Periphyton enhances arsenic release and methylation at the soil-water interface of paddy soils

Periphyton is ubiquitous in rice paddy fields, however its role in paddy soil arsenic (As) biogeochemistry remains unexplored. In this study, microcosm incubations and extensive field sampling were used to better understand the roles of periphyton on As mobility and transformation at the soil-water interface. Microcosm incubations revealed that periphyton on the paddy soil surface enhanced As release to water and increased methylated As contents at the soil-water interface. Experimental additions of dissolved phosphate did not significantly affect these processes. The presence of periphyton increased the dissolved organic carbon (DOC) content of the surface soil which may have played a role in the increased As mobility. However, the increase in methylated As species at the soil-water interface is indicative of detoxification processes of As by periphyton. The results from the field study revealed a high abundance and diversity of As biotransformation and detoxification genes in periphyton. Genera of Kineosporia, Limisphaera, Ornatilinea, Ktedonosporobacter and Anaerolinea played key roles in shaping arsM harboring microbe communities in field periphyton. These results highlight the importance of periphyton in the behavior of As in paddy soils and can potentially facilitate improved management of As contamination in paddy soils.