Depth-Resolved Metagenomics Reveals Nitrate- and Sulfate-Mediated Arsenic Transformation in Lake Tosen, a Stratified Saline Lake on the Tibetan Plateau

High-altitude saline lakes on the Tibetan Plateau host some of the world’s most arsenic-rich waters, yet controls on arsenic (As) transformation remain poorly resolved. By integrating depth-resolved geochemistry with metagenomic profiling in Lake Tosen, we identified a redox framework for As cycling that is not fully explained by the Fe-centric model derived from groundwater and freshwater. In summer, oxic-suboxic-anoxic stratification supported three distinct pathways: (i) oxic waters exhibited a microbial detoxification-oxidation loop sustaining As(V) dominance; (ii) suboxic waters were associated with nitrate-supported As(III) oxidation linked to denitrifying taxa; and (iii) anoxic sulfidic waters showed sulfate-reduction-related As transformation, with sulfur cycling likely modifying the fate and mobility of released As under sulfidic conditions. Meanwhile, canonical Fe(III)-respiration markers were not prominent, and saturation indices indicated stable goethite/hematite under in situ conditions. Together, these findings indicate that, under the observed stratified conditions in Lake Tosen, nitrate- and sulfate-associated pathways were more closely linked to arsenic transformation than canonical Fe(III)-respiration signals. This nonclassical coupling may become increasingly relevant as stratification, nitrate loading, and salinization intensify, with implications for As risk prediction in endorheic basins under changing climates.