A detailed investigation of iron complexation by organic ligands in the Western Tropical South Pacific Ocean

17 May 2024

Léo Mahieu and his co-workers (2024, see reference below) present the conditional concentration (LFe) and binding-strength (log K) of iron-binding ligands in the Western Tropical South Pacific Ocean during the GEOTRACES TONGA cruise (GPpr14). Based on the analysis of 103 samples, they found a high mean LFe of 5.2 ± 1.2 nMeqFe, predominantly consisting of intermediate strength L2 ligands (84%; mean log K of 11.6 ± 0.4), consistent with complexation by humic-like substances. However, only 30% of dissolved iron (DFe) in these samples is within humic complexes and ligands were in great excess of DFe in the mixed layer as in deep waters, suggesting poor stabilization of DFe inputs in this area.

The authors interpret that high LFe (up to 9 nMeqFe) in samples close to hydrothermal sites could be due to detoxification strategies from plankton communities toward hydrothermally-fueled toxic trace metals other than Fe. In addition, competition between Fe and competing metals for ligand binding sites could favor DFe oxidation and precipitation near hydrothermal vents and explain the absence of strong Fe stabilization.

**Figure 1.** Map of the area studied during the TONGA cruise (GPpr14), with main geographical characteristics, current directions, and sampling locations. The sampling locations were separated by regions for drawing the section plots presented in Figure 2 and 3.

**Figure 2.** Section plots of DFe (A), fluorescent humic-like substances (HS-like; B) and electroactive humic-like substances (L_FeHS, C). The DFe concentrations are increased in the Lau Basin, weakly correlated to the variations in HS-like, both impacted by remineralization of the DOM. The L_FeHS are very low in the Lau Basin, except for surface increases related to biological production.

**Figure 3.** Section plots of L_Fe (A), excess L_Fe (with regard to DFe concentration, eL_Fe; B), and log K (C). The L_Fe increase in deep waters of the Lau Basin is explained by the increase in DFe, supposedly related to remineralization of the DOM. eL_Fe reveals strong L_Fe production in deep waters at station 2 and station 6, potentially related to hydrothermal activity, while the absence of DFe stabilization (Figure 2A) suggests that these ligands are not specific to Fe. Log K values corresponds to humic substances in the vast majority of the transect.

Reference:

Mahieu, L., Whitby, H., Dulaquais, G., Tilliette, C., Guigue, C., Tedetti, M., Lefevre, D., Fourrier, P., Bressac, M., Sarthou, G., Bonnet, S., Guieu, C., & Salaün, P. (2024). Iron-binding by dissolved organic matter in the Western Tropical South Pacific Ocean (GEOTRACES TONGA cruise GPpr14). Frontiers in Marine Science, 11. Access the paper: 10.3389/fmars.2024.1304118

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