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

Abstract

There is a widely recognized link between iron bioavailability and phytoplankton growth in large tracts of the World Ocean. Iron bioavailability is thus pivotal to the removal of atmospheric carbon via the ocean's 'biological pump.' To evaluate future scenarios of global climate change, we must therefore understand better how the bioavailability of iron in seawater is linked to processes controlling its supply, chemical speciation and removal from the water column. Much of the research on iron inputs to the open ocean has focused on atmospheric, benthic (shelf sediments) and hydrothermal (midocean) sources of iron. The conventional wisdom has been that riverine iron sources are negligible because many of the major world rivers exhibit extensive removal of dissolved iron by flocculation processes during estuarine mixing. However, recent studies have revealed that a fraction of iron associated with peatland-derived humic and fulvic acids may survive aggregation processes in the freshwater-seawater mixing zone, and thus be exported offshore. This review is a synthesis of available data and information for and against the hypothesis that land-derived humic substances exert a significant control on the marine biogeochemical cycle of iron. From the outset, it is shown that this hypothesis can neither be verified nor disproved at present, in part due to analytical difficulties in characterizing the all-important marine colloidal phase. Evidence is then presented on the likely chemical nature and structure of iron-binding humic ligands along with implications for the lateral transport of iron in surface waters and its participation in carbon stabilization in marine sediments. This is followed by a discussion of photochemically and microbially mediated processes acting on terrestrial humic substances and a discussion of how terrestrial humic substances may influence the biological availability of iron. The review finishes with a presentation of measurement technologies and approaches that could be used to assess (i) how iron-binding ligands in seawater relate to land-derived humic substances and (ii) how terrestrial humics may influence the global carbon cycle indirectly by influencing the processes that control the supply and maintain the pool of 'dissolved' iron in the ocean.

Organic iron complexes enhance iron transport capacity along estuarine salinity gradients of Baltic estuaries

“…Fe transport capacity varied from 0.7 % to 24 % among the rivers. The high Fe transport capacity for most of the rivers studied go along with the existing literature showing that high-latitude DOC-rich rivers exhibit higher Fe-carrying capacities (Krachler et al, 2005;Muller, 2018). DOC was little affected by increasing salinity, as previously observed in high-latitude rivers with high DOC concentrations (Herzog et al, 2017;Forsgren et al, 1996).…”

Coastal waters contamination by mining tailings: What triggers the stability of iron in the dissolved and soluble fractions?

“…The SRFA standard solution was saturated with Fe prior to use to ensure the voltammetric signal reflected the contribution of all Fe-binding groups (Sukekava et al 2018). Previous work (Laglera and van den Berg 2009) has shown that the binding capacity of SRFA is 16.7 nM Fe/mg FA. We used this value as a guide to saturate the SRFA standard.…”

“…Assuming a ratio of 16.7 nmol Fe (mg FA) −1 for the binding capacity of humic substances (Laglera and van den Berg 2009), the maximum concentration of Fe-binding by humic substances would have been 0.2-5.8 nmol L −1 (Table 3). Based on previous studies, most of dissolved Fe would have been anticipated to occur as organic complexes, mainly with humic substances (Laglera and van den Berg 2009). In reality, the binding capacity of humic substances could only explain a maximum of between 0.2% and 13% (median is 2%) of the dissolved Fe (0.22 μm) ( Table 3).…”

“…These compounds are different in their origin, conditional stability and dispersal patterns from coastal to offshore oceanic regions (Hassler et al 2017). In estuarine and coastal waters, there is mounting evidence that humic substances dominate the bulk of organic ligands (e.g., Laglera and van den Berg 2009). These humic substances have been shown to be mostly of terrestrial origin (e.g., Laglera and van den Berg 2009) and although they undergo extensive removal within estuaries through coagulation and flocculation processes (Sholkovitz et al 1978), a significant proportion makes it through the estuarine trapping (Muller 2018).…”

Particle aggregation, pH changes and metal behaviour during estuarine mixing: review and integration

“…However, with regard to the truly dissolved Fe fraction (typically a much lower concentration than the solid Fe phase), found it behaved conservatively in the Ob and Yenisey estuaries. Some dissolved and colloidal Fe-binding constituents of aquatic humic substances also appear not to be subject to coagulation along the estuarine salinity gradient (Merschel et al 2017;Muller 2018). Hence, although it seems likely much particulate Fe is deposited to estuarine sediments, a portion can still be transported through estuaries to the coastal ocean (Sanders et al 2015).…”