbial metabolites and intracellular
proteins. The yeast was harvested from a yeast extract-peptonedextrose (YPD) medium containing 0, 10, 100 and 250 ppm ofCeNPs. It was incubated for 120 h in 1mM NaCl solution at
three different pH values: 3, 5 and 7. The yeast released organic
matter, P, K and Mg into the NaCl solution at all pH values,
even without CeNPs. Distinct differences were detected in thereleased organic species and intracellular proteins after exposure
to CeNPs. High-performance liquid chromatography revealed
that the yeast expressed or suppressed various organic species
after exposure to CeNPs. Although CeNPs did not havecytotoxicity for the yeast, the results of the peptide massfingerprint analysis of the intracellular protein revealed that
Eno2p, a glycolysis enzyme, was expressed after exposure toCeNPs. These results suggest that nanoparticles have thepotential to alter microbial metabolism, leading to change of thecomposition of the substances released in the surroundingenvironment.
Ce phosphate nano-particles are participate in Ce oxidation
Sorption of Ce by mixtures of synthetic Mn oxides andmicrobial cells of Pseudomonas fluorescens was examined toelucidate the role of microorganisms on Ce(III) oxidation in theenvironment [2]. The mixtures sorbing Ce by exposure tosolutions containing 1.0x10-4 or 1.0x10-5 mol L-1 Ce(III) wereanalyzed by scanning electron microscopy energy dispersive Xray spectroscopy (SEM-EDS) and micro-X-ray fluorescence(micro-XRF) at synchrotron facilities. A Ce LIII-edge micro
XANES spectra analysis was also performed to determine theoxidation states of Ce adsorbed to the Mn oxides and microbial
cells in the mixtures. The distribution ratios (Kd) of Ce between
the individual solids and solution rose with increasing in pH ofthe solution. The Kd was nearly the same in the mixturescontaining varying amounts of microbial cells. SEM-EDS andmicro-XRF analyses showed that Ce was sorbed by both MnO 2and microbial cells (1.7x10-1 or 3.3x10-1 g·L-1). In addition,nano-particles containing Ce and P grew up on the surface of themicrobial cells. XANES analysis showed that lower fractions ofCe(III) were oxidized to Ce(IV) in the mixtures containing
greater amounts of microbial cells. Micro-XANES analysis
revealed that Ce was present as Ce(III) on the microbial cells
and as Ce(IV) on Mn oxides. These results strongly suggest thatthe association of Ce(III) with the microbial cell surface and theformation of Ce phosphate nano-particles are responsible forsuppressing the oxidation of Ce(III) to Ce(IV) in the mixtures.
Yeast accumulates radioactive Cs competitively withminerals
In concerned with Fukushima Daiichi Nuclear Power Plant
Accident, it is important to elucidate migration behavior ofradioactive Cs deposited on the ground for determination of thedose rate and the fate of the radioactive Cs in the terrestrialenvironment. The migration of radioactive Cs in theenvironment is affected by minerals and fungal microorganisms.
However, the accumulation of radioactive Cs by fungal
microorganisms in the presence of minerals and role ofmicroorganisms in this migration are not completely understood.
The accumulation of Cs by unicellular fungus ofSaccharomyces cerevisiae in the presence of minerals has beenstudied to elucidate the role of microorganisms against themigration of radioactive Cs in the environment. Two types ofexperiments were employed. The one is the experiment using
stable Cs to examine the effect of a carbon source on theaccumulation of Cs. The other one is accumulation experimentof radioactive Cs from agar medium containing 137Cs andmineral supplements such as zeolite, vermiculite and so on. In
the former experiments, the Cs-accumulated cells were analyzed
by scanning electron microscopy equipped with energy
dispersive X-ray analysis (SEM-EDS). In the latter one, theradioactivity in the yeast cells was measured by anautoradiography technique.
When a carbon source was present, higher amounts of Csaccumulated in the cells than in the resting condition without acarbon source. Analysis by SEM-EDS showed that no mineralformed on the cell surface. These results indicate that the yeast
cells accumulate Cs by adsorption on the cell surface andintracellular accumulation. In the case of the presence ofminerals in the agar medium, the radioactivity in the yeast cells
was in the order of mica > smectite, illite >> vermiculite,phlogopite, zeolite. This order is inversely correlated to the ratioof the concentration of radioactive Cs between the minerals andthe medium solution. These results strongly suggest that theyeast accumulates radioactive Cs competitively with minerals.
References[1] K. Pedersen, In: Fredrickson, J.K., Fletcher, M. (Eds.),
Subsurface Microbiology and Biogeochemistry. Wiley Liss Inc.,New York (2001).
[2] T. Ohnuki, Geochim. Cosmochim. Acta 163, 1 (2015).