First Quantification of the Controlling Role of Humic Substances in the Transport of Iron Across the Surface of the Arctic Ocean

Abstract

One of the main reasons behind our current lack of understanding of iron cycling in the oceans is our inability to characterize the ligands that control iron solubility, photosensitivity, reactivity, and bioavailability. We currently lack consensus about the nature and origin of these ligands. Here, we present the first field application of a new methodological development that allows the selective quantification of the fraction of Fe complexed to humic substances (HS). In the HS-rich surface Arctic waters, including the Fe-rich Transpolar Drift (TPD), we found that HS iron binding groups were largely occupied by iron (49%). The overall contribution of Fe−HS complexes to DFe concentrations was substantial at 80% without significant differences between TPD and non-TPD waters. Stabilization and transport of large concentrations of DFe across the surface of the Arctic Ocean are due to the formation of high concentrations of Fe−HS complexes. Competition of Arctic Fe−HS complexes with desferrioxamine and EDTA indicated that their stability constants are considerably higher than the stability constants previously found for riverine HS in temperate estuaries and HS standard material. This is the first case of identification of the ligand-dominating iron speciation over a specific region of the global ocean.

Sources of Fe-binding organic ligands in surface waters of the western Antarctic Peninsula

“…Similarly, as detailed above for the shelf stations, the exudation of EPS from diatoms and haptophytes could be an important addition to the organic ligand pool. Moreover, polysaccharide ligands will be released by microbial cells during the bloom as well as via grazing (Sato et al, 2007;Laglera et al, 2019b) and viral lysis (Poorvin et al, 2011;Slagter et al, 2016).…”

“…In the HNLC Southern Ocean, the availability of Fe has a direct impact on the early spring phytoplankton bloom, and thus on primary productivity (Moore et al, 2013). The Fe limitation in the Southern Ocean could thus have a direct effect on the amount of atmospheric CO2 sequestration (Le Quéré et al, 2016;Arrigo et al, 2008;Raven and Falkowski, 1999) to the deep ocean via the biological pump (De La Rocha, 2006;Lam et al, 2011).…”

“…Other ligand types, such as polysaccharide compounds, are passively generated in situ from microbial excretion and grazing (Sato et al, 2007;Laglera et al, 2019b). The polysaccharides, such as exo-polymeric substances (EPS), are excreted abundantly by a large number of microbial cells, especially in surface water covered by with sea-ice (Norman et al, 2015;Lannuzel et al, 2015).…”

“…Although EPS are relatively labile macromolecules, they can be present in up to micromolar concentrations in seawater, showing the potential to outcompete stronger binding siderophores (Hassler et al, 2017). In addition, humic substances (HS) or HS-like substances from various origins constitute another type of ligand (Krachler et al, 2015;Laglera et al, 2019a;Whitby et al, 2020). Typically, HS are derived from remineralization and degradation of organic matter (Burkhardt et al, 2014).…”

“…HS-like substances have also shown to be part of Fe binding ligands in biologically refractory deep ocean dissolved organic matter (rDOM) with low Fe-bioavailability. However, photodegradation of rDOM was shown to increase the Fe bioavailability making Fe bound to such substances an important source in HNLC areas where upwelling plays a role (Hassler et al, 2020;Lauderdale et al, 2020;Whitby et al, 2020;Laglera et al, 2019a).…”