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Copper speciation in the upper marine water column is dominated by strong ligands thought to be of recent biological origin. Cultures of the marine cyanobacteria Synechococcus spp., a ubiquitous and important group of phytoplankton highly sensitive to Cu toxicity, were previously shown to produce chelators comparable in strength to those detected in the water column. Here we show that cultures of Synechococcus exposed to toxic concentrations of Cu produce an extracellular ligand with a binding constant comparable to constants for ligands found in the water column. Coordination of Cu by this compound decreases the concentration of free cupric ion (the toxic form) in the culture media to levels that do not inhibit growth. A tight linear correlation between chelator and Cu concentration suggests that production of this substance may be regulated by the concentration of free Cu in the media in a feedback mechanism. Similarly, the concentrations of Cu and Cu-binding ligands in the water column are often closely related. These results suggest that cyanobacteria modify Cu chemistry in seawater, creating conditions more favorable for growth.
Limnology and Oceanography