The effect of extracellular polymeric substances (EPS) of iron-oxidizing bacteria (Ochrobactrum EEELCW01) on mineral transformation and arsenic (As) fate.

Extracellular polymeric substances (EPS) are an important medium for communication and material exchange between iron-oxidizing bacteria and the external environment and could induce the iron (oxyhydr) oxides production which reduced arsenic (As) availability. The main component of EPS secreted by iron-oxidizing bacteria (Ochrobactrum EEELCW01) was composed of polysaccharides (150.76-165.33 mg/g DW) followed by considerably smaller amounts of proteins (12.98-16.12 mg/g DW). Low concentrations of As (100 or 500 µmol/L) promoted the amount of EPS secretion. FTIR results showed that EPS was composed of polysaccharides, proteins, and a miniscule amount of nucleic acids. The functional groups including -COOH, -OH, -NH, -C=O, and -C-O played an important role in the adsorption of As. XPS results showed that As was bound to EPS in the form of As3+. With increasing As concentration, the proportion of As3+ adsorbed on EPS increased. Ferrihydrite with a weak crystalline state was only produced in the system at 6 hr during the mineralization process of Ochrobactrum sp. At day 8, the minerals were composed of goethite, galena, and siderite. With the increasing mineralization time, the main mineral phases were transformed from weakly crystalline hydrous iron ore into higher crystallinity siderite (FeCO3) or goethite (α-FeOOH), and the specific surface area and active sites of minerals were reduced. It can be seen from the distribution of As elements that As is preferentially adsorbed on the edges of iron minerals. This study is potential to understand the biomineralization mechanism of iron-oxidizing bacteria and As remediation in the environment.

Arsenic Transformation in Soil-Rice System Affected by Iron-Oxidizing Strain (Ochrobactrum sp.) and Related Soil Metabolomics Analysis.

Iron-oxidizing bacteria (FeOB) could oxidize Fe(II) and mediate biomineralization, which provides the possibility for its potential application in arsenic (As) remediation. In the present study, a strain named Ochrobactrum EEELCW01 isolated previously, was inoculated into paddy soils to investigate the effect of FeOB inoculation on the As migration and transformation in paddy soils. The results showed that inoculation of Ochrobactrum sp. increased the proportion of As in iron-aluminum oxide binding fraction, which reduced the As bioavailability in paddy soils and effectively reduced the As accumulation in rice tissues. Moreover, the inoculation of iron oxidizing bacteria increased the abundance of KD4-96, Pedosphaeraceae and other bacteria in the soils, which could reduce the As toxicity in the soil through biotransformation. The abundance of metabolites such as carnosine, MG (0:0/14:0/0:0) and pantetheine 4'-phosphate increased in rhizosphere soils inoculated with FeOB, which indicated that the defense ability of soil-microorganism-plant system against peroxidation caused by As was enhanced. This study proved that FeOB have the potential application in remediation of As pollution in paddy soil, FeOB promotes the formation of iron oxide in paddy soil, and then adsorbed and coprecipitated with arsenic. On the other hand, the inoculation of Ochrobactrum sp. change soil microbial community structure and soil metabolism, increase the abundance of FeOB in soil, promote the biotransformation process of As in soil, and enhance the resistance of soil to peroxide pollution (As pollution).

Microbial driven iron reduction affects arsenic transformation and transportation in soil-rice system.

The microbe-driven iron cycle plays an important role in speciation transformation and migration of arsenic (As) in soil-rice systems. In this study, pot experiments were used to investigate the effect of bacterial iron (Fe) reduction processes in soils on As speciation and migration, as well as on As uptake in soil-rice system. During the rice growth period, pH and electrical conductivity (EC) in soil solutions initially increased and then decreased, with the ranges of 7.4-8.8 and 116.3-820 mS cm-1, respectively. The concentrations of Fe, total As and As(III) showed an increasing trend in the rhizosphere and non-rhizosphere soil solutions with the increasing time. Fe concentrations were significantly positively correlated with total As and As(III) concentrations (***p < 0.001) in the soil solutions. The abundances of the arsenate reductase gene (arsC) and the As(III) S-adenosylmethionine methyltransferase gene (arsM) in rhizosphere soils were higher than those in non-rhizosphere soils, while the abundance of the Fe-reducing bacteria (Geo) showed an opposite trend. Moreover, it showed that the Geo abundance was significantly positively correlated with that of the arsC (***p < 0.001) and arsM (**p < 0.01) genes, respectively. The abundances of Geo, arsC and arsM genes were significantly positively correlated with the concentrations of Fe, total As and As(III) in the soil solutions (*p < 0.05). Moreover, the abundances of arsC and arsM genes were significantly negatively correlated with total As and As(III) in rice grains (*P < 0.05). These results showed that the interaction of bacterial Fe reduction process and radial oxygen loss from roots promoted the reduction and methylation of As, and then decreased As uptake by rice, which provided a theoretical basis for alleviating As pollution in paddy soils.

Effect of sulfur-iron modified biochar on the available cadmium and bacterial community structure in contaminated soils.

Cadmium contamination in paddy soils has aroused increasing concern around the world, and biochar has many positive properties, such as large specific surface areas, micro porous structure for the heavy metal immobilization in soils. However there are few studies on sulfur-iron modified biochar as well as its microbiology effects. The purpose of this study was to evaluate the Cd immobilization effects of sulfur or sulfur-iron modified biochar and its related microbial community changes in Cd-contaminated soils. SEM-EDX analysis confirmed that sulfur and iron were loaded on the raw biochar successfully. Sulfur-modified biochar (S-BC) and sulfur-iron modified biochar (SF-BC) addition increased pH value and the content of soil organic matter, and also decreased DTPA-extractable Cd. There was a negative significant correlation between organic matter content and the available Cd (P < 0.05). During a 45-d incubation period, the fractions of Cd are mainly with the exchangeable (25.16-35.79%) and carbonate (22.01-25.10%) fractions. Compared with the control, the concentrations of exchangeable Cd in soil were significantly (P < 0.05) decreased by 12.54%, 29.71%, 18.53% under the treatments of BC, S-BC, SF-BC respectively. The S-BC and SF-BC treatments significantly (P < 0.05) increased Chao1, observed, Shannon and Simpson diversity indices compared with the control and biochar treatments. Meanwhile, the relative abundance of Proteobacteria, Bacteroidetes, and Actinobacteria increased, whereas the abundance of Acidobacteria and Germmatimonadetes decreased. Capsule: Sulfur-modified and sulfur-iron modified biochar applications decreased the available Cd and changed the microbial community.

Remediation of arsenic-contaminated paddy soil by iron-modified biochar.

Arsenic contamination in paddy soils has aroused global concern due to its threats to food security and human health. Biochar modified with different iron materials was prepared for arsenic (As) immobilization in contaminated soils. Soil incubation experiments were carried to investigate the effects of biochar modified with Fe-oxyhydroxy sulfate (Biochar-FeOS), FeCl3 (Biochar-FeCl3), and zero-valent iron (Biochar-Fe) on the pH, NaHCO3-extractable As concentrations, and the As fractions in soils. The scanning electron microscope and X-ray diffraction analysis demonstrated that iron was successfully loaded onto the surface or embedded into the pores of the biochar. Addition of Biochar-FeOS, Biochar-FeCl3, and Biochar-Fe had no significant effects on the soil pH but significantly decreased the contents of NaHCO3-extractable As in soils by 13.95-30.35%, 10.97-28.39%, and 17.98-35.18%, respectively. Biochar-FeOS, Biochar-FeCl3, and Biochar-Fe treatments decreased the concentrations of non-specifically sorbed and specifically sorbed As fractions in soils, and increased the amorphous and poorly crystalline, hydrated Fe, Al oxide-bound, and residual As fractions. Compared with the other iron-modified biochars, Biochar-FeOS showed the most effective immobilization and has the potential for the remediation of As-contaminated paddy soils.