Floating macrophyte growth and decomposition greatly affects the exogenous antimony mobility and microbial community...

Anthropogenic antimony (Sb) contamination in aquatic systems poses persistent ecological risks, yet the role of floating macrophyte life-cycle processes in regulating Sb migration and speciation remains poorly understood. In this study, a mesocosm experiment was conducted to investigate how the growth and decomposition of Alternanthera philoxeroides (AP) influence Sb mobility and transformation following exogenous Sb(V) input. Results show that Sb was ultimately sequestered in sediments, which acted as a dynamic regulator rather than a passive sink, controlling Sb retention and long-term reactivity. Rapid surface adsorption was followed by progressive downward migration driven by redox-sensitive remobilization and re-adsorption onto deeper mineral phases, with Sb predominantly associated with amorphous and poorly crystalline Fe/Al (hydr)oxides (67.3-84.1%). Growth of AP accelerated Sb removal from the water column mainly through indirect, DOM-mediated sequestration rather than direct plant uptake, while simultaneously enhancing the vertical redistribution of bioavailable Sb within sediments. In contrast, AP removal followed by decomposition caused pronounced physical and biogeochemical disturbances. These disturbances induced transient reducing conditions, organic matter release, and a marked increase in pH (up to 9.14), collectively promoting Sb remobilization and Sb(III) release into the overlying water. As a result, Sb(III) concentrations were up to 67-fold higher than those in the unvegetated control. Exogenous Sb strongly reshaped sediment microbial communities, selectively enriching metal-tolerant taxa such as Actinomycetota (genus Streptomyces) and favoring functional traits related to Sb detoxification and elemental cycling. Metagenomic evidence indicates that Sb resistance, coupled with coordinated C, N, P, and S cycling functions, enables the indigenous microbiome to actively regulate Sb speciation and mobility, particularly under organic matter inputs derived from macrophyte growth and decomposition. These findings demonstrate that floating macrophytes exert process-level control over Sb cycling, with life-cycle-mediated biogeochemical feedbacks governing its mobility, speciation, and persistence in water-sediment systems.