The role of humic-type ligands in the bioavailability and stabilization of dissolved iron in the Western Tropical South Pacific Ocean



Dulaquais, Gabriel, Fourrier, Pierre, Guieu, Cecile, Mahieu, Leo, Riso, Ricardo, Salaun, Pascal ORCID: 0000-0001-9525-3382, Tilliette, Chloe and Whitby, Hannah ORCID: 0000-0002-0064-3052
(2023) The role of humic-type ligands in the bioavailability and stabilization of dissolved iron in the Western Tropical South Pacific Ocean. FRONTIERS IN MARINE SCIENCE, 10.

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Abstract

<jats:p>The high N<jats:sub>2</jats:sub> fixation rate observed in the Lau Basin of the western tropical South Pacific Ocean (WTSP) is fueled by iron (Fe) released from shallow hydrothermal systems. Understanding Fe bioavailability is crucial but the controls on the stability and bioavailability of hydrothermal Fe inputs are still poorly understood. Here, we provide new data on the spatial and vertical distribution of the soluble ubiquitous humic-like ligands (L<jats:sub>FeHS</jats:sub>) and their associated dissolved Fe (DFe) in the WTSP, including in samples near hydrothermal vents. Our data show that L<jats:sub>FeHS</jats:sub> are heterogenous ligands with binding sites of both strong and intermediate strengths. These ligands are primarily produced in surface waters and partially mineralized in mesopelagic waters. A substantial fraction of DFe was complexed by L<jats:sub>FeHS</jats:sub> (mean ~30%). The DFe complexed by L<jats:sub>FeHS</jats:sub> is likely bioavailable to phytoplankton and L<jats:sub>FeHS</jats:sub> stabilized Fe released by the mineralization of sinking biomass. However, unsaturation of L<jats:sub>FeHS</jats:sub> by Fe suggest that part of DFe is not available for complexation with L<jats:sub>FeHS</jats:sub>. Possible reasons are competition between DFe and other metals, such as dissolved copper, or the inability of L<jats:sub>FeHS</jats:sub> to access colloidal DFe. The study of two volcanic sites indicates that L<jats:sub>FeHS</jats:sub> were not produced in these hydrothermal systems. At the active site (DFe ~50 nmol L<jats:sup>-1</jats:sup>), L<jats:sub>FeHS</jats:sub> can only partially solubilize the hydrothermal DFe released in this area (1~5.5% of the total DFe). We performed controlled laboratory experiments which show that the observed low solubilization yield result from the inability of L<jats:sub>FeHS</jats:sub> to solubilize aged Fe oxyhydroxides (FeOx - a kinetically mediated process) and to form stable complexes with Fe(II) species. Our study provides new understanding of the role of L<jats:sub>FeHS</jats:sub> on the bioavailability and stabilization of hydrothermal DFe.</jats:p>

Item Type: Article
Uncontrolled Keywords: iron, organic complexation, humic substances, hydrothermal vents, Pacific Ocean
Divisions: Faculty of Science and Engineering > School of Environmental Sciences
Depositing User: Symplectic Admin
Date Deposited: 21 Sep 2023 12:33
Last Modified: 29 Sep 2023 06:50
DOI: 10.3389/fmars.2023.1219594
Open Access URL: https://doi.org/10.3389/fmars.2023.1219594
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URI: https://livrepository.liverpool.ac.uk/id/eprint/3172937