Biosilica of the diatom species Thalassiosira pseudonana is used as hard template for the synthesis of silicon carbide-derived carbons. The typical species-specific macroporous structure is retained during the nanocasting-chlorine treatment process and the resulting materials exhibit very high specific surface areas up to 2300 m² g^-1. Bioinspired carbons show very high capacities in mercury adsorption from aqueous solution compared to reference materials.