Effects of iron-reducing bacteria on arsenic releasing from iron plaque on rice root surface

Iron plaque on rice roots controls arsenic (As) movement from soils to roots in the paddy fields. Although it is well known that iron-reducing bacteira are able to dissolve iron oxides, their role in rice rhizosphere is unclear. In this study, rice roots collected from polluted paddy fields and ferrihydrite adsorbing As or phosphate (P) were used as iron sources to support biological iron reduction. Arsenic is released through the Fe oxides reduction processes catalyzed by Shewanella oneidensis MR-1. Experiments showed that the Fe plaque was quickly reduced and released into the solution in the presence of Shewanella oneidensis MR-1. Arsenic in Fe plaque was also simultaneously released and the concentration reached 18 μmol·L^-1 within 48 hours. As concentration then gradually decreased and was reduced by about 60% after 120 hours. The As-loaded ferrihydrite was used to simulate the process of As release from iron plaque. Results showed that microbial absorption was one of the main causes of the decline of arsenic in solution, and implied that promoting the rhizosphere microorganism growth could reduce the risk of As in the soil environment.