Copper-binding ligands in the NE Pacific

Type Article
Date 2018-08
Language English
Author(s) Whitby HannahORCID1, Posacka Anna M.2, Maldonado Maria T.2, Van Den Berg Constant M. G.3
Affiliation(s) 1 : LEMAR, Technopole Brest Iroise, F-29280 Plouzane, France.
2 : Univ British Columbia, Vancouver, BC V6T 1Z4, Canada.
3 : Univ Liverpool, Liverpool L69 3GP, Merseyside, England.
Source Marine Chemistry (0304-4203) (Elsevier Science Bv), 2018-08 , Vol. 204 , P. 36-48
DOI 10.1016/j.marchem.2018.05.008
WOS© Times Cited 35
Keyword(s) Copper speciation, Seawater, Thiols, Humic substances, Cathodic stripping voltammetry, Thiourea, Glutathione, Organic ligands

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 (L-1 and L-2) were detected at both stations: the stronger L-1 ligand pool with log K'(cu2+)(L1) 15.0-16.5 and the weaker L-2 ligand pool with log K'(cu2)(+L2) 11.6-13.6. The L-1 class bound on average 94% of dCu, with the ratio between L-1 and dCu constant and close to unity (1.15 = [L-1]:[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 (L-T) and when combined with the two thiols contributed to up to 32% of L-T. 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 L-1, along with the high log K'(cu)(2+)(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.

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Supplementary Fig. 1. An example DP-CSV scan of sample P26, 75 m in the presence of borate/ammonia buffer, (A) with no added copper, with an extended deposition time of 300 ... 155 KB Open access
Supplementary Fig. 2. The relationship between the concentrations of thiols and L2, colour coded to show variation with depth. Top panels (a and b) are P4, bottom panels (c and d) are ... 138 KB Open access
Supplementary Fig. 3. The relationship between copper-binding humic substances (HSCu) with (a) L1 and (b) L2 for P4, and with (c) L1 and (d) L2 for P26. HSCu converted to nM using binding capacity ... 147 KB Open access
Supplementary Fig. 4. The variation with depth for TU, GSH and HSCu as a percentage of LT for (a) station P4 and (b) station P26 137 KB Open access
Supplementary Fig. 5. The variation with depth of the sum of the concentrations of TU, GSH and HSCu as a percentage of the total ligand concentration (LT) for the two stations, P4 (blue) and P26 (red) 88 KB Open access
Supplementary Fig. 6. Example titration curve and fittings in ProMCC software for sample P26, 50 m. Shown are the 14 points that were used to fit the titration, with each point the average peak height 169 KB Open access
Supplementary Table 1. The voltammetric peak position of the three potential thiol peaks in sample P26, 75 m after background subtraction, and their response to additions of copper, thiourea and ... 1 297 KB Open access
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