Spent culture media from the marine diatom P. tricornutum was analyzed by LC-FT-ICR MS at 21 T to characterize CuLs. Eleven CuLs were detected and assigned molecular formulae, which were confirmed through an analysis of their 15 N, C, and O isotopologues. The ligands are highly oxidized and rich in nitrogen, with O:C and N:C ratios of 0.18-0.30 and 0.11-0.13, respectively, and the Cu ion was in the +2 oxidation state. Tandem MS 2 and MS 3 spectra of CuLs and their metal-free analogues yielded diagnostic ions from fragmentation pathways characteristic of tri-and tetrapyrroles. These ligands are tri-dentate and bind Cu through coordination with nitrogen, similar to a CuL previously characterized from seawater. However, unlike the single CuL previously characterized from seawater, the CuLs isolated from P. tricornutum media were specific for Cu and did not bind Ni. Draft structures of P. tricornutum CuLs show strong similarities to known chlorophyll catabolites, suggesting that the CuLs characterized here maybe widely produced by marine photoautotrophs.