Novel copper-binding and nitrogen-rich thiols produced and exuded by Emiliania huxleyi

In surface seawater, dissolved copper (Cu) is predominantly bound by a small pool of strong ligands, with further complexation by a larger pool of weaker ligands. Although the chemical structure of both classes of ligands observed in the open ocean remains unknown, it is believed that the stronger ligands are of biological origin, and it has been hypothesized that they are thiols. Using precolumn derivatization high-performance liquid chromatography electrospray ionization ion-trap mass spectrometry, we found that the ubiquitous coccolithophorid Emiliania huxleyi constitutively produces two previously uncharacterized thiols, arginine-cysteine and glutamine-cysteine, in high intracellular concentrations. E. huxleyi exudes these novel thiols, along with cysteine, in response to increased Cu concentrations in the growth media. Furthermore, stable Cu (I) complexes with the exuded thiols were observed in the growth media using matrix-assisted laser desorption time-of-flight mass spectrometry following size exclusion chromatography. Additionally, E. huxleyi appears to utilize these novel thiols in nitrogen storage and assimilation, as they are rapidly synthesized upon nitrogen addition to a nitrogen-depleted culture. The identification of specific ligands produced by this organism will facilitate further linkages between this likely source and the pools of Cu-binding ligands observed in surface seawater, while the discovery of novel thiols signifies the presence of unique and previously unknown biochemical pathways in E. huxleyi.