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.