Periphyton-Associated Arsenic-Transforming Functional Potential at Paddy Soil–Water Interfaces

Arsenic (As) contamination in paddy soils poses a persistent threat to food safety and human health through rice consumption. The environmental fate of As in flooded paddy soils is governed by complex physicochemical and microbially transformations, yet the role of biologically active interfaces remains poorly understood. Periphyton, a ubiquitous and metabolically active biofilm at the paddy soil–water interface, may play a critical role in regulating As speciation and behavior. Herein, we integrated biofilm development, microscopic and elemental characterization, bacterial community profiling, and As-transforming functional gene analysis to elucidate the role of periphyton in As transformation potential. Results showed that periphyton formed a heterogeneous three-dimensional matrix composed of microbial aggregates, extracellular matrix-like materials, and mineral particles. Periphyton-associated As was 2.37 mg kg−1 in the irradiated-soil sample and 1.16 mg kg−1 in the natural-soil sample, whereas dissolved As in the overlying water was 2.06 and 5.85 μg L−1, respectively. The natural-soil composite showed higher bacterial richness and diversity indices than the irradiated-soil composite, with 2615 versus 1124 observed ASVs and Shannon indices of 8.63 versus 7.22. Proteobacteria and Cyanobacteria dominated both communities. The coexistence of aioA, arrA, arsC, and arsM indicated the potential for concurrent As(III) oxidation, As(V) reduction, detoxification, and methylation. In the natural-soil composite, aioA, arrA, and arsM reached 1.84 × 106, 1.99 × 106, and 2.63 × 106 copies g−1 sample, respectively. These values were 2.4-, 9.8-, and 7.2-fold higher than those in the irradiated-soil composite, whereas arsC remained relatively stable. These findings indicate that periphyton provides a structured microbial interface with the genetic potential for multiple As transformation pathways. We propose that paddy periphyton represents a structurally and microbiologically complex interface linking microbial assembly with As-transforming functional potential.