Combination of the endophytic manganese-oxidizing bacterium Pantoea eucrina SS01 and biogenic Mn oxides: An efficient and sustainable complex in degradation and detoxification of malachite green

Roles of naturally occurring biogenic iron-manganese oxides (BFMO) in PMS-based environmental remediation: A complete electron transfer pathway

Bisphenol A (BPA) is a pervasive endocrine disruptor that enters the environment through anthropogenic activities, posing significant risks to ecosystems and human health. Advanced oxidation processes (AOPs) are promising methods for the removal of organic microcontaminants in the environment. Biogenic manganese oxides (BMO) are reported as catalysts due to their transition metal nature, and are also readily generated by manganese-oxidizing microorganisms in the natural environment, and therefore their roles and effects in AOPs-based environmental remediation should be investigated. However, biogenic iron-manganese oxides (BFMO) are actually generated rather than BMO due to the coexistence of ferrous ions which can be oxidized to iron oxides. Therefore, this study produced BFMO originating from a highly efficient manganese-oxidizing fungus Cladosporium sp. XM01 and chose peroxymonosulfate (PMS) as a typical oxidant for the degradation of bisphenol A (BPA), a model organic micropollutant. Characterization results indicate that the formed BFMO was amorphous with a low crystallinity. The BFMO/PMS system achieved a high degradation performance that 85 % BPA was rapidly degraded within 60 min, and therefore the contribution of BFMO cannot be ignored during PMS-based environmental remediation. Different from the findings of previous studies (mostly radicals and singlet oxygen), the degradation mechanism was first proven as a 100 % electron-transfer pathway mediated by high-valence Mn under acidic conditions provided by PMS. The findings of this study provide new insights into the degradation mechanisms of pollutants using biogenic metal oxides in PMS activation and the contribution of their coexistence in AOPs-based environmental remediation.

Catalase-peroxidase StKatG2 from Salinicola tamaricis: a versatile Mn(II) oxidase that decolorizes malachite green

Manganese (Mn) oxidation processes have garnered significant attention recently due to their potential for degrading organic pollutants. These processes are primarily catalyzed by Mn(II) oxidases. Salinicola tamaricis F01, an endophytic bacterium derived from wetland plants, has demonstrated Mn(II)-oxidizing capacity. In this study, a catalase-peroxidase, StKatG2, was cloned and overexpressed in Escherichia coli from the strain F01. The purified recombinant StKatG2 exhibited Mn(II)-oxidizing activity with K m and K cat values of 2.529 mmol/L and 2.82 min-1, respectively. Optimal catalytic conditions for StKatG2 were observed at pH 7.5 and 55°C, with 45.1% activity retention after an 8-h exposure to 80°C. The biogenic manganese oxides produced by StKatG2 exhibited mixed-valence states with Mn(II), including Mn(III), Mn(IV), and Mn(VII). Furthermore, StKatG2 demonstrated superior decolorization efficiency for malachite green (MG), achieving decolorization rates of 73.38% for 20 mg/L MG and 60.08% for 50 mg/L MG, while degrading MG into 4-(dimethylamino)benzophenone. Therefore, the catalase-peroxidase StKatG2 exhibits multifunctionality in Mn(II)-oxidizing activity and has the potential to serve as an environmentally friendly enzyme for MG removal.

Denitrification performance of the nitrate-dependent manganese redox strain Dechloromonas sp. YZ8 under copper ion (Cu(Ⅱ)) stress: Promotion mechanism and immobilization efficacy

A novel nitrate-dependent manganese (Mn) redox strain was isolated and identified as Dechloromonas sp.YZ8 in this study. The growth conditions of strain YZ8 were optimized by kinetic experiments. The nitrate (NO3--N) removal efficiency was 100.0 % at 16 h at C/N of 2.0, pH of 7.0, and Mn(II) or Mn(IV) addition of 10.0 or 500.0 mg L-1, along with an excellent Mn redox capacity. Transmission electron microscopy supported the Mn redox process inside and outside the cells of strain YZ8. When strain YZ8 was exposed to different concentrations of copper ion (Cu(II)), it turned out that moderate amounts of Cu(II) increased microbial activity and metabolic activities. Moreover, it was discovered that the appropriate amount of Cu(II) promoted the conversion of Mn(IV) and Mn(II) to Mn(III) and improved electron transfer capacity in the Mn redox system, especially the Mn redox process dominated by Mn(IV) reduction. Then, δ-MnO2 and bio-manganese oxides (BMO) produced during the reaction process have strong adsorption of Cu(II). The surface valence changes of δ-MnO2 before and after the reaction and the production of BMO, Mn(III)-rich intermediate black manganese ore (Mn3O4), and Mn secondary minerals together confirmed the Mn redox pathway. The study provided new insights into the promotion mechanism and immobilization effects of redox-coupled denitrification of Mn in groundwater under Cu(II) stress.

Advances in Research on Bacterial Oxidation of Mn(II): A Visualized Bibliometric Analysis Based on CiteSpace

Manganese (Mn) pollution poses a serious threat to the health of animals, plants, and humans. The microbial-mediated Mn(II) removal method has received widespread attention because of its rapid growth, high efficiency, and economy. Mn(II)-oxidizing bacteria can oxidize toxic soluble Mn(II) into non-toxic Mn(III/IV) oxides, which can further participate in the transformation of other heavy metals and organic pollutants, playing a crucial role in environmental remediation. This study aims to conduct a bibliometric analysis of research papers on bacterial Mn(II) oxidation using CiteSpace, and to explore the research hotspots and developmental trends within this field between 2008 and 2023. A series of visualized knowledge map analyses were conducted with 469 screened SCI research papers regarding annual publication quantity, author groups and their countries and regions, journal categories, publishing institutions, and keywords. China, the USA, and Japan published the most significant number of research papers on the research of bacterial Mn(II) oxidation. Research hotspots of bacterial Mn(II) oxidation mainly focused on the species and distributions of Mn(II)-oxidizing bacteria, the influencing factors of Mn(II) oxidation, the mechanisms of Mn(II) oxidation, and their applications in environment. This bibliometric analysis provides a comprehensive visualized knowledge map to quickly understand the current advancements, research hotspots, and academic frontiers in bacterial Mn(II) oxidation.

Characteristics of biological manganese oxides produced by manganese-oxidizing bacteria H38 and its removal mechanism of oxytetracycline

Oxytetracycline (OTC) is widely used in clinical medicine and animal husbandry. Residual OTC can affect the normal life activities of microorganisms, animals, and plants and affect human health. Microbial remediation has become a research hotspot in the environmental field. Manganese oxidizing bacteria (MnOB) exist in nature, and the biological manganese oxides (BMO) produced by them have the characteristics of high efficiency, low cost, and environmental friendliness. However, the effect and mechanism of BMO in removing OTC are still unclear. In this study, Bacillus thuringiensis strain H38 of MnOB was obtained, and the conditions for its BMO production were optimized. The optimal conditions were determined as follows: optimal temperature = 35 °C, optimal pH = 7.5, optimal Mn(Ⅱ) initial concentration = 10 mmol/L. The results show that BMO are irregular or massive, mainly containing MnCO3, Mn2O3, and MnO2, with rich functional groups and chemical bonds. They have the characteristics of small particle size and large specific surface area. OTC (2.5 mg/L) was removed when the BMO dosage was 75 μmol/L and the solution pH was 5.0. The removal ratio was close to 100 % after 12 h of culture at 35 °C and 150 r/min. BMO can adsorb and catalyze the oxidation of OTC and can produce ·O2-, ·OH, 1O2, and Mn(Ⅲ) intermediate. Fifteen products and degradation pathways were identified, and the toxicity of most intermediates is reduced compared to OTC. The removal mechanism was preliminarily clarified. The results of this study are convenient for the practical application of BMO in OTC pollution in water and for solving the harm caused by antibiotic pollution.

Biodegradation and Decolorization of Crystal Violet Dye by Cocultivation with Fungi and Bacteria

Microbial degradation of dyes is vital to understanding the fate of dyes in the environment. In this study, a fungal strain A-3 and a bacterial strain L-6, which were identified as Aspergillus fumigatus and Pseudomonas fluorescens, respectively, had been proven to efficiently degrade crystal violet (CV) dye. The decolorization of CV dye by fungal and bacterial cocultivation was investigated. The results showed that the decolorization rate of cocultures was better than monoculture (P. fluorescens in L-6 (PF), and that of A. fumigatus A-3 (AF)). Furthermore, enzymatic analysis further revealed that Lac, MnP, Lip, and NADH-DCIP reductases were involved in the biodegradation of CV dyes. UV-visible spectroscopy, Fourier transform infrared (FT-IR) spectroscopy, and gas chromatography-mass spectrometry (GC-MS) were used to examine the degradation products. GC-MS analysis showed the presence of 4-(dimethylamino) benzophenone, 3-dimethylaminophenol, benzyl alcohol, and benzaldehyde, indicating that CV was degraded into simpler compounds. The phytotoxicity tests revealed that CV degradation products were less toxic than the parent compounds, indicating that the cocultures detoxified CV dyes. As a result, the cocultures are likely to have a wide range of applications in the bioremediation of CV dyes.

Production of extracellular superoxide radical in microorganisms and its environmental implications: A review

Extracellular superoxide radical (O2•-) is ubiquitous in microbial environments and has significant implications for pollutant transformation. Microbial extracellular O2•- can be produced through multiple pathways, including electron leakage from the respiratory electron transport chain (ETC), NADPH oxidation by the transmembrane NADPH oxidase (NOX), and extracellular reactions. Extracellular O2•- significantly influences the geochemical processes of various substances, including toxic metals and refractory organic pollutants. On one hand, extracellular O2•- can react with variable-valence metals and detoxify certain highly toxic metals, such as As(III), Cr(VI), and Hg(II). On the other hand, extracellular O2•- can directly or indirectly (via Bio-Fenton) degrade many organic pollutants, including a variety of emerging contaminants. In this work, we summarize the production mechanisms of microbial extracellular O2•-, review its roles in the transformation of environmental pollutants, and discuss the potential applications, limiting factors, and future research directions in this field.

A Review of Manganese-Oxidizing Bacteria (MnOB): Applications, Future Concerns, and Challenges

Groundwater serving as a drinking water resource usually contains manganese ions (Mn2+) that exceed drinking standards. Based on the Mn biogeochemical cycle at the hydrosphere scale, bioprocesses consisting of aeration, biofiltration, and disinfection are well known as a cost-effective and environmentally friendly ecotechnology for removing Mn2+. The design of aeration and biofiltration units, which are critical components, is significantly influenced by coexisting iron and ammonia in groundwater; however, there is no unified standard for optimizing bioprocess operation. In addition to the groundwater purification, it was also found that manganese-oxidizing bacteria (MnOB)-derived biogenic Mn oxides (bioMnOx), a by-product, have a low crystallinity and a relatively high specific surface area; the MnOB supplied with Mn2+ can be developed for contaminated water remediation. As a result, according to previous studies, this paper summarized and provided operational suggestions for the removal of Mn2+ from groundwater. This review also anticipated challenges and future concerns, as well as opportunities for bioMnOx applications. These could improve our understanding of the MnOB group and its practical applications.

Towards BioMnOx-mediated intra/extracellular electron shuttling for doxycycline hydrochloride metabolism in Bacillus thuringiensis

Doxycycline hydrochloride (DCH) could be continuously removed by Bacillus thuringiensis S622 with the in-situ biogenic manganese oxide (BioMnOx) via oxidizing/regenerating. The DCH removal rate was significantly increased by 3.01-fold/1.47-fold at high/low Mn loaded via the integration of biological (intracellular/extracellular electron transfer (IET/EET)) and abiotic process (BioMnOx, Mn(III) and •OH). BioMnOx accelerated IET via activating coenzyme Q to enhance electrons transfer (ET) from complex I to complex III, and as an alternative electron acceptor for respiration and provide another electron transfer transmission channel. Additionally, EET was also accelerated by stimulating to secrete flavins, cytochrome c (c-Cyt) and flavin bounded with c-Cyt (Flavins & Cyts). To our best knowledge, this is the first report about the role of BioMnOx on IET/EET during antibiotic biodegradation. These results suggested that Bacillus thuringiensis S622 incorporated with BioMnOx could adopt an alternative strategy to enhance DCH degradation, which may be of biogeochemical and technological significance.

Combined Process of Biogenic Manganese Oxide and Manganese-Oxidizing Microalgae for Improved Diclofenac Removal Performance: Two Different Kinds of Synergistic Effects

Biogenic manganese oxides (Bio-MnOx) have attracted considerable attention for removing pharmaceutical contaminants (PhCs) due to their high oxidation capacity and environmental friendliness. Mn-oxidizing microalgae (MnOMs) generate Bio-MnOx with low energy and organic nutrients input and degrade PhCs. The combined process of MnOMs and Bio-MnOx exhibits good prospects for PhCs removal. However, the synergistic effects of MnOMs and Bio-MnOx in PhCs removal are still unclear. The performance of MnOMs/Bio-MnOx towards diclofenac (DCF) removal was evaluated, and the mechanism was revealed. Our results showed that the Bio-MnOx produced by MnOMs were amorphous nanoparticles, and these MnOMs have a good Mn²⁺ tolerance and oxidation efficiency (80–90%) when the Mn²⁺ concentration is below 1.00 mmol/L. MnOMs/Bio-MnOx significantly promotes DCF (1 mg/L) removal rate between 0.167 ± 0.008 mg/L·d (by MnOMs alone) and 0.125 ± 0.024 mg/L·d (by Bio-MnOx alone) to 0.250 ± 0.016 mg/L·d. The superior performance of MnOMs/Bio-MnOx could be attributed to the continuous Bio-MnOx regeneration and the sharing of DCF degradation intermediates between Bio-MnOx and MnOMs. Additionally, the pathways of DCF degradation by Bio-MnOx and MnOMs were proposed. This work could shed light on the synergistic effects of MnOMs and Bio-MnOx in PhCs removal and guide the development of MnOMs/Bio-MnOx processes for removing DCF or other PhCs from wastewater.