The role of dissimilatory arsenate reducing bacteria in the biogeochemical cycle of arsenic based on the physiological and functional analysis of Aeromonas sp. O23A

Hydrogen gas oxidation-driven reductive mobilization of arsenic in solid phase contributing to arsenite contamination in groundwater: Insights from metagenomic and microcosm analyses

Hydrogen gas (H2) is naturally produced by biological and non-biological reactions in various environmental niches. However, the influence of H2 on microbial processes that cause the mobilization and release of arsenic from solid phase into groundwater remains to be resolved. Given that dissimilatory arsenate [As(V)]-respiring prokaryotes (DARPs) have been demonstrated to significantly contribute to the formation of As-contaminated groundwater, our study specifically examined the interactions between H2 and DARPs. We prepared an enriched DARP population from As-contaminated soils. Metagenomic analyses of the DARP population revealed that approximately 46.7 % of the qualified DARPs' MAGs contain at least one type I Ni-Fe hydrogenase. The Ni-Fe hydrogenase proteins in DARPs show unique diversity. Functional assays indicate that the DARP population exhibited notable activity in oxidizing H2 while concurrently reducing As(V) under strictly anaerobic conditions. Arsenic release assays indicate that the DARP population is highly proficient at catalyzing the reductive mobilization of arsenic in scorodite, using hydrogen as the electron donor. These findings offer the initial evidence that H2 can directly promote the formation of arsenic-contaminated groundwater mediated by DARPs, a biogeochemical process that has long been overlooked. Therefore, this study increases our insight into the microbial mechanisms involved in the formation of arsenic-contaminated groundwater.

Antimony resistant bacteria isolated from Budúcnosť adit (Pezinok-Kolársky vrch deposit) in western Slovakia

Potentially toxic elements (PTE), such as antimony (Sb), are dangerous putative contaminants for ground and surface waters around abandoned mines and ore deposits in Slovakia. Nearby mines antimony is commonly coprecipitated in ochre sediments precipitated from Fe-rich drainage waters and, therefore, these sites function as natural scavengers of this metalloid. Bacteria are well known to contribute to the process of redox state maintenance, biosorption and bioaccumulation of antimony and, consequently, to antimony precipitation or release from iron oxides complexes. Here we isolated 48 bacterial strains from circumneutral hydrous ferric oxides (HFO) rich iron ochres accumulated in the waters running from tailing pounds nearby Budúcnosť mine, Pezinok, Slovakia and polluted with high, but fluctuating, concentrations of antimony (130 μg/l Sb in water and 2317 mg/kg Sb in iron ochre in average). The isolated strains were V1-V9 16S rRNA sequenced and the resulting taxonomic affiliations of isolated strains were compared with taxonomy assignments obtained by V4 16S rRNA next generation sequencing approach, including two independent NGS analysis pipelines and different taxonomy classifiers ((IDTAXA (RDP, GTDB, SILVA, CONTAX), MEGAN (NCBI), RDP a SILVAngs). A Sb resistant subgroup of isolated strains (Pseudomonas A60B, Pseudomonas A59, Pseudomonas A28, Aeromonas A21, Aeromonas A13, Aeromonas A60A, Acinetobacter A14, Buttiauxella A58, Shewanella A20A a Yersinia A68), well growing at high Sb concentration (300 mg/l Sb), was tested for an ability of the strains to retain Sb from cultivation media. Based on ICP-MS measurements of the dried biomasses we concluded that all the strains can retain antimony from growth media to some extent, with strains Shewanella A20A, Buttiauxella A58, Yersinia A68 and Aeromonas A60A being the most effective.

Controlling factors of iron plaque formation and its adsorption of cadmium and arsenic throughout the entire life cycle of rice plants

Iron (Fe) plaque, which forms on the surface of rice roots, plays a crucial role in immobilizing heavy metal(loids), thus reducing their accumulation in rice plants. However, the principal factors influencing Fe plaque formation and its adsorption capacity for heavy metal(loid)s throughout the rice plant's lifecycle remain poorly understood. Thus, this study investigated the dynamics of Fe plaque formation and its ability to adsorb cadmium (Cd) and arsenic (As) across different growth stages, aiming to identify the key drivers behind these processes. The findings reveal that the rate of radial oxygen loss (ROL) and the abundance of plaque-associated microbes are the primary drivers of Fe plaque formation, with their relative importance ranging from 1.4% to 81%. Similarly, the adsorption of As by Fe plaque is principally determined by the rate of ROL and the quantity of Fe plaque, with subsequent effects from the total Fe in rhizospheric soil, arsenate-reducing bacteria, and organic matter-degrading bacteria. The relative importance of these factors ranges from 6.0% to 11.7%. By contrast, the adsorption of Cd onto Fe plaque is primarily affected by competition for adsorption sites with ammonium in soils and the presence of organic matter-degrading bacteria, contributing 25.5% and 23.5% to the adsorption process, respectively. These findings provide significant insights into the development of Fe plaque and its absorption of heavy metal(loid)s throughout the lifecycle of rice plants.

Bioprospecting CAZymes repertoire of Aspergillus fumigatus for eco-friendly value-added transformations of agro-forest biomass

BACKGROUND

Valorizing waste residues is crucial to reaching sustainable development goals and shifting from a linear fossil-based economy to a circular economy. Fungal cell factories, due to their versatility and robustness, are instrumental in driving the bio-transformation of waste residues. The present work isolated a potent strain, i.e., Aspergillus fumigatus (ZS_AF), from an ancient Złoty Stok gold mine, which showcased distinctive capabilities for efficient hydrolytic enzyme production from lignocellulosic wastes.

RESULTS

The present study optimized hydrolytic enzyme production (cellulases, xylanases, and β-glucosidases) from pine sawdust (PSD) via solid-state fermentation using Aspergillus fumigatus (ZS_AF). The optimization, using response surface methodology (RSM), produced a twofold increase with maximal yields of 119.41 IU/gds for CMCase, 1232.23 IU/gds for xylanase, 63.19 IU/gds for β-glucosidase, and 31.08 IU/gds for FPase. The secretome profiling validated the pivotal role of carbohydrate-active enzymes (CAZymes) and auxiliary enzymes in biomass valorization. A total of 77% of carbohydrate-active enzymes (CAZymes) were constituted by glycoside hydrolases (66%), carbohydrate esterases (9%), auxiliary activities (3%), and polysaccharide lyases (3%). The saccharification of pretreated wheat straw and PSD generated high reducing sugar yields of 675.36 mg/g and 410.15 mg/g, respectively.

CONCLUSION

These findings highlight the significance of an efficient, synergistic, and cost-effective arsenal of fungal enzymes for lignocellulosic waste valorization and their potential to contribute to waste-to-wealth creation through solid-waste management. The utilization of Aspergillus fumigatus (ZS_AF) from an unconventional origin and optimization strategies embodies an innovative approach that holds the potential to propel current waste valorization methods forward, directing the paradigm toward improved efficiency and sustainability.

Microbial biochemical pathways of arsenic biotransformation and their application for bioremediation

Arsenic is a ubiquitous toxic metalloid, the concentration of which is beyond WHO safe drinking water standards in many areas of the world, owing to many natural and anthropogenic activities. Long-term exposure to arsenic proves lethal for plants, humans, animals, and even microbial communities in the environment. Various sustainable strategies have been developed to mitigate the harmful effects of arsenic which include several chemical and physical methods, however, bioremediation has proved to be an eco-friendly and inexpensive technique with promising results. Many microbes and plant species are known for arsenic biotransformation and detoxification. Arsenic bioremediation involves different pathways such as uptake, accumulation, reduction, oxidation, methylation, and demethylation. Each of these pathways has a certain set of genes and proteins to carry out the mechanism of arsenic biotransformation. Based on these mechanisms, various studies have been conducted for arsenic detoxification and removal. Genes specific for these pathways have also been cloned in several microorganisms to enhance arsenic bioremediation. This review discusses different biochemical pathways and the associated genes which play important roles in arsenic redox reactions, resistance, methylation/demethylation, and accumulation. Based on these mechanisms, new methods can be developed for effective arsenic bioremediation.

Comparative genomic analysis provides insights into taxonomy and temperature adaption of Aeromonas salmonicida

Aeromonas salmonicida has long been known as psychrophiles since it is mainly isolated from cold water fish, and recent reports have revealed the existence of mesophilic strains isolated from warm sources. However, the genetic differences between mesophilic and psychrophilic strains remain unclear due to few complete genomes of mesophilic strain are available. In this study, six A. salmonicida (2 mesophilic and 4 psychrophilic) were genome-sequenced, and comparative analyses of 25 A. salmonicida complete genomes were conducted. The ANI values and phylogenetic analysis revealed that 25 strains formed three independent clades, which were referred as typical psychrophilic, atypical psychrophilic and mesophilic groups. Comparative genomic analysis showed that two chromosomal gene clusters, related to lateral flagella and outer membrane proteins (A-layer and T2SS proteins), and insertion sequences (ISAs4, ISAs7 and ISAs29) were unique to the psychrophilic groups, while the complete MSH type IV pili were unique to the mesophilic group, all of which may be considered as lifestyle-related factors. The results of this study not only provide new insights into the classification, lifestyle adaption and pathogenic mechanism of different strains of A. salmonicida, but also contributes to the prevention and control of disease caused by psychrophilic and mesophilic A. salmonicida.

Functional genomic analysis of an efficient indole degrading bacteria strain Alcaligenes faecalis IITR89 and its biodegradation characteristics

Indole is a nitrogenous heterocyclic aromatic pollutant often detected in various environments. An efficient indole degrading bacterium strain IITR89 was isolated from River Cauvery, India, and identified as Alcaligenes faecalis subsp. phenolicus. The bacterium was found to degrade ~ 95% of 2.5 mM (293.75 mg/L) of indole within 18 h utilizing it as a sole carbon and energy source. Based on metabolite identification, the metabolic route of indole degradation is indole → (indoxyl) → isatin → (anthranilate) → salicylic acid → (catechol) → (Acetyl-CoA) → and further entering into TCA cycle. Genome sequencing of IITR89 revealed the presence of gene cluster dmpKLMNOP, encoding multicomponent phenol hydroxylase; andAbcd gene cluster, encoding anthranilate 1,2-dioxygenase ferredoxin subunit (andAb), anthranilate 1,2-dioxygenase large subunit (andAc), and anthranilate 1,2-dioxygenase small subunit (andAd); nahG, salicylate hydroxylase; catA, catechol 1,2-dioxygenase; catB, cis, cis-muconate cycloisomerase; and catC, muconolactone D-isomerase which play an active role in indole degradation. The findings strongly support the degradation potential of strain IITR89 and its possible application for indole biodegradation.

Geogenic arsenic and arsenotrophic microbiome in groundwater from the Hetao Basin

High arsenic (As) in groundwater is an environmental issue of global concern, which is closely related to microbe-mediated As biogeochemical cycling. However, the distribution of genes related to As cycling and underlying microbial As biogeochemical processes in high As groundwater remain elusive. Hence, we profiled the As cycling genes (arsC, arrA, and aioA genes) and indigenous microbial communities in groundwater from a typical high As area, the Hetao Basin from China, using amplicon sequencing and qPCR techniques. Here, we revealed the significant difference in microbial community structure between low As groundwater samples (LG) and high As groundwater samples (HG). Acinetobacter, Thiovirga, Hydrogenophaga, and Sulfurimonas were dominant in LG, while Aquabcterium, Acinetobacter, Sphingomonas, Pseudomonas, Desulfomicrobium, Hydrogenophaga, and Nitrospira were predominant in HG. Shannon and Chao indices of the microbial communities in HG were significantly higher than those of in LG. Alpha diversity and abundance of arsC and arrA genes were higher than those of aioA genes. The significant positive correlation was uncovered between the abundances of arsC and aioA genes, suggesting the cooccurrence of As functional genes in groundwater. Sphingopyxis, Agrobacterium, Klebsiella, Hoeflea, and Aeromonas represented the dominant taxa within the As (V) reducers communities. Distance-based redundancy analysis showed that ORP, pH, As_tot, Mn, and DOC were the key factors shaping the diverse microbial populations, while ORP, S^2-, As(III), Fe(II), NH₄⁺, pH, Mn, SO₄^2-, As(V), temperature, and P as the main drivers affecting arsenotrophic microbiota. This work provides an insight into microbial communities linked to As biogeochemical processes in high As groundwater, playing a fundamental role in groundwater As cycling.

Characterization of Three Novel Virulent Aeromonas Phages Provides Insights into the Diversity of the Autographiviridae Family

In this study, we isolated and characterized three novel virulent Autographiviridae bacteriophages, vB_AspA_Bolek, vB_AspA_Lolek, and vB_AspA_Tola, which infect different Aeromonas strains. These three host-pathogen pairs were derived from the same sampling location-the arsenic-containing microbial mats of the Zloty Stok gold mine. Functional analysis showed they are psychrotolerant (4-25 °C), albeit with a much wider temperature range of propagation for the hosts (≤37 °C). Comparative genomic analyses revealed a high nucleotide and amino acid sequence similarity of vB_AspA_Bolek and vB_AspA_Lolek, with significant differences exclusively in the C-terminal region of their tail fibers, which might explain their host range discrimination. The protein-based phage network, together with a phylogenetic analysis of the marker proteins, allowed us to assign vB_AspA_Bolek and vB_AspA_Lolek to the Beijerinckvirinae and vB_AspA_Tola to the Colwellvirinae subfamilies, but as three novel species, due to their low nucleotide sequence coverage and identity with other known phage genomes. Global comparative analysis showed that the studied phages are also markedly different from most of the 24 Aeromonas autographiviruses known so far. Finally, this study provides in-depth insight into the diversity of the Autographiviridae phages and reveals genomic similarities between selected groups of this family as well as between autographiviruses and their relatives of other Caudoviricetes families.

Synergy between indigenous bacteria and extracellular electron shuttles enhances transformation and mobilization of Fe(III)/As(V)

The reduction of Fe(III) by metal-reducing bacteria through extracellular electron transfer (EET) is a critical link in the biogeochemical cycle of As/Fe, and humic substances are believed to play a role in this process. In this study, the indigenous As-resistant bacterium Bacillus D2201 isolated from the Datong Basin was responsible for the valence transition of Fe and As in the groundwater environment. The bacterium has both the arsC gene for intracellular arsenate reduction and an EET pathway for transferring electrons to an electrode or Fe(III). Chronoamperometry showed that 3.0- and 10.2-fold increases in the output current density were achieved by injecting 0.05 and 0.5 mM AQDS with an inoculation of Bacillus D2201. Interestingly, Fe(III) bio-reduction is not only regulated by AQDS, but also by As(V) stimulation. The increase in pyruvate consumption and levels of intracellular glutathione (GSH) suggest that As pressure promotes cell metabolism and the consumption of electron donors for Fe(III) reduction with strain D2201. The reduction and dissolution of Fe(III) mineral regulated by AQDS dominated the release and mobilization of As. Compared with the AQDS-free treatment, 5.5-, 6.6-, and 7.2-fold increases in the amounts of Fe(II) were released with the addition of 0.1, 0.5, and 1 mM AQDS, respectively, and approximately 2.6-, 2.8-, and 3.2-fold increases in the As(V) levels were observed under the same conditions. These insights have profound environmental implications with respect to the effect of AQDS and As stress on EET and Fe(III) reduction in arsenic-resistant bacteria.

Arsenic and antimony co-contamination influences on soil microbial community composition and functions: Relevance to arsenic resistance and carbon, nitrogen, and sulfur cycling

Microorganisms can mediate arsenic (As) and antimony (Sb) transformation and thus change the As and Sb toxicity and mobility. The influence of As and Sb on the innate microbiome has been extensively characterized. However, how microbial metabolic potentials are influenced by the As and Sb co-contamination is still ambiguous. In this study, we selected two contrasting sites located in the Shimen realgar mine, the largest realgar mine in Asia, to explore the adaptability and response of the soil microbiome to As and Sb co-contamination and the impact of co-contamination on microbial metabolic potentials. It is observed that the geochemical parameters, including the As and Sb fractions, were the driving forces that reshaped the community composition and metabolic potentials. Bacteria associated with Bradyrhizobium, Nocardioides, Sphingomonas, Burkholderia, and Streptomyces were predicted to be tolerant to high concentrations of As and Sb. Co-occurrence network analysis revealed that the genes related to C fixation, nitrate/nitrite reduction, N fixation, and sulfate reduction were positively correlated with the As and Sb fractions, suggesting that As and Sb biogeochemical cycling may interact with and benefit from C, N, and S cycling. The results suggest that As and Sb co-contamination not only influences As-related genes, but also influences other genes correlated with microbial C, N, and S cycling.

An indigenous bacterium Bacillus XZM for phosphate enhanced transformation and migration of arsenate

A number of arsenate-reducing bacteria respire adsorbed As(V), producing As(III) and thus contributing to arsenic mobilization from the solid phase to the aqueous phase. Two arsenate reducing genes, arsC and arrA, were both amplified in an indigenous bacterium Bacillus XZM isolated from high arsenic aquifer sediments. The effect of phosphate input on this novel bacterium in terms of mediating the biogeochemical behavior of arsenic was investigated for the first time. The results show bacterial growth and arsenate reduction appear to increase with the addition of phosphate. Input of 1 mM phosphate reduced the negative effects of As(V) on bacterial growth, resulting in 55-60% greater biomass production compared to lower phosphate inputs (0.01 and 0.1 mM). The data of real-time quantitative PCR (qPCR) indicated arsenate was involved in the expressions of two arsenate reductase genes (arsC and arrA genes) in indigenous bacterium Bacillus XZM. Overall, the addition of phosphate (from 0.1 to 1 mM) resulted in a doubling of arsenate bio-desorption from the sediment into the aqueous medium. Oxidation-reduction potential, as an environmental indicator of the bacterial reduction of metals, declined to -200 mV in the presence of strain XZM and 1 mM phosphate in the microcosm. Phosphate input enhanced arsenic biomigration, indicating the effect of phosphate concentration should be considered when studying the biogeochemical behavior of arsenic.

Functional Analysis of a Polluted River Microbiome Reveals a Metabolic Potential for Bioremediation

The objective of this study is to understand the functional and metabolic potential of the microbial communities along the Apatlaco River and highlight activities related to bioremediation and its relationship with the Apatlaco’s pollutants, to enhance future design of more accurate bioremediation processes. Water samples were collected at four sampling sites along the Apatlaco River (S1–S4) and a whole metagenome shotgun sequencing was performed to survey and understand the microbial metabolic functions with potential for bioremediation. A HMMER search was used to detect sequence homologs related to polyethylene terephthalate (PET) and polystyrene biodegradation, along with bacterial metal tolerance in Apatlaco River metagenomes. Our results suggest that pollution is a selective pressure which enriches microorganisms at polluted sites, displaying metabolic capacities to tolerate and transform the contamination. According to KEGG annotation, all sites along the river have bacteria with genes related to xenobiotic biodegradation. In particular, functions such as environmental processing, xenobiotic biodegradation and glycan biosynthesis are over-represented in polluted samples, in comparison to those in the clean water site. This suggests a functional specialization in the communities that inhabit each perturbated point. Our results can contribute to the determination of the partition in a metabolic niche among different Apatlaco River prokaryotic communities, that help to contend with and understand the effect of anthropogenic contamination.

Genomic Analysis of Shewanella sp. O23S-The Natural Host of the pSheB Plasmid Carrying Genes for Arsenic Resistance and Dissimilatory Reduction

Shewanella sp. O23S is a dissimilatory arsenate reducing bacterial strain involved in arsenic transformations within the abandoned gold mine in Zloty Stok (SW Poland). Previous physiological studies revealed that O23S may not only release arsenic from minerals, but also facilitate its immobilization through co-precipitation with reduced sulfur species. Given these uncommon, complementary characteristics and the application potential of the strain in arsenic-removal technologies, its genome (~5.3 Mbp), consisting of a single chromosome, two large plasmids (pSheA and pSheB) and three small plasmid-like phages (pSheC-E) was sequenced and annotated. Genes encoding putative proteins involved in heavy metal transformations, antibiotic resistance and other phenotypic traits were identified. An in-depth comparative analysis of arsenic respiration (arr) and resistance (ars) genes and their genetic context was also performed, revealing that pSheB carries the only copy of the arr genes, and a complete ars operon. The plasmid pSheB is therefore a unique natural vector of these genes, providing the host cells arsenic respiration and resistance abilities. The functionality of the identified genes was determined based on the results of the previous and additional physiological studies, including: the assessment of heavy metal and antibiotic resistance under various conditions, adhesion-biofilm formation assay and Biolog^TM metabolic preferences test. This combined genetic and physiological approach shed a new light on the capabilities of O23S and their molecular basis, and helped to confirm the biosafety of the strain in relation to its application in bioremediation technologies.

The whole genome insight on condition-specific redox activity and arsenopyrite interaction promoting As-mobilization by strain Lysinibacillus sp. B2A1

A gram-positive spore former, Lysinibacillus sp. B2A1 was isolated from a high arsenic containing groundwater of Beimen2A well, Chianan Plain area, Southwestern Taiwan. Noteworthy, in the subsurface-mimicking anoxic incubation with a Na-lactate amendment system, this isolate could interact with arsenic-source mineral arsenopyrite and enhance arsenic mobilization. Further, the isolate showed elevated levels of arsenic resistance, 200 mM and 7.5 mM for arsenate and arsenite, respectively. Lysinibacillus sp. B2A1 demonstrated condition-specific redox activities including salient oxic oxidation of arsenite and anoxic reduction of arsenate. The elevated rate of As(III) oxidation (V_max = 0.13 μM min^-1 per 10⁶ cells, K_m = 15.3 μM) under oxic conditions was potent. Correlating with stout persistence in an arsenic-rich niche, remarkably, the lesser toxic effects of arsenic ions on bacterial sporulation frequency and germination highlight this strain's ability to thrive under catastrophic conditions. Moreover, the whole genome analysis elucidated diverse metal redox/resistance genes that included a potential arsenite S-adenosylmethyltransferase capable of mitigating arsenite toxicity. Owing to its arsenic resistance, conditional redox activities and ability to interact with arsenic minerals leading to arsenic mobilization, the presence of such spore-forming strains could be a decisive indication towards arsenic mobilization in subsurface aquifers having a high concentration of soluble arsenic or its source minerals.

Impacts of Human Activities on the Composition and Abundance of Sulfate-Reducing and Sulfur-Oxidizing Microorganisms in Polluted River Sediments

Water system degradation has a severe impact on daily life, especially in developing countries. However, microbial changes associated with this degradation, especially changes in microbes related to sulfur (S) cycling, are poorly understood. In this study, the abundance, structure, and diversity of sulfate-reducing microorganisms (SRM) and sulfur-oxidizing microorganisms (SOM) in the sediments from the Ziya River Basin, which is polluted by various human interventions (urban and agricultural activities), were investigated. Quantitative real-time PCR showed that the S cycling-related (SCR) genes (dsrB and soxB) were significantly elevated, reaching 2.60 × 10⁷ and 1.81 × 10⁸ copies per gram of dry sediment, respectively, in the region polluted by human urban activities (RU), and the ratio of dsrB to soxB abundance was significantly elevated in the region polluted by human agricultural activities (RA) compared with those in the protected wildlife reserve (RP), indicating that the mechanisms underlying water system degradation differ between RU and RA. Based on a 16S rRNA gene analysis, human interventions had substantial effects on microbial communities, particularly for microbes involved in S cycling. Some SCR genera (i.e., Desulfatiglans and Geothermobacter) were enriched in the sediments from both RA and RU, while others (i.e., Desulfofustis and Desulfonatronobacter) were only enriched in the sediments from RA. A redundancy analysis indicated that NH₄⁺-N and total organic carbon significantly influenced the abundance of SRM and SOM, and sulfate significantly influenced only the abundance of SRM. A network analysis showed high correlation between SCR microorganisms and other microbial groups for both RU and RA, including those involved in carbon and metal cycling. These findings indicated the different effects of different human interventions on the microbial community composition and water quality degradation.

Biomineralization mechanism of U(VI) induced by Bacillus cereus 12-2: The role of functional groups and enzymes

Current studies reveal that the biomineralization of U(VI) by anaerobes normally produces nano-sized U(IV) minerals that can easily re-migrate/re-oxidize, while the biomineralization of U(VI) by aerobes has been constrained because the general mechanism has not yet been fully characterized. The biomineralization of U(VI) by Bacillus cereus 12-2 was investigated in this work. The maximum biosorption capability of intact cells was 448.68 mg U/g biomass (dry weight) at pH 5, while a decrease over 60% was induced when phosphate, amino, and especially carboxyl groups were shielded. X-ray diffraction, electron microscopy, and tracing the concentration of soluble intracellular U(VI) demonstrated that extracellular amorphous uranium particles can directly enter cells as solid, and about 10 nm-sized (NH₄)(UO₂)PO₄·3H₂O was formed subsequently. It was also revealed that the biosorption capability was not affected by a high uranium concentration, while biomineralization was inhibited, suggesting that a high concentration of heavy metals may inhibit the enzyme activity involved in biomineralization. Besides, U(VI) could trigger the overexpression of proteins with a molecular weight of 22 kD, including various phosphatases, kinases, and other enzymes that are related to metabolism and stimulus response, which may contribute to the intracellular transformation of U(VI) compounds from amorphous to crystalline phase. Taken together, the immobilization of U(VI) by B. cereus 12-2 contains two major steps: (1) fast immobilization of U(VI) on the cell surface as amorphous compounds, in which the carboxyl groups served as the predominant coordination functional groups and (2) transport of amorphous particles to cells directly and enzyme-related formation of uramphite.

Redox-stat bioreactors for elucidating mobilisation mechanisms of trace elements: an example of As-contaminated mining soils

The environmental fate of major (e.g. C, N, S, Fe and Mn) and trace (e.g. As, Cr, Sb, Se and U) elements is governed by microbially catalysed reduction-oxidation (redox) reactions. Mesocosms are routinely used to elucidate trace metal fate on the basis of correlations between biogeochemical proxies such as dissolved element concentrations, trace element speciation and dissolved organic matter. However, several redox processes may proceed simultaneously in natural soils and sediments (particularly, reductive Mn and Fe dissolution and metal/metalloid reduction), having a contrasting effect on element mobility. Here, a novel redox-stat (R_cont) bioreactor allowed precise control of the redox potential (159 ± 11 mV, ~ 2 months), suppressing redox reactions thermodynamically favoured at lower redox potential (i.e. reductive mobilisation of Fe and As). For a historically contaminated mining soil, As release could be attributed to desorption of arsenite [As(III)] and Mn reductive dissolution. By contrast, the control bioreactor (R_nat, with naturally developing redox potential) showed almost double As release (337 vs. 181 μg g^-1) due to reductive dissolution of Fe (1363 μg g^-1 Fe²⁺ released; no Fe²⁺ detected in R_cont) and microbial arsenate [As(V)] reduction (189 μg g^-1 released vs. 46 μg g^-1 As(III) in R_cont). A redox-stat bioreactor thus represents a versatile tool to study processes underlying mobilisation and sequestration of other trace elements as well.

Analysis of the Genome and Mobilome of a Dissimilatory Arsenate Reducing Aeromonas sp. O23A Reveals Multiple Mechanisms for Heavy Metal Resistance and Metabolism

Aeromonas spp. are among the most ubiquitous microorganisms, as they have been isolated from different environmental niches including waters, soil, as well as wounds and digestive tracts of poikilothermic animals and humans. Although much attention has been paid to the pathogenicity of Aeromonads, the role of these bacteria in environmentally important processes, such as transformation of heavy metals, remains to be discovered. Therefore, the aim of this study was a detailed genomic characterization of Aeromonas sp. O23A, the first representative of this genus capable of dissimilatory arsenate reduction. The strain was isolated from microbial mats from the Zloty Stok mine (SW Poland), an environment strongly contaminated with arsenic. Previous physiological studies indicated that O23A may be involved in both mobilization and immobilization of this metalloid in the environment. To discover the molecular basis of the mechanisms behind the observed abilities, the genome of O23A (∼5.0 Mbp) was sequenced and annotated, and genes for arsenic respiration, heavy metal resistance (hmr) and other phenotypic traits, including siderophore production, were identified. The functionality of the indicated gene modules was assessed in a series of minimal inhibitory concentration analyses for various metals and metalloids, as well as mineral dissolution experiments. Interestingly, comparative analyses revealed that O23A is related to a fish pathogen Aeromonas salmonicida subsp. salmonicida A449 which, however, does not carry genes for arsenic respiration. This indicates that the dissimilatory arsenate reduction ability may have been lost during genome reduction in pathogenic strains, or acquired through horizontal gene transfer. Therefore, particular emphasis was placed upon the mobilome of O23A, consisting of four plasmids, a phage, and numerous transposable elements, which may play a role in the dissemination of hmr and arsenic metabolism genes in the environment. The obtained results indicate that Aeromonas sp. O23A is well-adapted to the extreme environmental conditions occurring in the Zloty Stok mine. The analysis of genome encoded traits allowed for a better understanding of the mechanisms of adaptation of the strain, also with respect to its presumable role in colonization and remediation of arsenic-contaminated waters, which may never have been discovered based on physiological analyses alone.