New Evidence of MnO x -Mediated Pb Sequestration by Estuarine Biofilms in a Sediment–Water Microcosm

Abstract Intertidal biofilms experience recurring Pb exposure due to pollutant accumulation and sediment remobilization, and yet the sources supplying Pb to biofilms and mechanisms governing its uptake remain ambiguous. Using biofilms from Sundarbans as a globally relevant model, we developed a sediment–water microcosm mimicking estuarine conditions and provided a dissolved Pb pulse in a spiked tank, while maintaining an unspiked control for comparison. Laboratory-controlled experiments measuring the Pb isotope and time-series data across growth medium, biofilm, and sediment compartments indicate that biofilm uptake is dominated by the dissolved pool, with Pb removal reaching ∼77% by Day 21. Sediment–water isotope exchange progressively modifies the dissolved-pool isotopic composition, which is subsequently reflected in biofilms through continued uptake. Pb exposure triggers strong extracellular polymeric substance (EPS) upregulation, establishing a reactive extracellular matrix that acts as the primary site where key processes occur and Pb is retained. Biogeochemical evidence further indicates that Pb was sorbed predominantly through mineral-associated pathways within the EPS matrix, including association with newly formed manganese oxide (MnOx) phases and capture in phosphate-bearing solids (pyromorphite), rather than by EPS chelation alone. Overall, this study provides a sediment-inclusive model mimicking estuarine conditions to identify where Pb in biofilms comes from and how biofilms cope with dissolved Pb exposure.