Rhizospheric iron and arsenic bacteria influence metalloid uptake by plant in rice paddies

Question The European Community has established specific limits for arsenic (As) concentration in rice grains (Commission Regulation (EU) 2015/1006). In rice rhizosphere, iron (Fe) and As metabolic processes carried out by microorganisms have a crucial role on As cycling in rice paddies. The aim was to evaluate the effect of different agronomic conditions on rice rhizosphere bacteria. Methods Rice plants were grown under: continuous flooding (CF), aerobic rice (AR) and 2nd internode elongation drainage (2IED). Microbial community was characterized by 16S rRNA pyro-sequencing and real time qPCR. Results Under CF, AsV and Fe increased in soil solution from 1.40 to 180 μg L-1 and from 0.75 to 51.1 mg L-1, respectively, and AsIII reached 40 μg L-1. In AR and in 2IED they remained negligible. In CF, inorganic As was accumulated in rice grains above the law limits for baby food production (100 μg Kg-1) whereas in AR and 2IED its content was below the limit. As processing bacteria increased under CF from 8% to 13% over time. Genes for AsIII oxidation (aioA) were the most abundant in all treatments, explaining the presence of AsV in soil solution. Genes for AsIII-methylation (arsM) were not influenced by the water regime, but under CF the presence of AsIII allowed the production of the organic As observed in rice grains. AsV reduction was primarily performed by arsC-carrying bacteria, since arrA were absent in all the treatments. Under CF, an increase of Fe-reducing bacteria from 4.6% to 8.7% was observed over time. Real time qPCR confirmed the promotion of Geobacteraceae in this condition, possibly explaining AsV dissolution from Fe minerals. In the other conditions, an increase of Fe-oxidizing bacteria likely influenced the reduction of As translocation in rice grains. Conclusions According to these data, in rice rhizosphere Fe-bacteria affected As solubilisation, whereas As-bacteria drove the speciation of the metalloid, influencing As translocation to rice grains.