Here we describe the design and properties of heavy metal biosensor for the measurement of bio-available heavy metals in aqueous environments. The biosensor protein includes an N-terminal, cyan variant of the green fluorescent protein (mCFP), an intervening metallothionein (MT), and a C-terminal yellow variant of the green fluorescent protein (mYFP) from jellyfish. The genes encoding this protein were codon-optimized for expression in Chlamydomonas and the modified green-fluorescent variants were engineered to reduce pH effects on fluorescence emission quantum yield and to reduce and-to-and dimerization. In the absence of heavy metals MT assumes an unfolded, randon-coil structure. Following metal binding MT folds into a compact dumbbell shaped protein. The metal metal-induced folding of MT brings the two fluorescent proteins mCFP (λ_ex= 440 nm and λ_em=485 nm) and mYFP (λ_ex= 515 nm and λ_em=527 nm) within 70Å of each other facilitating fluorescence resonance energy transfer (FRET) between the two fluorophores, resulting in a shift of the relative ratio of yellow and blue fluorescence. We have expressed this heavy metal biosensor in a cell wall-less and arg^- strain of Chlamydomonas reinhardtii. Fluorescence analyses confirm that the protein is expressed in the algae. When grown in TAP media the ratio of yellow to blue fluorescence is ~ 1.0. The experiments show the effects of Cd²⁺, Cu²⁺ and Pb²⁺ ions on the fluorescence emission spectra of the biosensor. These results point to the importance of microalgal biosensor and could be used in biomonitoring heavy metals in various water bodies.