Linking Chitin Degradation to Arsenic Methylation under Oxic Conditions: A Critical Driver of Arsenic Cycling in Paddy Soils

Aerobic microbial arsenic (As) methylation exhibits significant efficiency, potentially influencing As fluxes and their biogeochemistry in paddy soils. However, the role of primary degraders initiating decomposition of complex biopolymers in As methylation and the underlying driving mechanisms remain largely unexplored. This study uncovers a direct metabolic connection between chitin, a major component of particulate organic matter in the soil, and As methylation facilitated by specialized aerobic chitin-degrading bacteria. Chitin markedly enhanced As methylation and its volatilization in paddy soils under aerobic conditions, resulting in 2.9- and 25-fold higher increases compared with the treatments with lignocellulosic residue and components, respectively. <i>Chitinophagaceae</i> members were enriched in the presence of chitin, with bacteria related to <i>Chitinophaga</i> likely serving as the key contributors. The direct coupling between As methylation and chitin hydrolysis was further confirmed using a representative <i>Chitinophaga</i> strain isolated from a paddy soil, which produced dimethylarsenate and trimethylarsenate as the major MeAs products. Comparative genomic analysis revealed that a considerable proportion of <i>Chitinophaga</i> found diverse soil environments possess complete functional genes for As methylation and chitin utilization, highlighting their potential for regulating As cycling broadly. These findings emphasize the chitin degradation-associated As methylation as a previously overlooked contributor to MeAs fluxes, enhancing our understanding of the interconnected biogeochemistry of As and carbon in soils.