Molecular Cloning and Heterologous Expression of Manganese(II)-Oxidizing Enzyme from Acremonium strictum Strain KR21-2

Formation of Biogenic Manganese Oxide Nodules on Hyphae of a New Fungal Isolate of Periconia That Immobilizes Aqueous Copper

Mn(II)-oxidizing microorganisms are considered to play significant roles in the natural geochemical cycles of Mn and other heavy metals because the insoluble biogenic Mn oxides (BMOs) that are produced by these microorganisms adsorb other dissolved heavy metals and immobilize them as precipitates. In the present study, a new Mn(II)-oxidizing fungal strain belonging to the ascomycete genus Periconia, a well-studied plant-associating fungal genus with Mn(II)-oxidizing activity that has not yet been exami-ned in detail, was isolated from natural groundwater outflow sediment. This isolate, named strain TS-2, was confirmed to oxidize dissolved Mn(II) and produce insoluble BMOs that formed characteristic, separately-located nodules on their hyphae while leaving major areas of the hyphae free from encrustation. These BMO nodules also adsorbed and immobilized dissolved Cu(II), a model analyte of heavy metals, as evidenced by elemental mapping ana-lyses of fungal hyphae-BMO assemblages using a scanning electron microscope with energy-dispersive X-ray spectroscopy (SEM-EDX). Analyses of functional genes within the whole genome of strain TS-2 further revealed the presence of multiple genes predicted to encode laccases/multicopper oxidases that were potentially responsible for Mn(II) oxidation by this strain. The formation of BMO nodules may have functioned to prevent the complete encrustation of fungal hyphae, thereby enabling the control of heavy metal concentrations in their local microenvironments while maintaining hyphal functionality. The present results will expand our knowledge of the physiological and morphological traits of Mn(II)-oxidizing Periconia, which may affect the natural cycle of heavy metals through their immobilization.

Effect of particle size on the colonization of biofilms and the potential of biofilm-covered microplastics as metal carriers

Upon release into the aquatic environment, the surface of microplastics (MPs) can be readily colonized by biofilms, which may enhance the adsorption of contaminants. In this study, industrial-grade polystyrene (PS) of about 4 mm in size (MP4000-1), food-grade PS of about 4 mm in size (MP4000-2), and Powder PS of about 75 μm in size (MP75) were co-cultured with a model freshwater fungus, namely Acremonium strictum strain KR21-2, for seven days to form biofilms on their surface. We also determined the changes in surface physicochemical properties of the biofilm-covered MPs (BMPs) and the heavy metal adsorption capacity of the original MPs and BMPs. The results revealed that the biofilms improve the adsorption of heavy metals on MPs, and the particle size of MPs plays a crucial role in biofilm colonization and adsorption of heavy metals by BMPs. MP75 can carry more biofilm on its surface than that of the two MP4000s and form heteroaggregates with biofilms. In addition, there were more functional groups on the surface of BMP75 than on the surface of the two BMP4000s, which could promote the electrostatic interaction and chemical association of heavy metals. Moreover, BMP75 exhibited a higher capacity to adsorb Cu and reduce Cr (VI), which may be related to the functional groups in its biofilm. Overall, this study showed that after biofilms colonization, BMPs of smaller size have more significant potential as a metal vector, and the particle size deserves more scientific attention during the risk assessment.

Simultaneous Sequestration of Co2+ and Mn2+ by Fungal Manganese Oxide through Asbolane Formation

Biogenic manganese oxides (BMOs) have attractive environmental applications owing to their metal sequestration and oxidizing abilities. Although Co readily accumulates into Mn oxide phases in natural environments, the Co2+ sequestration process that accompanies the enzymatic Mn(II) oxidation of exogenous Mn2+ remains unknown. Therefore, we prepared newly formed BMOs in a liquid culture of Acremonium strictum KR21-2 and conducted repeated sequestration experiments in a Mn2+/Co2+ binary solution at pH 7.0. The sequestration of Co2+ by newly formed BMOs (~1 mM Mn) readily progressed in parallel with the oxidation of exogenous Mn2+, with higher efficiencies than that in single Co2+ solutions when the initial Co2+ concentrations (0.16–0.8 mM) were comparable to or lower than the exogenous Mn2+ concentration (~0.8 mM). This demonstrates a synergetic effect on Co sequestration. Powder X-ray diffraction showed a typical pattern for asbolane only when newly formed BMOs were treated in Mn2+/Co2+ binary systems, implying that the enzymatic Mn(II) oxidation by newly formed BMOs favored asbolane formation. Cobalt K-edge X-ray absorption near-edge structure measurements showed that both Co(II) and Co(III) participated in the formation of the asbolane phase in the binary solutions, whereas most of the primary Co2+ was sequestered as Co(III) in the single Co2+ solutions, which partly explains the synergetic effects on Co sequestration efficiency in the binary solutions. The results presented here provide new insights into the mechanism of Co interaction with Mn oxide phases through asbolane formation by enzymatic Mn(II) oxidation under circumneutral pH conditions.

Mechanisms of Manganese(II) Oxidation by Filamentous Ascomycete Fungi Vary With Species and Time as a Function of Secretome Composition

Manganese (Mn) oxides are among the strongest oxidants and sorbents in the environment, and Mn(II) oxidation to Mn(III/IV) (hydr)oxides includes both abiotic and microbially-mediated processes. While white-rot Basidiomycete fungi oxidize Mn(II) using laccases and manganese peroxidases in association with lignocellulose degradation, the mechanisms by which filamentous Ascomycete fungi oxidize Mn(II) and a physiological role for Mn(II) oxidation in these organisms remain poorly understood. Here we use a combination of chemical and in-gel assays and bulk mass spectrometry to demonstrate secretome-based Mn(II) oxidation in three phylogenetically diverse Ascomycetes that is mechanistically distinct from hyphal-associated Mn(II) oxidation on solid substrates. We show that Mn(II) oxidative capacity of these fungi is dictated by species-specific secreted enzymes and varies with secretome age, and we reveal the presence of both Cu-based and FAD-based Mn(II) oxidation mechanisms in all 3 species, demonstrating mechanistic redundancy. Specifically, we identify candidate Mn(II)-oxidizing enzymes as tyrosinase and glyoxal oxidase in Stagonospora sp. SRC1lsM3a, bilirubin oxidase in Stagonospora sp. and Paraconiothyrium sporulosum AP3s5-JAC2a, and GMC oxidoreductase in all 3 species, including Pyrenochaeta sp. DS3sAY3a. The diversity of the candidate Mn(II)-oxidizing enzymes identified in this study suggests that the ability of fungal secretomes to oxidize Mn(II) may be more widespread than previously thought.

Biogenic manganese oxides combined with 1-hydroxybenzotriazol and an Mn(II)-oxidizing enzyme from Pleosporales sp. Mn1 oxidize 3,4-dimethoxytoluene to yield 3,4-dimethoxybenzaldehyde

Using soil samples, we screened for microbes that produce biogenic manganese oxides (BMOs) and isolated Mn(II)-oxidizing fungus, namely Pleosporales sp. Mn1 (Mn1). We purified the Mn(II)-oxidizing enzyme from intracellular extracts of Mn1. The enzyme oxidized Mn(II) most effectively at pH 7.0 and 45 °C. The N-terminal amino acid sequence of the purified enzyme possessed homology with multicopper oxidases in fungi. The properties of the enzyme and the effects of the pH and inhibitors on the Mn(II)-oxidization activity suggested that the enzyme is a member of the multicopper oxidase family. The X-ray diffraction pattern of the BMOs produced by Mn1 showed a strong correlation with that of a typical poorly crystalized vernadite (δ-MnO₂). Since BMOs are some of the most reactive materials in the environment, we investigated a potential new application of BMOs as oxidation catalysts. We confirmed that BMOs oxidized aromatic methyl groups when combined with the purified enzyme and a mediator, 1-hydroxybenzotriazole (HBT). BMO oxidation of 3,4-dimethoxytoluene achieved a better yield than that of abiotic MnO₂ and white-rot fungus laccase under acidic and neutral pH conditions. Under neutral pH, the BMOs oxidized 3,4-dimethoxytoluene to yield 200-fold more 3,4-dimethoxybenzaldehyde than that of abiotic MnO₂. This is the first report to reveal that BMOs combined with a Mn(II)-oxidizing enzyme and mediator can oxidize aromatic hydrocarbons to yield corresponding aldehydes.

Preferential Elimination of Ba2+ through Irreversible Biogenic Manganese Oxide Sequestration

Biogenic manganese oxides (BMOs) formed in a culture of the Mn(II)-oxidizing fungus Acremonium strictum strain KR21-2 are known to retain enzymatic Mn(II) oxidation activity. Consequently, these are increasingly attracting attention as a substrate for eliminating toxic elements from contaminated wastewaters. In this study, we examined the Ba2+ sequestration potential of enzymatically active BMOs with and without exogenous Mn2+. The BMOs readily oxidized exogenous Mn2+ to produce another BMO phase, and subsequently sequestered Ba2+ at a pH of 7.0, with irreversible Ba2+ sequestration as the dominant pathway. Extended X-ray absorption fine structure spectroscopy and X-ray diffraction analyses demonstrated alteration from turbostratic to tightly stacked birnessite through possible Ba2+ incorporation into the interlayer. The irreversible sequestration of Sr2+, Ca2+, and Mg2+ was insignificant, and the turbostratic birnessite structure was preserved. Results from competitive sequestration experiments revealed that the BMOs favored Ba2+ over Sr2+, Ca2+, and Mg2+. These results explain the preferential accumulation of Ba2+ in natural Mn oxide phases produced by microbes under circumneutral environmental conditions. These findings highlight the potential for applying enzymatically active BMOs for eliminating Ba2+ from contaminated wastewaters.