Production of copper-complexing ligands in response to elevated concentrations of copper in marine Synechococcus sp

Synechococcus spp. (marine cyanobacteria) are extremely sensitive to copper toxicity and can produce high-affinity ligands of unknown structure which form complexes with free cupric ion. These ligands may contribute to the biological control of the levels of free cupric ions in surface seawater. Synechococcus spp. are known to produce metallothioneins (MT) in response to cadmium and zinc stress. In the present study the hypothesis that marine Synechococcus produce MT in response to copper was tested. Three marine Synechococcus spp. i.e., PCC 7001, PCC 73109, and PCC 7003, were exposed to different concentrations of CUSO 4 for various time periods. Size exclusion chromatography, atomic absorption spectrophotometry, and reverse phase HPLC were used to isolate an intracellular copper binding component (CBC) of low molecular weight (<6,500 Da). Synechococcus PCC 73109 produced an intracellular CBC after exposure to ≥ 8 muM CuSO4 for 2 hr. The intracellular CBC was shown not to be MT, phytochelatin or a siderophore. It is not a peptide; it contains lysine and an unidentified UV 254-absorbing component. This compound is a novel copper-binding ligand previously not reported in Synechococcus spp. and may function as a component of a copper export system.