PRODUCTION OF STRONG, EXTRACELLULAR CU CHELATORS BY MARINE CYANOBACTERIA IN RESPONSE TO CU STRESS

Copper speciation in the upper marine water column is dominated by str ong ligands thought to be of recent biological origin. Cultures of the marine cyanobacteria Synechococcus spp., a ubiquitous and important g roup of phytoplankton highly sensitive to Cu toxicity, were previously shown to produce chelators comparable in strength to those detected i n the water column. Here we show that cultures of Synechococcus expose d to toxic concentrations of Cu produce an extracellular ligand with a binding constant comparable to constants for ligands found in the wat er column. Coordination of Cu by this compound decreases the concentra tion of free cupric ion (the toxic form) in the culture media to level s that do not inhibit growth. A tight linear correlation between chela tor and Cu concentration suggests that production of this substance ma y be regulated by the concentration of free Cu in the media in a feedb ack mechanism. Similarly, the concentrations of Cu and Cu-binding liga nds in the water column are often closely related. These results sugge st that cyanobacteria modify Cu chemistry in seawater, creating condit ions more favorable for growth.