Spatiotemporal patterns of arsenic and its microbial arsenic transformation in the Pearl River Estuary

Estuarine ecosystems are critical zones for arsenic (As) biogeochemical cycling, yet the spatiotemporal distribution and microbial transformation mechanisms of As in these dynamic environments remain poorly understood. This study integrated geochemical analyses with metagenomic and metatranscriptomic approaches to investigate As distribution and microbial transformation mechanisms in Pearl River Estuary (PRE). Our results revealed distinct spatiotemporal patterns of As in the PRE. As in sediment were significantly higher in the western region and exhibited a clear decreasing gradient from upstream to downstream. As(V) was the dominant species in both sediments and water, while organic As remained below detection limits. Seasonally, As concentrations peaked in winter and spring. Microbial community analysis showed that highly diverse microbial taxa capable of transforming As were detected, with Proteobacteria identified as the dominant phylum. Among key functional genes, arsM exhibited the highest abundance and transcription level, indicating substantial methylation potential throughout the estuary. Notably, metagenome-assembled genome (MAG) analysis uncovered a previously undocumented metabolic transition along the estuarine gradient, shifting from As(V) reduction coupled with methylation and efflux in upstream to As(III) oxidation with a more diversified strategy in mid-downstream. This systematic study clarified the distribution and microbial transformation mechanisms of As in the PRE, advancing our understanding of As biogeochemical cycling in estuarine ecosystems.