Effect of soil microorganisms on arsenite oxidation in paddy soils under oxic conditions

Abstract

The contribution of abiotic and biotic processes to the oxidation of arsenite [As(III)] when anaerobic paddy soils were shifted to oxic conditions was investigated. Soil slurries were first incubated under reducing conditions to allow indigenous arsenic (As) to be dissolved into the liquid phase as As(III), and were then switched to oxic incubation conditions with shaking. One day after switching to oxic incubation, As(III) almost disappeared from the liquid phase without any increase of arsenate [As(V)], suggesting that dissolved As(III) was coprecipitated or adsorbed on the soil solid phase. X-ray adsorption near-edge structure (XANES) analysis revealed that the predominant species of As in the solid phase before the oxic incubation was As(III), ranging from 74 to 85% of total As. After 1 d of the oxic incubation, the proportion of As(III) decreased to 46-47%. This oxidation step was an abiotic process and 28-38% of As(III) was oxidized per day on average. However, the abiotic oxidation ceased within 24 h probably due to passivation of reactive sites on the mineral surface. Afterward, a second slow oxidation step (2.5-2.8% per day on average) became predominant. Interestingly, this step was microbiologically mediated, since it did not occur in sterilized soil slurries. Determination of putative arsenite oxidase gene (aioA) sequences suggested that arsenite-oxidizing bacteria are actually present in our soil slurries. Our results suggest that microbial As(III) oxidation accounts for more than 30% of total As(III) oxidation, and thus it is an important process especially after abiotic oxidation ceases.

Effects of Iron Amendments on the Speciation of Arsenic in the Rice Rhizosphere after Drainage

“…In their experiment, O 2 was supplied to a soil slurry via bubbling air. The oxidation speed of As-3 in our pot experiment was much slower than the speed estimated by Dong et al [33]. This difference was attributed to the slow and heterogeneous O 2 diffusion in the soil and Fe plaque of the rhizosphere in the rice-cultivated pot.…”

“…After drainage at harvest, part of the As-3 was oxidized over time, the proportion of As-S was decreased and the proportion of As-5 was increased. Dong et al [33] showed that the rate of oxidation of As-3 was 28-38% per day based on a microcosm experiment. In their experiment, O 2 was supplied to a soil slurry via bubbling air.…”

Impact of Arsenite on the Bacterial Community Structure and Diversity in Soil

“…At various time intervals, soil slurry was centrifuged, and arsenic speciation in the supernatant was determined using high performance liquid chromatography (HPLC) (L-7000, Hitachi, Tokyo, Japan) with an Aminex HPX-87H ion exclusion column (Bio-Rad Laboratories, Hercules, CA, USA) under UV detection at 212 nm. In soil slurry spiked with 50 and 500 μM As(III), arsenic speciation and the total concentration of arsenic in the supernatant were determined by HPLC connected with a quadrupole inductively coupled plasma mass spectrometer (ICPMS) as described previously ( 8 , 45 ) because arsenic concentrations less than 1,000 μM cannot be measured by HPLC. An Inertsil ODS-3 column (4.6 × 150 mm; GL Science, Tokyo, Japan) was used for the separation of As(III) and As(V).…”

“…The PCR products were cloned into the pMD 20-T vector using the Mighty TA-Cloning Kit (Takara Bio, Otsu, Japan) and transformed into DH5α chemically competent Escherichia coli (Takara Bio), according to the manufacturer’s instructions. Thirty to forty clones were randomly chosen from each soil slurry and were sequenced as described previously ( 8 ). The gene sequences obtained were aligned using the Clustal W program in MEGA version 6.0 ( 44 ) and clustered into operational taxonomic units (OTU), followed by grouping at more than 97% similarity based on different distances using Mothur ( http://www.mothur.org/ ).…”

“…Another study also showed the abiotic oxidation of As(III) by a poorly crystalline manganese oxide mineral occurring in soils ( 16 ). We recently reported that microbial As(III) oxidation accounted for more than 30% of total As(III) oxidation in natural paddy soils to which no exogenous arsenic was added ( 8 ). These findings suggest that the capacity for As(III) oxidation is widespread in natural soil microbial communities.…”