Synergistic effect of silicon availability and salinity on metal adsorption in a common estuarine diatom

•Diatoms shaped by Si enrichment or low salinity accumulated more Cd and Cu.•Si-enriched cells under low salinity showed the highest instantaneous metal influx.•The adsorption capacity of diatoms is mediated by cell wall physicochemical traits.•Lower Si availability and salinity led to rougher cell walls with larger surface area.•Higher silicification level decreased cell surface potential with more silanol groups.