Although nearly all Cu dissolved in seawater is bound to organic ligands, the structures of these ligands remain uncharacterized. To the best of our knowledge there is only one published high resolution MS 2 spectrum for a CuL detected in the Eastern Tropical South Pacific (ETSP). Compositionally, the ETSP ligand, [C20H21O8S2N4] + , is similar to the ligands we characterized from P. tricornutum in that it has four N atoms and an H:C ratio of 1.0, which falls within the range of H:C ratios found in this study (0.96 to 1.09). However, there are two significant differences: the ETSP ligand contains two S heteroatoms and was found to bind Ni as well as Cu, while the ligands from P. tricornutum did not contain S and were specific to Cu. Additionally, the O:C and N:C ratios of the ETSP ligand, 0.4 and 0.19, respectively, were above the ranges found in this study (0.18 to 0.30 and 0.11 to 0.13, respectively). These compositional differences indicate that the ETSP ligand is more protein-like than the ligands in this study. Chlorophyll catabolites of higher plants readily form complexes with metals such as Zn 2+ , Ni 2+ , and Cu 2+ and it has been suggested that this may point to a biological role of heavy metal detoxification. Given the structural similarities between the CuLs in P. tricornutum media extracts and chlorophyll catabolites, it is possible that CuLs may likewise play a role in Cu detoxification. In order to determine this, the strength of the CuL complexes needs to be determined and further experiments with cultures grown under different Cu concentrations up to the toxicity threshold of P. tricornutum need to be made. The structural information provided by our study may help with these studies and in further characterizing CuLs in environmental samples.