In surface seawater many metals are complexed by organic ligands that have been hypothesized to be of recent biological origin. Ligands containing sulfhydryl groups may be important for some metals such as Cu, Cd and Hg. Phytochelatin, a small sulfhydryl-containing peptide that is synthesized in response to trace metal stress, is exuded by marine phytoplankton in response to metal stress. We have examined the production, exudation and degradation rates of phytochelatin in seawater and have proposed a model to predict dissolved phytochelatin concentrations in seawater. In addition, we have recently discovered two previously unknown thiol-containing compounds in the culture medium of the species Emiliania huxleyi following Cu addition and also found that this organism constitutively contains these two compounds (arginine-cysteine and glutamine-cysteine) in high intracellular concentrations. We found that E. huxleyi exudes these thiols, along with cysteine, in response to increased copper concentrations in the growth media. Furthermore, stable Cu (I) complexes with the exuded thiols were observed in the growth media. Recent field measurements of dissolved thiols suggest that a suite of biogenic thiols may be involved in metal complexation in surface seawater.