Living microbes can accumulate and exhibit distinct responses to different metal ions by strong coordination between bio-macromolecules and metal ions. Thus carefully selected bio-macromolecules could possibly serve as sensitive biologic probes to discriminate toxic metal ions. C-Phycocyanin (C-PC), a water-soluble phycobiliprotein widespread in algae, was selected as a unique biologic probe. Heavy-metal ions coordinated with C-PC, altered greatly secondary structures and fluorescence of C-PC, and also induced supramolecular fibrillation, in contrast to inert response of other metal ions.