Iron complexation by phenolic ligands in seawater

Abstract

Iron is an essential micronutrient for phytoplankton and can limit primary production in the ocean. Fe chemistry is highly controlled by its interaction with organic complexes (>99%). It is still unknown which organic compounds produced by cells have the ability to bind Fe. Within the pool of organic ligands, polyphenols are known to be exudated by marine diatoms and, in this study, the role of three polyphenols ((±) – catechin, sinapic acid and gallic acid) was studied in terms of dissolved Fe complexation via kinetic and titration approaches, and also their role as a source of Fe(II) in seawater. The results demonstrated that these three polyphenols are weak L2-type Fe-binding ligands according to the conditional stability constant, computed by using the kinetic approach (log K′Fe′L = 8.86–9.2), where the formation rate constant (kf) was 3.1·105–4.2·105 M−1 s−1 and the dissociation rate constant (kd) was 2.43·10−4–4.4·10−4 s−1. The conditional stability was also computed from the titration approach with log K′Fe′L from 8.6 to 9.5. These studied ligands also regenerated Fe(II) in seawater from 0.05% to 11.92%. The results obtained in this study suggest that polyphenols increase the persistence of dissolved Fe and should be considered as an important Fe-binding ligands in seawater to better understand the global biogeochemical cycles. This article is part of a special issue entitled: “Cycles of trace elements and isotopes in the ocean – GEOTRACES and beyond” - edited by Tim M. Conway, Tristan Horner, Yves Plancherel, and Aridane G. González.