Photoreduction of Terrigenous Fe‐Humic Substances Leads to Bioavailable Iron in Oceans

Abstract

Humic substances (HS) are important iron chelators responsible for the transport of iron from freshwater systems to the open sea, where iron is essential for marine organisms. Evidence suggests that iron complexed to HS comprises the bulk of the iron ligand pool in near‐coastal waters and shelf seas. River‐derived HS have been investigated to study their transport to, and dwell in oceanic waters. A library of iron model compounds and river‐derived Fe‐HS samples were probed in a combined X‐ray absorption spectroscopy (XAS) and valence‐to‐core X‐ray emission spectroscopy (VtC‐XES) study at the Fe K‐edge. The analyses performed revealed that iron complexation in HS samples is only dependent on oxygen‐containing HS functional groups, such as carboxyl and phenol. The photoreduction mechanism of FeIII‐HS in oceanic conditions into bioavailable aquatic FeII forms, highlights the importance of river‐derived HS as an iron source for marine organisms. Consequently, such mechanisms are a vital component of the upper‐ocean iron biogeochemistry cycle.

Characterization of Iron and Organic Carbon Colloids in Boreal Rivers and Their Fate at High Salinity

“…The X‐ray absorption near edge structure (XANES) data for the aggregated sample from River Lyckeby were similar to the water samples from Rivers Helge and Mörrum, although the double peak in the first derivative was more distinct, indicating an even higher contribution of Fe(oxy)hydroxides (Figure 2). The low contribution of Fe‐OM complexes in the aggregates suggests that the chelate formation indicated by the Fe‐C path contributes to the stability of the Fe at higher salinity (Blazevic et al, 2016; Herzog et al, 2017). Aggregation is caused by weakening of repulsive forces as a result of a compression of the diffuse double layer of the colloids.…”

Exploring the Potential Role of Terrestrially Derived Humic Substances in the Marine Biogeochemistry of Iron

“…Because of this continuum, it seems reasonable to use the term humic, to refer collectively to humic and fulvic acids. Additional support for this terminology comes from recent findings that all humic and fulvic substances appear to contain the same functional groups, i.e., carboxylic, phenolic, and carbonyl, only their abundances differ (Blazevic et al, 2016;Schellekens et al, 2017). On the basis of such findings, Orlowska et al (2017b) synthesized a range of model compounds in an attempt to represent the main components of humic substances, namely lignin decomposition products.…”

“…Theoretical approaches to the classification of metals and ligands in solution tell us that Fe 3+ is a hard Lewis acid which will tend to form strong complexes with correspondingly hard ligands such as OH − (Morel and Hering, 1993;Stumm and Morgan, 1996;Luther, 2016). Recent XAS analyses of terrigenous humic substances have indeed shown that iron complexation was only dependent on oxygen containing groups such as carbonyl and phenol (Blazevic et al, 2016;Orlowska et al, 2017a). This, in turn, is consistent with the view that lignin products are important in maintaining iron in solution (Murphy et al, 2008;Krachler et al, 2012) and also that functional groups residing in the proximity of aromatic structures within the humics are especially significant (Fujii et al, 2014).…”

β-O-4 type dilignol compounds and their iron complexes for modeling of iron binding to humic acids: synthesis, characterization, electrochemical studies and algal growth experiments

“…The proposed mechanism of iron release in natural humic acid complexes includes the photoreduction of Fe(III) to Fe(II). 36 Fe(II) has low affinity to AHS, dissociates from the complex and can be uptaken by the microorganisms. Although Fe(II) undergoes oxidation in oxygenated seawater, the steady state concentration of Fe(II) remains higher due to this reduction process.…”

“…In our previous studies, we synthesized simple iron complexes with different coordination motifs and ligand scaffolds and investigated them for their suitability as model compounds for humic acids iron complexes. 37 The model compounds were characterized and investigated by various analytical techniques (cyclic voltammetry, EPR, EXAFS, 36 UV-Vis spectroscopy etc.) in addition to algal batch culture studies on chlorophyte and haptophyte unicellular algae species.…”