Iron-reducing bacteria in rice rhizosphere contribute to arsenic mobilization under flooded conditions

Rice is among the crops mostly affected by arsenic contamination. Whereas in arsenic contaminated soils (> 40 mg kg-1) rice farming contributes to population exposure, in rice field soils with low arsenic content agronomic conditions represent an issue for arsenic bioavailability. Iron-reducing bacterial populations (Geobacteraceae and Shewanellaceae) inhabiting rice rhizosphere were assessed in box plots (total arsenic 18.4 mg kg-1) cultivated with different water regimes, in order to evidence their role in the release of iron and consequently arsenic from root ferric iron plaques. Geobacteraceae and Shewanellaceae 16S rRNA genes significantly increased in rice rhizosphere from the order of 105 to 106 copies (g dry weight)-1 under flooding, while no significant increase occurred in aerobic rice. Fluorescence in situ hybridization in rhizoplane evidenced a similar trend related to the active populations. This increase was concomitant with the release of arsenic in soil solution under continuous flooding: from 1.40 μg L-1 to 190 μg L-1 over the cropping cycle, whereas in aerobic rice no significant release occurred. Analogously, ferrous iron increased from 0.75 mg L -1 to 51.1 mg L-1 and dissolved organic C from 8.9 to 79.9 mg L-1 until late reproductive stage. In aerobic rice soluble iron remained almost negligible and dissolved organic C never exceeded 30 mg L-1. Arsenic content in rice grains was 237 μg kg-1 under flooding and 4.67 μg kg-1 in aerobic rice, thus reflecting this solubilisation trend. These outcomes evidence that iron-reducing bacteria promote the arsenic release from iron plaques under flooded conditions in non-contaminated rice fields thus contributing to grain contamination.