Dissimilatory arsenate-respiring prokaryotes catalyze the dissolution, reduction and release of arsenic from paddy soils into groundwater: implication for the effect of sulfate

Functional features of a novel Sb(III)- and As(III)-oxidizing bacterium: Implications for the interactions between bacterial Sb(III) and As(III) oxidation pathways

Antimony (Sb) and arsenic (As) share similar chemical characteristics and commonly coexist in contaminated environments. It has been reported that the biogeochemical cycles of antimony and arsenic affect each other. However, there is limited understanding regarding microbial coupling between the biogeochemical processes of antimony and arsenic. Here, we aimed to solve this issue. We successfully isolated a novel bacterium, Shinella sp. SbAsOP1, which possesses both Sb(III) and As(III) oxidase, and can effectively oxidize both Sb(III) and As(III) under aerobic and anaerobic conditions. SbAsOP1 exhibits greater aerobic oxidation activity for the oxidation of As(III) or Sb(III) compared to its anaerobic activity. SbAsOP1 also significantly catalyzes the oxidative mobilization of solid-phase Sb(III) under aerobic conditions. The activity of SbAsOP1 in oxidizing solid Sb(III) is 3 times lower than its activity in oxidizing soluble form. It is noteworthy that, in the presence of both Sb(III) and As(III) under aerobic conditions, either As(III) or Sb(III) significantly inhibits the oxidation of Sb(III) or As(III), respectively. In comparison, under anaerobic conditions and in the coexistence of Sb(III) and As(III), As(III) significantly inhibits Sb(III) oxidation, whereas Sb(III) almost completely inhibits As(III) oxidation. These findings suggest that under both aerobic and anaerobic conditions, SbAsOP1 demonstrates a partial preference for Sb(III) oxidation. Additionally, bacterial oxidations of Sb(III) and As(III) mutually inhibit each other to varying degrees. These observations gain a novel understanding of the interplay between the biogeochemical processes of antimony and arsenic.

Biological effect of phosphate on the dissimilatory arsenate-respiring bacteria-catalyzed reductive mobilization of arsenic from contaminated soils

Dissimilatory arsenate-respiring prokaryotes (DARPs) are considered to be the major drive of the reductive mobilization of arsenic from solid phases. However, it is not fully understood how phosphate, a structural analog of arsenate, affects the DARPs-mediated arsenic mobilization. This work aimed to address this issue. As-contaminated soils were collected from a Shimen Realgar Mine-affected area. We identified a unique diversity of DARPs from the soils, which possess high As(V)-respiring activities using one of multiple small organic acids as the electron donor. After elimination of the desorption effect of phosphate on the As mobilization, the supplement of additional 10 mM phosphate to the active slurries markedly increased the microbial community-mediated reductive mobilization of arsenic as revealed by microcosm tests; this observation was associated to the fact that phosphate significantly increased the As(V)-respiratory reductase (Arr) gene abundances in the slurries. To confirm this finding, we further obtained a new DARP strain, Priestia sp. F01, from the samples. We found that after elimination of the chemical effect of phosphate, the supplement of 10 mM phosphate to the active slurries resulted in an 82.2% increase of the released As(III) in the solutions, which could be contributed to that excessive phosphate greatly increased the Arr gene abundance, and enhanced the transcriptional level of arrA gene and the bacterial As(V)-respiring activity of F01 cells. Considering that phosphate commonly coexists with As in the environment, and is a frequently-used fertilizer, these findings are helpful for deeply understanding why As concentrations in contaminated groundwater are dynamically fluctuated, and also provided new knowledge on the interactions between the biogeochemical processes of P and As.

Effects of Calcium on Arsenate Adsorption and Arsenate/Iron Bioreduction of Ferrihydrite in Stimulated Groundwater

The reduction and transformation of arsenic-bearing ferrihydrite by arsenate-iron reducing bacteria is one of the main sources of arsenic enrichment in groundwater. During this process the coexistence cations may have a considerable effect. However, the ionic radius of calcium is larger than that of iron and shows a low affinity for ferrihydrite, and the effect of coexisting calcium on the migration and release of arsenic in arsenic-bearing ferrihydrite remains unclear. This study mainly explored the influence of adsorbed Ca²⁺ on strain JH012-1-mediated migration and release of arsenate in a simulated groundwater environment, in which 3 mM ferrihydrite and pH 7.5. Ca²⁺ were pre-absorbed on As(V)-containing ferrihydrite with a As:Fe ratio of 0.2. Solid samples were analyzed by X-ray diffraction (XRD), scanning electron microscopic (SEM), Fourier transform infrared spectroscopy (FTIR), and X-ray photoelectron spectroscopy (XPS). The results show that calcium and arsenate can synergistically adsorb on ferrihydrite due to the electrostatic interactions, and the adsorbed Ca²⁺ mainly exists on the surface through the outer-sphere complex. Adsorbed Ca²⁺ entering the stimulated groundwater was easily disturbed and led to an extra release of 3.5 mg/L arsenic in the early stage. Moreover, adsorbed Ca²⁺ inhibited biogenic ferrous ions from accumulating on ferrihydrite. As a result, only 12.30% Fe(II) existed in the solid phase, whereas 29.35% existed without Ca²⁺ adsorption. Thus, the generation of parasymplesite was inhibited, which is not conducive to the immobilization of arsenic in groundwater.

Understanding Microbial Arsenic-Mobilization in Multiple Aquifers: Insight from DNA and RNA Analyses

Biogeochemical processes critically control the groundwater arsenic (As) enrichment; however, the key active As-mobilizing biogeochemical processes and associated microbes in high dissolved As and sulfate aquifers are poorly understood. To address this issue, the groundwater-sediment geochemistry, total and active microbial communities, and their potential functions in the groundwater-sediment microbiota from the western Hetao basin were determined using 16S rRNA gene (rDNA) and associated 16S rRNA (rRNA) sequencing. The relative abundances of either sediment or groundwater total and active microbial communities were positively correlated. Interestingly, groundwater active microbial communities were mainly associated with ammonium and sulfide, while sediment active communities were highly related to water-extractable nitrate. Both sediment-sourced and groundwater-sourced active microorganisms (rRNA/rDNA ratios > 1) noted Fe(III)-reducers (induced by ammonium oxidation) and As(V)-reducers, emphasizing the As mobilization via Fe(III) and/or As(V) reduction. Moreover, active cryptic sulfur cycling between groundwater and sediments was implicated in affecting As mobilization. Sediment-sourced active microorganisms were potentially involved in anaerobic pyrite oxidation (driven by denitrification), while groundwater-sourced organisms were associated with sulfur disproportionation and sulfate reduction. This study provides an extended whole-picture concept model of active As-N-S-Fe biogeochemical processes affecting As mobilization in high dissolved As and sulfate aquifers.

A novel biofilm bioreactor derived from a consortium of acidophilic arsenite-oxidizing bacteria for the cleaning up of arsenite from acid mine drainage

Arsenite (As(III)) was considered to be of great concern in acid mine drainage (AMD). A promising approach for cleaning up of arsenite from AMD is microbial oxidation of As(III) followed by adsorptions. However, there is virtually no research about the acidophilic bioreactor for As(III) oxidation so far. In this study, we formed a new biofilm bioreactor with a consortium of acidophilic As(III) oxidation bacteria. It is totally chemoautotrophic, with no need to add any carbon or other materials during the operations. It works well under pH 3.0-4.0, capable of oxidizing 1.0-20.0 mg/L As(III) in 3.0-4.5 h, respectively. A continuous operation of the bioreactor suggests that it is very stable and sustainable. Functional gene detection indicated that the biofilms possessed a unique diversity of As(III) oxidase genes. Taken together, this acidophilic bioreactor has great potential for industrial applications in the cleaning up of As(III) from AMD solution.

Epiphytic bacterial community enhances arsenic uptake and reduction by Myriophyllum verticillatum

Microbes play an important role in the biotransformation of arsenic (As) speciation in various environments. Nevertheless, whether epiphytic bacteria that attached on submerged macrophytes have the potential to influence As speciation still remains unclear. In this study, sterile or nonsterile Myriophyllum verticillatum was cultured with arsenite (As(III)) or arsenate (As(V)) to investigate the impact of epiphytic bacterial community on As uptake, transformation, and efflux. Results showed that both sterile and nonsterile M. verticillatum did not display substantial As(III) oxidation, suggesting that neither M. verticillatum nor epiphytic bacterial community has the capacities of As(III) oxidation. However, sterile M. verticillatum exhibited capacity for As(V) reduction, and the presence of epiphytic bacterial community substantially enhanced the proportions of As(III) in the medium (from 39.91 to 98.44%), indicating that epiphytic bacterial community contributes significantly to As(V) reduction in the medium. The presence of epiphytic bacterial community elevated As accumulation (by up to 2.06-fold) in plants when exposed to As(V). Results also showed that epiphytic bacterial community contributed little to As(III) efflux. Quantitative PCR of As metabolism genes revealed the dominance of the respiratory As(V) reductase genes (arrA) in epiphytic bacterial community, which might play a significant role in As(V) reduction in aquatic environments. Phylogenetic analysis of the arrA genes revealed the widely distribution and diversity of As(V)-respiring bacteria. These results highlighted the substantial impact of the epiphytic bacterial community associated with submerged aquatic macrophytes on As biogeochemistry in wetland and water environments.

Microbial sulfate reduction facilitates seasonal variation of arsenic concentration in groundwater of Jianghan Plain, Central China

Bacterial sulfate reduction (BSR) plays a vital but complex role in regulating groundwater arsenic concentration. A quarterly hydro-biogeochemical investigation was conducted to clarify how BSR participated in arsenic dynamics in the geogenic As-contaminated alluvial aquifers of the Jianghan Plain, central Yangtze River Basin. Anthropogenic input of sulfate was identified in the transitional season with higher Cl concentrations and Cl/Br molar ratios compared to the monsoon season. Seasonal increase of S(-II) and Fe(II) concentrations in monsoon season suggests the co-occurrence of iron and sulfate reduction. Quantitative analysis of dsrB gene abundance revealed the corresponding variations between dsrB gene abundance (up to 1.2 × 10⁷ copies L^-1) and Fe(II) in groundwater. High-throughput sequencing of the dsrB gene identified a considerable proportion of sequences in the sulfate-reducing bacterial community was affiliated with Desulfobulbus (22.7 ± 20.8%) and Desulfocapsa (11.5 ± 11.9%). Moreover, the relative abundance of Desulfocapsa increased with the Fe(II) in the groundwater (R = 0.78, P < 0.01). These results suggest that microbially-mediated sulfate reduction facilitated the abiotic reduction of As-bearing Fe-oxides in the monsoon season after anthropogenic input of sulfate in the transitional season under oscillating redox conditions in the groundwater systems. The present research provides new insights into the critical role of BSR in the seasonal redox cycling of iron and variation of As in the aquifer systems, which are not only applicable in the central Yangtze River basin, but also to other similar As-rich alluvial aquifers worldwide.

Microbial reactions and environmental factors affecting the dissolution and release of arsenic in the severely contaminated soils under anaerobic or aerobic conditions

The soils near the abandoned Shimen Realgar Mine are characterized by containing extremely high contents of total and soluble arsenic. To determine the microbial reactions and environmental factors affecting the mobilization and release of arsenic from soils phase into pore water, we collected 24 soil samples from the representative points around the abandoned Shimen Realgar Mine. They contained 8310.84 mg/kg total arsenic and 703.21 mg/kg soluble arsenic in average. The soluble arsenic in the soils shows significant positive and negative correlations with environmental SO₄^2-/TOC/pH/PO₄^3-, and Fe/Mn, respectively. We found that diverse dissimilatory As(V)-respiring prokaryotes (DARPs) and As(III)-oxidizing bacteria (AOB) exist in all the examined soil samples. The activities of DARPs led to 65-1275% increase of soluble As(III) in the examined soils after 21.0 days of anaerobic incubation, and the microbial dissolution and releases of arsenic show significant positive and negative correlations with the environmental pH/TN and NH₄⁺/PO₄^3-, respectively. In comparison, the activities of AOB led to 24-346% inhibition of the dissolved oxygen-mediated dissolution of arsenic in the soils, and the AOB-mediated releases of As(V) show significant positive and negative correlations with the environmental SO₄^2- and pH/NH₄⁺, respectively. The microbial communities of 24 samples contain 54 phyla of bacteria that show extremely high diversities. Total arsenic, TOC, NO₃^- and pH are the key environmental factors that indirectly controlled the mobilization and release of arsenic via influencing the structures of the microbial communities in the soils. This work gained new insights into the mechanism for how microbial communities catalyze the dissolution and releases of arsenic from the soils with extremely high contents of arsenic.

Unique diversity and functions of the arsenic-methylating microorganisms from the tailings of Shimen Realgar Mine

Microbial arsenic (As) methylation plays important roles in the As biogeochemical cycle. However, little is known about the diversity and functions of As-methylating microorganisms from the tailings of a Realgar Mine, which is characterized as containing extremely high concentrations of As. To address this issue, we collected five samples (T1-T5) from the tailings of Shimen Realgar Mine. Microcosm assays without addition of exogenous As and carbon indicated that all the five samples possess significant As-methylating activities, producing 0.8-5.7 μg/L DMAs^V, and 1.1-10.7 μg/L MMAs^V with an exception of T3, from which MMAs^V was not detectable after 14.0 days of incubation. In comparison, addition of 20.0 mM lactate to the microcosms significantly enhanced the activities of these samples; the produced DMAs^V and MMAs^V are 8.0-39.7 μg/L and 5.8-38.3 μg/L, respectively. The biogenic DMAs^V shows significant positive correlations with the Fe concentrations and negative correlations with the total nitrogen concentrations in the environment. A total of 63 different arsM genes were identified from the five samples, which code for new or new-type ArsM proteins, suggesting that a unique diversity of As-methylating microbes are present in the environment. The microbial community structures of the samples were significantly shaped by the environmental total organic carbon, total As contents and NO₃^- contents. These data help to better understand the microorganisms-catalyzed As methylation occurred in the environment with extremely high contents of As.