Salinity-dependent toxicities of zinc oxide nanoparticles to the marine diatom Thalassiosira pseudonana

•Increasing salinity enhanced the aggregation and decreased the Zn2+ release.•Toxicity of ZnO–NPs to the marine diatom decreased with increasing salinity.•Toxicity of ZnO–NPs to the marine diatom was partly due to dissolved Zn2+.•ZnO–NPs aggregates attached on the algal surface.•Interaction between ZnO–NPs and the cell surface may explain the toxicity.