Hiroshi Tsuno

Manganese(IV) Oxide Production by Acremonium sp. Strain KR21-2 and Extracellular Mn(II) Oxidase Activity

ABSTRACT Ascomycetes that can deposit Mn(III, IV) oxides are widespread in aquatic and soil environments, yet the mechanism(s) involved in Mn oxide deposition remains unclear. A Mn(II)-oxidizing ascomycete, Acremonium sp. strain KR21-2, produced a Mn oxide phase with filamentous nanostructures. X-ray absorption near-edge structure (XANES) spectroscopy showed that the Mn phase was primarily Mn(IV). We purified to homogeneity a laccase-like enzyme with Mn(II) oxidase activity from cultures of strain KR21-2. The purified enzyme oxidized Mn(II) to yield suspended Mn particles; XANES spectra indicated that Mn(II) had been converted to Mn(IV). The pH optimum for Mn(II) oxidation was 7.0, and the apparent half-saturation constant was 0.20 mM. The enzyme oxidized ABTS [2,2′-azinobis(3-ethylbenzothiazoline-6-sulfonic acid)] (pH optimum, 5.5; Km, 1.2 mM) and contained two copper atoms per molecule. Moreover, the N-terminal amino acid sequence (residues 3 to 25) was 61% identical with the corresponding sequence of an Acremonium polyphenol oxidase and 57% identical with that of a Myrothecium bilirubin oxidase. These results provide the first evidence that a fungal multicopper oxidase can convert Mn(II) to Mn(IV) oxide. The present study reinforces the notion of the contribution of multicopper oxidase to microbially mediated precipitation of Mn oxides and suggests that Acremonium sp. strain KR21-2 is a good model for understanding the oxidation of Mn in diverse ascomycetes.

Production of Biogenic Manganese Oxides by Anamorphic Ascomycete Fungi Isolated from Streambed Pebbles

We characterized the production of biogenic Mn oxides by four anamorphic ascomycete fungi isolated from streambed pebbles with Mn oxide coatings. Based on the 18S rRNA gene sequences, one strain was related to members of the order Xylariales and the other three were within distinct lineages of the Pleosporales. These strains oxidized Mn(II) to deposit Mn oxides when their growth approached the stationary phase. The fungal Mn oxides showed X-ray diffraction patterns typical of poorly crystalline vernadite (δ -MnO2), and X-ray absorption near-edge structure spectroscopy confirmed that the Mn phases consisted predominantly of Mn(IV). Mn(II) oxidation in the four strains proceeded enzymatically. The Mn(II)-oxidizing proteins were inhibited by azide and o-phenanthroline, and the proteins also oxidized typical laccase substrates including 2,2′-azinobis(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS), showing the role of laccase or a laccase-like metalloenzyme. The mineralogical traits of the biogenic Mn oxides, and the participation of laccase-like enzymes, are in accordance with our previous results obtained with one Hypocreales ascomycete. In conclusion, phylogenetically diverse ascomycetes may use this common enzymatic system to produce solid Mn phases similar to δ -MnO2.

Influence of multi-electron excitation on EXAFS spectroscopy of trivalent rare-earth ions and elucidation of change in hydration number through the series

Abstract We have made a detailed study of the extended X-ray absorption fine spectra (EXAFS) at the K edge of aqueous Y ion and at L3 edges of aqueous lanthanide ions and thereby elucidated the systematic changes in their hydration structures. Anomalous peaks arising from double-electron excitation (2p, 4d → 5d, 5d) appear in the EXAFS signals of La3+-Tb3+ between 5-7 Å−1. We established a removal process of double-electron excitation from EXAFS spectra. Using that process, we confirmed that the intensity and energy position of the extracted double-electron excitation are comparable to previously reported data. The presence of double-electron excitation engenders a smaller error than the errors estimated in the fitting process. Consequently, double-electron excitation does not seriously affect the determination of the structures of REE3+ aquo ions in the first coordination sphere. Subsequent EXAFS analyses of hydrated REE3+ ions suggest that the hydration numbers, the interatomic distances, and the Debye-Waller factors decrease from 9.7, 2.55 Å, and 9.0 × 10−3 Å2 for La3+ to 7.9, 2.31 Å, and 5.7 × 10−3 Å2 for Lu3+. These parameters change as a sigmoid curve with increasing atomic number. The hydration structures of REE3+ ions are inferred to change from the nonahydrated structure for La3+-Nd3+ to the octahydrated structure for Tb3+-Lu3+ through intermediate structures for Sm3+, Eu3+, and Gd3+. In addition, the hydration state of Y3+ closely resembles that of Ho3+ because the two have almost identical ionic radii.

Coordination study of rare earth elements on Fe oxyhydroxide and Mn dioxides: Part II. Correspondence of structural change to irregular variations of partitioning coefficients and tetrad effect variations appearing in interatomic distances

Abstract Experimental distribution coefficients of rare earth elements (REEs) between Fe oxyhydroxides (FeOOH) and Mn dioxide (δ-MnO2) and solutions [KD(REE)] exhibit anomalous variations: preferential uptake of light REEs by Mn dioxide, a step-like trend in KD(REE) in the Er-Tm-Yb-Lu region, and fractionation of KD(Y) from KD(Ho). Extended X-ray absorption fine structure (EXAFS) spectroscopy was applied to determine coordination states of Er, Tm, Yb, Lu, and Y adsorbed onto FeOOH and δ-MnO2 to assess structural changes around the REE site. The structures obtained, combined with previously determined structures of light REEs-sorbed Fe and Mn samples, corresponded to variations found in KD(REE). The structural parameters in the first coordination sphere suggest that La, Pr, and Nd adsorbed onto δ-MnO2 have a distorted tenfold-coordination sphere and differ greatly from La-, Pr-, Nd-, and Sm-sorbed FeOOH, which have a mixture of eightfold- and ninefold-coordination spheres. In contrast, heavy REEs including Y adsorbed onto Fe and Mn samples’ local structures have an eightfold-coordination sphere. The preferential uptakes of light REEs by δ-MnO2 are explained by the structural change. The irregular variations of heavy REEs and Y fractionation from Ho in KD(REE) do not, however, correspond to any change found in the coordination sphere. During characterization of the first coordination sphere, the W-type tetrad effect appears in the series variation of interatomic distances of REE3+aq and REE-sorbed FeOOH and δ-MnO2. The occurrence of a tetrad effect indicates that the interatomic distances relate not only to the electrostatic field but also to a quantum field.

Coordination study of rare earth elements on Fe oxyhydroxide and Mn dioxides: Part I. Influence of a multi-electron excitation on EXAFS analyses of La, Pr, Nd, and Sm

Abstract Coordination states of rare earth elements (REEs) adsorbed by iron oxyhydroxide (FeOOH) and manganese dioxide (δ-MnO2) (REE = La, Pr, Nd, and Sm) were determined using extended X-ray absorption fine structure (EXAFS) spectroscopy. Multi-electron excitation, resulting in double-electron excitation (2p, 4d → 5d, 5d) for REE-LIII edge EXAFS spectra, possibly causes a considerable error in EXAFS analyses for light REEs. To obtain reliable structural parameters this study elucidates the effects of double-electron excitation on the local structure determination of light REEs on poorly crystallized FeOOH and δ-MnO2. For this study, anomalous features attributable to excitation are superimposed on EXAFS signals of La, Pr, Nd, and Sm samples in the 5.0-7.0 Å-1 k range. The relative intensity of the double-electron excitation to the LIII adsorption edge is <1.1%. Consequently, the double-electron excitation engenders a smaller error than those estimated in fitting for Pr, Nd, and Sm samples. However, significant correction is necessary for the determination of local coordination states of La samples: interatomic distances are 0.007-0.036 Å shorter after correction. The EXAFS analyses of REE-sorbed Fe samples show that adsorbed La, Pr, Nd, and Sm have a mixture of eightfold and ninefold coordination structures and form inner sphere complexes at the FeOOH surface. The determined structural parameters of light REE-sorbed δ-MnO2 suggest that adsorbed light REEs have distorted tenfold-coordination spheres consisting of six short and four long REE-O bonds and form inner sphere complexes. Their coordination structures are more disordered than those of aquo ions and FeOOH samples.

Preliminary Evaluation of Local Structure and Speciation of Lanthanoids in Aqueous Solution, Iron Hydroxide, Manganese Dioxide, and Calcite Using the L3-Edge X-ray Absorption Near Edge Structure Spectra

We elucidate the application of L3-edge X-ray absorption near edge structure (XANES) spectra to the local structural analysis of lanthanoids in aqueous solution, iron hydroxide, manganese dioxide, and calcium carbonate. The L3-edge XANES spectra of lanthanoid compounds showed sharp white lines. The full width at half-maximum (FWHM) values of lanthanoid aqua ions exhibited a convex tetrad curve in the series variation across the lanthanoid series. The variation is attributable to 4f electron orbitals and can be explained by the refined spin-pairing energy theory. For each lanthanoid, the FWHM values of lanthanoid compounds roughly decreased with increasing local coordination numbers. However, they did not faithfully reflect the local coordination sphere of the lanthanoid complex having a high and distorted coordination sphere and were rather sensitive to their chemical forms. The relationship between the magnitude of the FWHM values was determined by the crystal field splitting or degeneracy of 5d orbitals. The systematic variation of FWHM can be explained by the ligand strength of the ligand molecules (-H2O0, -O-, -OH-, -CO32-, -Cl-, and -O2-) that cause the crystal field splitting. Therefore, the FWHM values of L3-edge XANES of lanthanoid compounds may be more useful in speciation analysis rather than structural analysis.