Transcriptional Activity of Arsenic-Reducing Bacteria and Genes Regulated by Lactate and Biochar during Arsenic Transformation in Flooded Paddy Soil

Organic substrates and biochar are important in controlling arsenic release from sediments and soils; however, little is known about their impact on arsenic-reducing bacteria and genes during arsenic transformation in flooded paddy soils. In this study, microcosm experiments were established to profile transcriptional activity of As­(V)-respiring gene (<i>arrA</i>) and arsenic resistance gene (<i>arsC</i>) as well as the associated bacteria regulated by lactate and/or biochar in anaerobic arsenic-contaminated paddy soils. Chemical analyses revealed that lactate as the organic substrate stimulated microbial reduction of As­(V) and Fe­(III), which was simultaneously promoted by lactate+biochar, due to biochar’s electron shuttle function that facilitates electron transfer from bacteria to As­(V)/Fe­(III). Sequencing and phylogenetic analyses demonstrated that both <i>arrA</i> closely associated with <i>Geobacter</i> (>60%, number of identical sequences/number of the total sequences) and <i>arsC</i> related to <i>Enterobacteriaceae</i> (>99%) were selected by lactate and lactate+biochar. Compared with the lactate microcosms, transcriptions of the bacterial 16S rRNA gene, <i>Geobacter</i> spp., and <i>Geobacter</i> <i>arrA</i> and <i>arsC</i> genes were increased in the lactate+biochar microcosms, where transcript abundances of <i>Geobacter</i> and <i>Geobacter</i> <i>arrA</i> closely tracked with dissolved As­(V) concentrations. Our findings indicated that lactate and biochar in flooded paddy soils can stimulate the active As­(V)-respiring bacteria <i>Geobacter</i> species for arsenic reduction and release, which probably increases arsenic bioavailability to rice plants