Copper-binding ligands in the NE Pacific

Copper distribution and speciation were determined at stations P4 and P26 along Line P as part of a GEOTRACES Process Study in the Northeast Pacific, at depths between 10 and 1400 m. Two ligand classes (L1 and L2) were detected at both stations: the stronger L1 ligand pool with log K'Cu2+L1 15.0-16.5 and the weaker L2 ligand pool with log K'Cu2+L2 11.6-13.6. The L1 class bound on average 94% of dCu, with the ratio between L1 and dCu constant and close to unity (1.15 = [L1]:[dCu]). The concentrations of total ligands exceeded those of dCu at all depths, buffering Cu2+ concentrations ([Cu2+]) to femtomolar levels (i.e. pCu 14.1-15.7). Measurements using cathodic stripping voltammetry also identified natural copper-responsive peaks, which were attributed to thiourea- and glutathione-like thiols (TU and GSH, respectively), and Cu-binding humic substances (HSCu). Concentrations of TU, GSH and HSCu were determined by standard addition of model compounds in an attempt to identify Cu-binding ligands. HSCu concentrations were generally higher at P26 than at P4, consistent with a marine origin of the humic material. Overall, HSCu contributed to 1-27% of the total L concentration (LT) and when combined with the two thiols contributed to up to 32% of LT. This suggests other ligand types are responsible for the majority of dCu complexation in these waters, such as other thiols. Some potential candidates for detected, but unidentified, thiols are cysteine, 3-mercaptopropionic acid and 2-mercaptoethanol, all of which bind Cu. Significant correlation between the concentrations of TU-like thiols and L1, along with the high log K'Cu2+L1 values, tentatively suggest that the electrochemical TU-type peak could be part of a larger, unidentified, high-affinity Cu compound, such as a methanobactin or porphyrin, with a stronger binding capability than typical thiols. This could imply that chalkophores may play a greater role in oceanic dCu complexation than previously considered.