Characterization of pH dependent Mn(II) oxidation strategies and formation of a bixbyite-like phase by Mesorhizobium australicum T-G1

Can xenobiotics support the growth of Mn(II)-oxidizing bacteria (MnOB)? A case of phenol-utilizing bacteria Pseudomonas sp. AN-1

Biogenic manganese oxides (BioMnOx) produced by Mn(II)-oxidizing bacteria (MnOB) have garnered considerable attention for their exceptional adsorption and oxidation capabilities. However, previous studies have predominantly focused on the role of BioMnOx, neglecting substantial investigation into MnOB themselves. Meanwhile, whether the xenobiotics could support the growth of MnOB as the sole carbon source remains uncertain. In this study, we isolated a strain termed Pseudomonas sp. AN-1, capable of utilizing phenol as the sole carbon source. The degradation of phenol took precedence over the accumulation of BioMnOx. In the presence of 100 mg L-1 phenol and 100 µM Mn(II), phenol was entirely degraded within 20 h, while Mn(II) was completely oxidized within 30 h. However, at the higher phenol concentration (500 mg L-1), phenol degradation reduced to 32% and Mn(II) oxidation did not appear to occur. TOC determination confirmed the ability of strain AN-1 to mineralize phenol. Based on the genomic and proteomics studies, the Mn(II) oxidation and phenol mineralization mechanism of strain AN-1 was further confirmed. Proteome analysis revealed down-regulation of proteins associated with Mn(II) oxidation, including MnxG and McoA, with increasing phenol concentration. Notably, this study observed for the first time that the expression of Mn(II) oxidation proteins is modulated by the concentration of carbon sources. This work provides new insight into the interaction between xenobiotics and MnOB, thus revealing the complexity of biogeochemical cycles of Mn and C.

Epidermal microorganisms contributed to the toxic mechanism of nZVI and TCEP in earthworms by robbing metal elements and nutrients

Disrupting effects of pollutants on symbiotic microbiota have been regarded as an important mechanism of host toxicity, with most current research focusing on the intestinal microbiota. In fact, the epidermal microbiota, which participates in the nutrient exchange between hosts and environments, could play a crucial role in host toxicity via community changes. To compare the contributions of intestinal and epidermal symbiotic microorganisms to host toxicity, this study designed single and combined scenarios of soil contamination [nano zero-valent iron (nZVI) and tris (2-chloroethyl) phosphate (TCEP)], and revealed the coupling mechanisms between intestinal/epidermal symbiotic bacterial communities and earthworm toxicological endpoints. Microbiome analysis showed that 15% of intestinal microbes were highly correlated with host endpoints, compared to 45% of epidermal microbes showing a similar correlation. Functional comparisons revealed that key species on the epidermis were mainly heterotrophic microbes with genetic abilities to utilize metal elements and carbohydrate nutrients. Further verifications demonstrated that when facing the co-contamination of nZVI and TCEP, certain symbiotic microorganisms became dominant and consumed zinc, copper, and manganese along with saccharides and amino acids, which may be responsible for the nutritional deficiencies in the host earthworms. The findings can enrich the understanding of the coupling relationship between symbiotic microorganisms and host toxicity, highlighting the importance of epidermal microorganisms in host resistance to environmental pollution.

Manganese Pollution and Its Remediation: A Review of Biological Removal and Promising Combination Strategies

Manganese (Mn), as a cofactor of multiple enzymes, exhibits great significance to the human body, plants and animals. It is also a critical raw material and alloying element. However, extensive employment for industrial purposes leads to its excessive emission into the environment and turns into a significant threat to the ecosystem and public health. This review firstly introduces the essentiality, toxicity and regulation of Mn. Several traditional physicochemical methods and their problems are briefly discussed as well. Biological remediation, especially microorganism-mediated strategies, is a potential alternative for remediating Mn-polluted environments in a cost-efficient and eco-friendly manner. Among them, microbially induced carbonate precipitation (MICP), biosorption, bioaccumulation, bio-oxidation are discussed in detail, including their mechanisms, pivotal influencing factors along with strengths and limitations. In order to promote bioremediation efficiency, the combination of different techniques is preferable, and their research progress is also summarized. Finally, we propose the future directions of Mn bioremediation by microbes. Conclusively, this review provides a scientific basis for the microbial remediation performance for Mn pollution and guides the development of a comprehensive competent strategy towards practical Mn remediation.

Abiotic transformation of atrazine in aqueous phase by biogenic bixbyite-type Mn2O3 produced by a soil-derived Mn(II)-oxidizing bacterium of Providencia sp

Recently, biogenic Mn oxides (BioMnO_x) are considered as the promising degradation agents for environmental organic contaminants. However, little information is available for the degradation of atrazine by BioMnO_x. In this work, BioMnO_x, generated by a soil-derived Mn(II)-oxidizing bacterium, Providencia sp. LLDRA6, was explored to degrade atrazine. To begin with, collective results from mineral characterization analyses demonstrated that this BioMnO_x was biogenic bixbyite-type Mn₂O₃. After that, purified biogenic Mn₂O₃ was found to exhibit a much higher removal efficiency for atrazine in aqueous phase, as compared to unpurified biogenic Mn₂O₃ and LLDRA6 biomass. During the atrazine removal by biogenic Mn₂O₃, six intermediate degradation products were discovered, comprising deethylatrazine (DEA), hydroxylatrazine (HA), deethylhydroxyatrazine (DEHA), ammeline, cyanuric acid, and 5-methylhexahydro-1,3,5-triazine-2-thione (MTT). Particularly, the intermediate, MTT, was considered as a new degradation product of atrazine, which was not described previously. Meanwhile, Mn(II) ions were released from biogenic Mn₂O₃, and on the surface of biogenic Mn₂O₃, the content of hydroxyl O species increased at the expense of that of lattice and water O species, but the fundamental crystalline structure of this Mn oxide remained unchanged. Additionally, no dissociative Mn(III) was found to involve in atrazine degradation. In summary, these results demonstrated that both the non-oxidative and oxidative reactions underlay the degradation of atrazine by biogenic Mn₂O₃.

Mineralogical and Genomic Constraints on the Origin of Microbial Mn Oxide Formation in Complexed Microbial Community at the Terrestrial Hot Spring

Manganese (Mn) oxides are widespread on the surface environments of the modern Earth. The role of microbial activities in the formation of Mn oxides has been discussed for several decades. However, the mechanisms of microbial Mn oxidation, and its role in complex microbial communities in natural environments, remain uncertain. Here, we report the geochemical, mineralogical, and metagenomic evidence for biogenic Mn oxides, found in Japanese hot spring sinters. The low crystallinity of Mn oxides, and their spatial associations with organic matter, support the biogenic origin of Mn oxides. Specific multicopper oxidases (MCOs), which are considered Mn-oxidizing enzymes, were identified using metagenomic analyses. Nanoscale nuggets of copper sulfides were, also, discovered in the organic matter in Mn-rich sinters. A part of these copper sulfides most likely represents traces of MCOs, and this is the first report of traces of Mn-oxidizing enzyme in geological samples. Metagenomic analyses, surprisingly, indicated a close association of Mn oxides, not only in aerobic but also in anaerobic microbial communities. These new findings offer the unique and unified positions of Mn oxides, with roles that have not been ignored, to sustain anaerobic microbial communities in hot spring environments.

Synthesis, optimization, and characterization of biogenic manganese oxide (BioMnOx) by bacterial isolates from mangrove soils with sorbents property towards different toxic metals

Manganese oxidizing bacteria, Bacillus mycoides and Bacillus subtilis were isolated from mangrove soils and optimized for the removal of Mn(II) with simultaneous production of biogenic manganese oxide (BioMnOx). The removal rate of Mn(II) was 90% in 48 h for B. mycoides and 72 h for B. subtilis under the optimized conditions at pH 7, temperature 37 °C, 120 rpm, with 1% inoculum containing 10 mM MnCl₂. Scanning electron microscopy (SEM), Transmission electron microscopy (TEM), Energy dispersive X-Ray analysis (EDAX), Fourier-transform infrared spectroscopy (FT-IR), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM) were used to characterize the synthesized biogenic manganese oxide. BioMnOx by Bacillus mycoides and Bacillus subtilis were identified as Bixbyite (Mn₂O₃) and Hausmannite (Mn₃O₄), respectively, with nano-sized monocrystalline nature. BioMnOx of Bacillus subtilis strain was more efficient in the removal of metals Zn and Co than BioMnOx of Bacillus mycoides except for mercury. The removal property of synthesized BioMnOx could be applied to treat multi-metal containing wastewater.

Manganese Stress Adaptation Mechanisms of Bacillus safensis Strain ST7 From Mine Soil

The mechanism of bacterial adaption to manganese-polluted environments was explored using 50 manganese-tolerant strains of bacteria isolated from soil of the largest manganese mine in China. Efficiency of manganese removal by the isolated strains was investigated using atomic absorption spectrophotometry. Bacillus safensis strain ST7 was the most effective manganese-oxidizing bacteria among the tested isolates, achieving up to 82% removal at a Mn(II) concentration of 2,200 mg/L. Bacteria-mediated manganese oxide precipitates and high motility were observed, and the growth of strain ST7 was inhibited while its biofilm formation was promoted by the presence of Mn(II). In addition, strain ST7 could grow in the presence of high concentrations of Al(III), Cr(VI), and Fe(III). Genome-wide analysis of the gene expression profile of strain ST7 using the RNA-seq method revealed that 2,580 genes were differently expressed under Mn(II) exposure, and there were more downregulated genes (n = 2,021) than upregulated genes (n = 559) induced by Mn stress. KAAS analysis indicated that these differently expressed genes were mainly enriched in material metabolisms, cellular processes, organism systems, and genetic and environmental information processing pathways. A total of twenty-six genes from the transcriptome of strain ST7 were involved in lignocellulosic degradation. Furthermore, after 15 genes were knocked out by homologous recombination technology, it was observed that the transporters, multicopper oxidase, and proteins involved in sporulation and flagellogenesis contributed to the removal of Mn(II) in strain ST7. In summary, B. safensis ST7 adapted to Mn exposure by changing its metabolism, upregulating cation transporters, inhibiting sporulation and flagellogenesis, and activating an alternative stress-related sigB pathway. This bacterial strain could potentially be used to restore soil polluted by multiple heavy metals and is a candidate to support the consolidated bioprocessing community.

Carbamazepine removal by the synergistic effect of manganese-oxidizing microalgae and biogenic manganese oxides

Forty strains of Mn-oxidizing microalgae (MnOMs) with different Mn²⁺ oxidation mechanisms were identified from two aquatic environments. Among them, three strains of isolates (Chlamydomonas sp. WH1-1, Chlamydomonas sp. WH1-4, and Chlorella sp. WH2-5) oxidize Mn²⁺ by increasing the ambient pH and by secreting Mn oxidation factors (e.g., superoxide-production enzymes and/or other Mn oxidases) into the extracellular environment at the same time. In carbamazepine (CBZ) removal by MnOMs and/or Bio-MnOx, the combination of MnOMs and Bio-MnOx significantly increased the CBZ (1 mg/L) removal efficiency from 36.05% (by MnOMs alone) and 20.11% (by Bio-MnOx alone) to 80.13% by two synergistic mechanisms. One of the synergistic mechanisms was confirmed as that the Mn²⁺ was re-oxidized by MnOMs to Bio-MnOx, which can promote the CBZ removal, and another was the mutual exchange of degradation products of CBZ as shared reactants between MnOMs and Bio-MnOx. The degradation intermediates of CBZ were analyzed using high-performance liquid chromatography-tandem mass spectrometry, based on which the CBZ degradation pathway by MnOMs and Bio-MnOx was proposed. These findings expand existing knowledge on the Mn²⁺ oxidation mechanisms of MnOMs, and indicate that MnOMs and their generated Bio-MnOx are promising for the removal of CBZ or other pharmaceutical contaminants from wastewater.

A Critical Review on the Multiple Roles of Manganese in Stabilizing and Destabilizing Soil Organic Matter

Manganese (Mn) is a biologically important and redox-active metal that may exert a poorly recognized control on carbon (C) cycling in terrestrial ecosystems. Manganese influences ecosystem C dynamics by mediating biochemical pathways that include photosynthesis, serving as a reactive intermediate in the breakdown of organic molecules, and binding and/or oxidizing organic molecules through organo-mineral associations. However, the potential for Mn to influence ecosystem C storage remains unresolved. Although substantial research has demonstrated the ability of Fe- and Al-oxides to stabilize organic matter, there is a scarcity of similar information regarding Mn-oxides. Furthermore, Mn-mediated reactions regulate important litter decomposition pathways, but these processes are poorly constrained across diverse ecosystems. Here, we discuss the ecological roles of Mn in terrestrial environments and synthesize existing knowledge on the multiple pathways by which biogeochemical Mn and C cycling intersect. We demonstrate that Mn has a high potential to degrade organic molecules through abiotic and microbially mediated oxidation and to stabilize organic molecules, at least temporarily, through organo-mineral associations. We outline research priorities needed to advance understanding of Mn-C interactions, highlighting knowledge gaps that may address key uncertainties in soil C predictions.

Biochemical properties of CumA multicopper oxidase from plant pathogen, Pseudomonas syringae

Multicopper oxidases have a wide range of substrate specificity to be involved in various physiological reactions. Pseudomonas syringae, a plant pathogenic bacterium, has a multicopper oxidase, CumA. Multicopper oxidases have ability to degrade plant cell wall component, lignin. Once P. syringae enter apoplast and colonize, they start to disrupt plant immunity. Therefore, deeper understanding of multicopper oxidases from plant pathogens, help to invent measures to prevent invasion into plant cell, which bring agricultural benefits. Several biochemical studies have reported lower activity of CumA compared with other multicopper oxidase called CotA. However, the mechanisms underlying the difference in activity have not yet been revealed. In order to acquire insight into them, we conducted a biophysical characterization of PsCumA. Our results show that PsCumA has weak type I copper EPR signal, which is essential for oxidation activity. We propose that difference in the coordination of copper ions may decrease reaction frequency.

Removal of Mn(II) by a nitrifying bacterium Acinetobacter sp. AL-6: efficiency and mechanisms

A nitrifying bacterium Acinetobacter sp. AL-6 showed a high efficiency of 99.05% for Mn(II) removal within 144 h when the Mn(II) concentration was 200 mg L^-1; meanwhile, 64.23% of NH₄⁺-N was removed. With the Mn(II) concentration increased from 25 to 300 mg L^-1, bacterial growth and Mn(II) removal were stimulated. However, due to the electron acceptor competition between Mn(II) oxidation and nitrification reactions, the increase in NH₄⁺-N concentration would inhibit Mn(II) removal. By measuring Mn metabolic form and locating oxidative active factors, it was proved that extracellular oxidation effect played a dominant role in the removal process of Mn(II). The self-regulation of pH during strain metabolism further promoted the occurrence of biological Mn oxidation. Characterization results showed that the Mn oxidation products were tightly attached to the surface of the bacteria in the form of flakes. The product crystal composition (mainly MnO₂ and Mn₂O₃), Mn-O functional group, and element level fluctuations confirmed the biological oxidation information. The changes of -OH, N-H, and -CH₂ groups and the appearance of new functional groups (such as C-H and C-O) provided more possibilities for Mn ion adsorption and bonding.

Enrichment of marine manganese-oxidizing microorganisms using polycaprolactone as a solid organic substrate

OBJECTIVE

Heterotrophic manganese (Mn)-oxidizing microorganisms responsible for biogenic manganese oxide (Bio-MnOx) production are fastidious. Their enrichment is not easily accomplished by merely adding a soluble organic substrate to non-sterile mixed cultures. The objective of this study was to evaluate polycaprolactone (PCL), an aliphatic polyester, as an effective solid organic substrate for the enrichment of marine Mn-oxidizing microorganisms.

RESULTS

We successfully obtained marine microbial enrichment with the capacity for dissolved Mn removal and MnOx production using PCL as a solid organic substrate. The removal of dissolved Mn by the Mn-oxidizing enrichment culture followed first-order kinetics with a rate constant of 0.014 h^-1. 16S rRNA gene amplicon sequencing analysis revealed that the Mn-oxidizing enrichment culture was highly dominated by operational taxonomic units related to the bacterial phyla Cyanobacteria, Planctomycetes, and Proteobacteria.

CONCLUSIONS

Our data demonstrate that PCL can serve as a potential substrate to enrich Mn-oxidizing microorganisms with the ability to produce MnOx under marine conditions.

A combined abiotic oxidation-precipitation process for rapid As removal from high-As(III)-Mn(II) acid mine drainage and low As-leaching solid products

A combination process of Fenton-like and catalytic Mn(II) oxidation via molecular oxygen-induced abio-oxidation of As(III)-Mn(II)-rich acid mine drainage (AMD) is developed to rapidly and efficiently remove As and obtain low As-leaching solids in this study. The effect of pH, temperature, oxygen flow rate and neutralization reagent on As removal was investigated. The results showed that pH was important to As removal efficiency, which achieved maximum in 0.25-2 h, but decreased from ∼100 % to ∼92.6 % with the increase of pH 5-9. pH, temperature and oxygen flow rate played key roles in As(III) oxidation. The increase of As(III) oxidized from 16.8 to 67.1% to 98.6-99.0 % occurred as increasing the pH 5-9, 25-95 °C and oxygen flow rate of 0-2.4 L min^-1. NaOH or Ca(OH)₂ as base was less important to As removal. The mechanism involved Fenton-like reaction between Fe(II) and O₂ for produced Fe(III) (oxy)hydroxide association with As(III + V) and Mn(II), catalytic Mn(II) oxidation for the formation of Mn(III, IV) oxides, and further As(III) oxidation by Mn(III, IV) oxides. As-bearing six-line ferrihydrite was the main solid product for low As-leaching fixation. pH 8, 95 °C and oxygen flow rate of 1.6 L min^-1 were optimal for As removal.

Microbiomes in a manganese oxide producing ecosystem in the Ytterby mine, Sweden: impact on metal mobility

Microbe-mediated precipitation of Mn-oxides enriched in rare earth elements (REE) and other trace elements was discovered in tunnels leading to the main shaft of the Ytterby mine, Sweden. Defining the spatial distribution of microorganisms and elements in this ecosystem provide a better understanding of specific niches and parameters driving the emergence of these communities and associated mineral precipitates. Along with elemental analyses, high-throughput sequencing of the following four subsystems were conducted: (i) water seeping from a rock fracture into the tunnel, (ii) Mn-oxides and associated biofilm; referred to as the Ytterby Black Substance (YBS) biofilm (iii) biofilm forming bubbles on the Mn-oxides; referred to as the bubble biofilm and (iv) fracture water that has passed through the biofilms. Each subsystem hosts a specific collection of microorganisms. Differentially abundant bacteria in the YBS biofilm were identified within the Rhizobiales (e.g. Pedomicrobium), PLTA13 Gammaproteobacteria, Pirellulaceae, Hyphomonadaceae, Blastocatellia and Nitrospira. These taxa, likely driving the Mn-oxide production, were not detected in the fracture water. This biofilm binds Mn, REE and other trace elements in an efficient, dynamic process, as indicated by substantial depletion of these metals from the fracture water as it passes through the Mn deposit zone. Microbe-mediated oxidation of Mn(II) and formation of Mn(III/IV)-oxides can thus have considerable local environmental impact by removing metals from aquatic environments.

A manganese-oxidizing bacterial consortium and its biogenic Mn oxides for dye decolorization and heavy metal adsorption

Manganese (Mn) contamination is a common environmental problem in the world and manganese oxidizing bacteria (MOB) play important roles in bioremediation of heavy metal and organic pollution. In this study, a novel MOB consortium AS containing core microbes of Sphingobacterium and Bacillus was acclimated from Mn-contaminated rivulet sediments. The MOB consortium AS presented good Mn(II) removal performance under 500-10,000 mg/L Mn(II), with Mn(II) removal capacities ranging from 481 to 3478 mg/L. In coexistence systems of Mn(II) and Fe(II), Ni(II), Cu(II), and Zn(II), the MOB consortium AS removed 98%, 91%, 99%, and 76% of Mn(II), respectively. Additionally, the MOB consortium AS could utilize multiple carbon sources (e.g., Chitosan, β-Cyclodextrin, and Phenanthrene) to remove Mn(II), with Mn(II) removal efficiencies ranging from 11% to 97%. Meanwhile, XRD, XPS, FTIR, SEM, and EDS analyses reflected that biogenic Mn oxides (bio-MnOx-C) contained C, O, Mn (Mn(II) and Mn(IV)) and embodied in rhodochrosite and birnessite. The bio-MnOx-C exhibited second-order kinetic reaction for removal of dye, with corresponding decolorization capacities of 22.0 mg/g for methylene blue and 23.8 mg/g for crystal violet. In addition, bio-MnOx-C showed adsorption capacities of 159.0 mg/g for Cu(II), 130.7 mg/g for Zn(II), and 123.3 mg/g for Pb(II). Overall, this study illustrates consortium AS and bio-MnOx-C have great potentials in remediation of pollution caused by heavy metals and organic pollutants.

Mn oxide formation by phototrophs: Spatial and temporal patterns, with evidence of an enzymatic superoxide-mediated pathway

Manganese (Mn) oxide minerals influence the availability of organic carbon, nutrients and metals in the environment. Oxidation of Mn(II) to Mn(III/IV) oxides is largely promoted by the direct and indirect activity of microorganisms. Studies of biogenic Mn(II) oxidation have focused on bacteria and fungi, with phototrophic organisms (phototrophs) being generally overlooked. Here, we isolated phototrophs from Mn removal beds in Pennsylvania, USA, including fourteen Chlorophyta (green algae), three Bacillariophyta (diatoms) and one cyanobacterium, all of which consistently formed Mn(III/IV) oxides. Isolates produced cell-specific oxides (coating some cells but not others), diffuse biofilm oxides, and internal diatom-specific Mn-rich nodules. Phototrophic Mn(II) oxidation had been previously attributed to abiotic oxidation mediated by photosynthesis-driven pH increases, but we found a decoupling of Mn oxide formation and pH alteration in several cases. Furthermore, cell-free filtrates of some isolates produced Mn oxides at specific time points, but this activity was not induced by Mn(II). Manganese oxide formation in cell-free filtrates occurred via reaction with the oxygen radical superoxide produced by soluble extracellular proteins. Given the known widespread ability of phototrophs to produce superoxide, the contribution of phototrophs to Mn(II) oxidation in the environment may be greater and more nuanced than previously thought.

Coupling the phenolic oxidation capacities of a bacterial consortium and in situ-generated manganese oxides in a moving bed biofilm reactor (MBBR)

Phenolic wastewater containing phenol and 4-chlorophenol pose a risk to the environment and to human health. Treating them using chemical-biological coupling method is challenging. In this study, manganese oxidizing bacteria (MnOB) were enriched in moving bed biofilm reactor (MBBR) using synthetic phenol wastewater (800 mg L^-1) to facilitate in situ production of biogenic manganese oxides (BioMnOx) after 90 days of operation. Then, 4-chlorophenol (4-CP) was added to the MBBR to simulate mixed phenolic wastewater. Comparing the MBBR (R1) without feeding Mn(II) and the MBBR with BioMnOx (R2) production, R2 exhibited robust phenol and 4-CP removal performance. 16S rRNA gene sequencing was employed to determine the microbial community. Subsequently, a batch experiment demonstrated that partly purified BioMnOx does not exhibits a capacity for phenol removal, but can efficiently remove 4-CP. Interestingly, 5-chloro-2-hydroxymuconic semialdehyde was found in the products of 4-CP degradation, which was the unique product of 4-CP degradation by catechol 2,3-dioxygenase (C23O). In both reactors, only catechol 1,2-dioxygenase (C12O) activity from microbes can be detected, indicating that the existence of BioMnOx provide an alternative pathway in addition to microbe driven 4-CP degradation. Overall, MBBR based MnOB enrichment under high phenol concentration was achieved, and 4-CP/phenol removal can be accelerated by in situ-formed BioMnOx. Considering the C23O-like activity of BioMnOx, our results suggest a new coupling strategy that involves nanomaterials and a microbial consortium.

Natural Attenuation of Mn(II) in Metal Refinery Wastewater: Microbial Community Structure Analysis and Isolation of a New Mn(II)-Oxidizing Bacterium Pseudomonas sp. SK3

Natural attenuation of Mn(II) was observed inside the metal refinery wastewater pipeline, accompanying dark brown-colored mineralization (mostly MnIVO2 with some MnIII2O3 and Fe2O3) on the inner pipe surface. The Mn-deposit hosted the bacterial community comprised of Hyphomicrobium sp. (22.1%), Magnetospirillum sp. (3.2%), Geobacter sp. (0.3%), Bacillus sp. (0.18%), Pseudomonas sp. (0.03%), and non-metal-metabolizing bacteria (74.2%). Culture enrichment of the Mn-deposit led to the isolation of a new heterotrophic Mn(II)-oxidizer Pseudomonas sp. SK3, with its closest relative Ps. resinovorans (with 98.4% 16S rRNA gene sequence identity), which was previously unknown as an Mn(II)-oxidizer. Oxidation of up to 100 mg/L Mn(II) was readily initiated and completed by isolate SK3, even in the presence of high contents of MgSO4 (a typical solute in metal refinery wastewaters). Additional Cu(II) facilitated Mn(II) oxidation by isolate SK3 (implying the involvement of multicopper oxidase enzyme), allowing a 2-fold greater Mn removal rate, compared to the well-studied Mn(II)-oxidizer Ps. putida MnB1. Poorly crystalline biogenic birnessite was formed by isolate SK3 via one-electron transfer oxidation, gradually raising the Mn AOS (average oxidation state) to 3.80 in 72 h. Together with its efficient in vitro Mn(II) oxidation behavior, a high Mn AOS level of 3.75 was observed with the pipeline Mn-deposit sample collected in situ. The overall results, including the microbial community structure analysis of the pipeline sample, suggest that the natural Mn(II) attenuation phenomenon was characterized by robust in situ activity of Mn(II) oxidizers (including strain SK3) for continuous generation of Mn(IV). This likely synergistically facilitated chemical Mn(II)/Mn(IV) synproportionation for effective Mn removal in the complex ecosystem established in this artificial pipeline structure. The potential utility of isolate SK3 is illustrated for further industrial application in metal refinery wastewater treatment processes.

Ripening of bananas using Bowdichia virgilioides Kunth leaves

Bananas are usually ripened with calcium carbide (CaC2), a dangerous substance that can cause food poisoning. The objective was to test the empirical ripening banana method using Bowdichia virgilioides leaves compared to carbide. Ripening tests were carried out using 'Pacovan' banana fruits with B. virgilioides leaves and carbide following the empirical method used by Borborema farmers, Paraíba, Brazil. Bowdichia virgilioides leaves induced increased respiration and ascorbic acid production and reduced acidity, chlorophyll and pH in banana fruits like CaC2. Leaves of B. virgilioides induce ripening of 'Pacovan' banana with safer and same results than with CaC2.

A review of the implications and challenges of manganese removal from mine drainage

Manganese (Mn) is the third most abundant transition metal in the Earth's crust. Decades of increasing worldwide mining activities have inevitably led to the release of large amounts of this metal into the environment. Mine drainage, either acidic or neutral, often contains high levels of Mn, which have potentially detrimental effects on ecosystems and receiving water bodies. This review provides a comprehensive assessment of the main implications and challenges of Mn treatment in mine drainage. With this aim, the beneficial and adverse effects of Mn on ecosystems and human health are presented first. A comparison of background and mine effluents Mn contents is also provided, further stressing the need for Mn removal from mine drainage. Several technical options to address Mn contamination in acid and neutral mine drainage, and the challenges associated with Mn removal, are subsequently discussed. Thus, this paper presents up-to-date knowledge on the available physicochemical and biological processes deemed operative in Mn removal during mine drainage treatment and their limitations considering the distinctive behavior of Mn. The discussion is further extended to passive treatment systems, which are the most commonly implemented systems for mine drainage treatment on abandoned or closed mine sites, and highlights both their design criteria and operation requirements, as well as the factors that influence Mn removal efficiency. Finally, new perspectives on future research and development needs are identified to address the challenges in Mn removal during mine drainage treatment.

Characterization of manganese oxidation by Brevibacillus at different ecological conditions

Bacterial Mn(II) oxidation plays an important role in the biogeochemical cycling of manganese and many trace metals. This study describes Mn(II) oxidation by two isolated manganese (Mn)-oxidizing strains that were identified and assigned as Brevibacillus brevis MO1 and Brevibacillus parabrevis MO2 based on physiochemical and phylogenetic characterizations. The ecological conditions influenced Mn(II) oxidation by both strains. Mn(II) stimulated the growth of strain MO2 while slightly inhibiting strain MO1. Mn(II)-oxidizing activity of two strains was enhanced with increase of initial pH, and maximum Mn(II)-oxidizing activity occurred at pH 8 for both strains (93.5%-94.0%). Brevibacillus showed the capability of mesophilic and psychrophilic Mn(II) oxidation. X-ray photoelectron spectroscopy (XPS) analysis indicated that the biogenic manganese oxides had an intermediate valence between 3 and 4. These results demonstrated that Brevibacillus, which is capable of oxidizing dissolved Mn(II), will be a suitable strain for exploring the mechanism of manganese oxidation in engineered and natural environments.

From urban municipalities to polar bioremediation: the characterisation and contribution of biogenic minerals for water treatment

Minerals of biological origin have shown significant potential for the separation of contaminants from water worldwide. This study details the contribution of biologically derived minerals to water treatment operations, with a focus on filtration media from urban municipalities and remote cold regions. The results support biofilm-embedded iron and manganese to be the building blocks of biogenic mineral development on activated carbon and nutrient-amended zeolites. The presence of similar iron and manganese oxidising bacterial species across all filter media supports the analogous morphologies of biogenic minerals between sites and suggests that biological water treatment processes may be feasible across a range of climates. This is the first time the stages of biogenic mineral formation have been aligned with comprehensive imaging of the biofilm community and bacterial identification; especially with respect to cold regions. Where biogenic mineral formation occurs on filter media, the potential exists for enhanced adsorption for a range of organic and inorganic contaminants and improved longevity of filter media beyond the adsorption or exchange capacities of the raw material.

Manganese in Marine Microbiology

The importance of manganese in the physiology of marine microbes, the biogeochemistry of the ocean and the health of microbial communities of past and present is emerging. Manganese is distributed widely throughout the global ocean, taking the form of an essential antioxidant (Mn²⁺), a potent oxidant (Mn³⁺) and strong adsorbent (Mn oxides) sequestering disproportionately high levels of trace metals and nutrients in comparison to the surrounding seawater. Manganese is, in fact, linked to nearly all other elemental cycles and intricately involved in the health, metabolism and function of the ocean's microbiome. Here, we briefly review the diversity of microbes and pathways responsible for the transformation of Mn within the three Mn pools and their distribution within the marine environment. Despite decades of interrogation, we still have much to learn about the players, mechanisms and consequences of the Mn cycle, and new and exciting discoveries are being made at a rapid rate. What is clear is the dynamic and ever-inspiring complexity of reactions involving Mn, and the acknowledgement that microorganisms are the catalytic engine driving the Mn cycle.

Effects of Manganese Oxide on Arsenic Reduction and Leaching from Contaminated Floodplain Soil

Reductive release of the potentially toxic metalloid As from Fe(III) (oxyhydr)oxides has been identified as an important process leading to elevated As porewater concentrations in soils and sediments. Despite the ubiquitous presence of Mn oxides in soils and their oxidizing power toward As(III), their impact on interrelated As, Fe, and Mn speciation under microbially reducing conditions remains largely unknown. For this reason, we employed a column setup and X-ray absorption spectroscopy to investigate the influence of increasing birnessite concentrations (molar soil Fe-to-Mn ratios: 4.8, 10.2, and 24.7) on As speciation and release from an As-contaminated floodplain soil (214 mg As/kg) under anoxic conditions. Our results show that birnessite additions significantly decreased As leaching. The reduction of both As and Fe was delayed, and As(III) accumulated in birnessite-rich column parts, indicating the passivation of birnessite and its transformation products toward As(III) oxidation and the precipitation of Fe(III)(oxyhydr)oxides. Microbial Mn reduction resulted in elevated soil pH values, which in turn lowered the microbial activity in the birnessite-enriched soil. We conclude that in Mn-oxide-rich soil environments undergoing redox fluctuations, the enhanced As adsorption to newly formed Fe(III) (oxyhydr)oxides under reducing conditions leads to a transient stabilization of As.

Production of Manganese Oxide Nanoparticles by Shewanella Species

Several species of the bacterial genus Shewanella are well-known dissimilatory reducers of manganese under anaerobic conditions. In fact, Shewanella oneidensis is one of the most well studied of all metal-reducing bacteria. In the current study, a number of Shewanella strains were tested for manganese-oxidizing capacity under aerobic conditions. All were able to oxidize Mn(II) and to produce solid dark brown manganese oxides. Shewanella loihica strain PV-4 was the strongest oxidizer, producing oxides at a rate of 20.3 mg/liter/day and oxidizing Mn(II) concentrations of up to 9 mM. In contrast, S. oneidensis MR-1 was the weakest oxidizer tested, producing oxides at 4.4 mg/liter/day and oxidizing up to 4 mM Mn(II). Analysis of products from the strongest oxidizers, i.e., S loihica PV-4 and Shewanella putrefaciens CN-32, revealed finely grained, nanosize, poorly crystalline oxide particles with identical Mn oxidation states of 3.86. The biogenic manganese oxide products could be subsequently reduced within 2 days by all of the Shewanella strains when culture conditions were made anoxic and an appropriate nutrient (lactate) was added. While Shewanella species were detected previously as part of manganese-oxidizing consortia in natural environments, the current study has clearly shown manganese-reducing Shewanella species bacteria that are able to oxidize manganese in aerobic cultures.

IMPORTANCE

Members of the genus Shewanella are well known as dissimilatory manganese-reducing bacteria. This study shows that a number of species from Shewanella are also capable of manganese oxidation under aerobic conditions. Characterization of the products of the two most efficient oxidizers, S. loihica and S. putrefaciens, revealed finely grained, nanosize oxide particles. With a change in culture conditions, the manganese oxide products could be subsequently reduced by the same bacteria. The ability of Shewanella species both to oxidize and to reduce manganese indicates that the genus plays a significant role in the geochemical cycling of manganese. Due to the high affinity of manganese oxides for binding other metals, these bacteria may also contribute to the immobilization and release of other metals in the environment.

Biological Low-pH Mn(II) Oxidation in a Manganese Deposit Influenced by Metal-Rich Groundwater

The mechanisms, key organisms, and geochemical significance of biological low-pH Mn(II) oxidation are largely unexplored. Here, we investigated the structure of indigenous Mn(II)-oxidizing microbial communities in a secondary subsurface Mn oxide deposit influenced by acidic (pH 4.8) metal-rich groundwater in a former uranium mining area. Microbial diversity was highest in the Mn deposit compared to the adjacent soil layers and included the majority of known Mn(II)-oxidizing bacteria (MOB) and two genera of known Mn(II)-oxidizing fungi (MOF). Electron X-ray microanalysis showed that romanechite [(Ba,H2O)2(Mn(4+),Mn(3+))5O10] was conspicuously enriched in the deposit. Canonical correspondence analysis revealed that certain fungal, bacterial, and archaeal groups were firmly associated with the autochthonous Mn oxides. Eight MOB within the Proteobacteria, Actinobacteria, and Bacteroidetes and one MOF strain belonging to Ascomycota were isolated at pH 5.5 or 7.2 from the acidic Mn deposit. Soil-groundwater microcosms demonstrated 2.5-fold-faster Mn(II) depletion in the Mn deposit than adjacent soil layers. No depletion was observed in the abiotic controls, suggesting that biological contribution is the main driver for Mn(II) oxidation at low pH. The composition and species specificity of the native low-pH Mn(II) oxidizers were highly adapted to in situ conditions, and these organisms may play a central role in the fundamental biogeochemical processes (e.g., metal natural attenuation) occurring in the acidic, oligotrophic, and metalliferous subsoil ecosystems.

IMPORTANCE

This study provides multiple lines of evidence to show that microbes are the main drivers of Mn(II) oxidation even at acidic pH, offering new insights into Mn biogeochemical cycling. A distinct, highly adapted microbial community inhabits acidic, oligotrophic Mn deposits and mediates biological Mn oxidation. These data highlight the importance of biological processes for Mn biogeochemical cycling and show the potential for new bioremediation strategies aimed at enhancing biological Mn oxidation in low-pH environments for contaminant mitigation.