Abstract: Arsenic (As) is a carcinogen that is widely distributed in the environment. Phyllosphere of duckweeds in paddy soil environments is a hotspot in arsenite (As(III)) oxidation which may have a substantial impact on As biogeochemistry in water environments. Previous work has shown that As(III) oxidizing bacterial communities in the phyllosphere of duckweeds were dominated by Rhodobacter species. In this study, Rhodobacter strain CZR27 that exhibited the ability to oxidize As(III) was isolated from the phyllosphere of duckweeds using an anaerobic phototrophic enrichment culture method. CZR27 is highly resistant to arsenate (As(V)), but more sensitive to As(III). Bioinformatic analysis revealed that CZR27 has three As resistance operons and an As(III) oxidase gene cluster. The As-resistance and As(III) oxidation ability by strain CZR27 and the underlying mechanisms were investigated. Oxidation of As(III) by the strain under anoxic phototrophic conditions coincided with bacterial expression of aioA, indicating participation of the gene in the As(III) oxidation process. Expression of ars genes after exposure to As(III) and As(V) was analyzed. Both As(III) and As(V) were effective inducers. There was no detection of As(III) in cell cultures incubated with As(V), and the genes to reduce As(V) were absent in the CZR27 genome, which indicates that nonenzymatic reduction of As(V) may provide sufficient As(III) for low-level induction of ars genes in strain CZR27. The results indicate that strain CZR27 has As-responsive genes that confer resistance to both As(III) and As(V). Quantitative PCR of aioA revealed the presence of As(III)-oxidizing bacteria in the phyllosphere of duckweeds. The abundance of CZR27 revealed by quantification of the genome-specific gene was significantly larger in the phyllosphere of duckweeds than other niches in the paddy soils. The prevelance of a specific bacterium in the phyllosphere of duckweeds may be useful for efficient and sustainable bioremediation of As-contaminated water.

Keywords: Arsenic; Rhodobacter spp.; Anoxygenic photosynthetic; Arsenite oxidase; Genome-specific gene; Duckweeds phyllosphere.