Continuous flooding selects for bacterial populations involved in arsenic cycle in rice rhizosphere

Arsenic contamination in rice is strictly related to water management. Recent outcomes evidenced that arsenic content and speciation in rice grains cannot be explained only by the metabolism of the plant, but also by the activity of the rhizosphere microbiome. Several microbial species have been reported to perform different arsenic transformations. Furthermore, the activity of iron-reducing bacteria could contribute to the solubilization of arsenic from soil iron oxides. To test the effect of water management on rhizospheric bacterial populations involved in arsenic cycle, rice plants were grown under different water regimes in a non-contaminated soil (total arsenic 18.4 mg kg-1). The microbial community was characterized by pyrosequencing of 16S rRNA and real time PCR quantification of iron-reducing bacteria 16S rRNA genes. Genes encoding for arsenite oxidase (aioA) and for arsenite efflux pump (ACR3) were also quantified. Under flooded conditions arsenate in soil solution increased from 1.40 μg L-1 to 190 μg L-1, whereas arsenite increased to 40 μg L-1. Arsenic release was negligible in aerobic rice. Metalloid accumulation in rice grains was 237 μg kg-1, contrary to 4.67 μg kg-1 measured in aerobic rice. Rhizospheric microbial populations involved in arsenic speciation were markedly selected under continuous flooding, with relative abundance from 8% in the soil before seeding to 13% before harvesting. In these conditions ACR3 gene copies increased and were two orders of magnitude higher with respect on aioA genes. Soluble ferrous iron increased from 0.75 mg L-1 to 51.1 mg L-1 under continuous flooding. In these conditions a concomitant increase of iron-reducing bacteria from 4.6% to 8.7% was observed over time. Pseudomonas sp. (4.2%) and Geobacter sp. (1.7%) contributed to this increase. Real time PCR confirmed the increase of Geobacteraceae 16S rRNA genes from 105 to 106 copies (g dry weight)-1. These outcomes indicate that continuous flooding leads to a positive selection in the rhizospheric community of bacterial populations involved in arsenic and iron cycles, promoting the release of arsenic from iron-oxides and the consequent contamination of rice grains. Acknowledgments: The financial support of the PhD school in Food Systems (University of Milan) and PRIN (2010JBNLJ7-004) was greatly appreciated.