Influences of arsenate and/or phosphate adsorption to ferrihydrite on iron-reducing and arsenic-reducing microbial communities in paddy soil revealed by rRNA-13C-acetate probing

Iron (oxyhydr)oxides [Fe(III)] are important adsorbents of arsenate [As(V)] and phosphate in paddy soils, and microbial Fe(III) reduction is hence central to biogeochemical cycles of arsenic and phosphorus. Nevertheless, how Fe(III) reducers and As(V) reducers at the community level respond to As(V) and phosphate adsorption in paddy soils remain unclear. Here, we explored the influences of arsenate and/or phosphate adsorption to ferrihydrite on active acetate-dependent Fe(III)-reducing and As(V)-reducing microbial communities in a paddy soil, using ¹³C-acetate-based rRNA-stable isotope probing (SIP). During anaerobic SIP incubations, the arsenate and/or phosphate adsorption to ferrihydrite retarded Fe(III) reduction to various extents, with arsenate alone or combined with phosphate having greater inhibitory effects than phosphate alone. 16S rRNA-based sequencing results revealed that the adsorption of arsenate alone or combined with phosphate markedly enriched several Fe(III) reducers that have also been found to enable arsenate reduction, especially Geobacter genus. This was coincided with the pronounced increment in transcript abundance of arsenate-respiring gene (arrA) induced by the presence of arsenate and further confirmed by cloning and sequencing result which indicated that Geobacter spp. harboring arrA gene were the major As(V) reducers herein. In contrast, the presence of arsenate led to a remarkable decline in other Fe(III) reducers, including Dechloromonas and Thermincola genera, which have rarely been reported to be related to arsenate transformation. Furthermore, all these identified Fe(III) reducers declined significantly by phosphate adsorption alone. Additionally, the adsorption of phosphate to ferrihydrite not only boosted the reduction of adsorbed As(V), but also enriched the respiratory As(V)-reducing microbes containing arrA gene. These findings demonstrate that the adsorption of arsenate and/or phosphate inhibits Fe(III) reduction while promotes As(V) reduction, and shifts the Fe(III)-reducing and As(V)-reducing microbial communities in paddy soils. Overall this study provides novel insights into intricate biogeochemical coupling between iron, arsenic and phosphorus in paddy soils.