Iron-Binding Ligands in the Southern California Current System: Mechanistic Studies

Abstract

The distributions of dissolved iron and organic iron-binding ligands were examined in water column profiles and deckboard incubation experiments in the southern California Current System (sCCS) along a transition from coastal to semi-oligotrophic waters. Analysis of the iron-binding ligand pool by competitive ligand exchange-adsorptive cathodic stripping voltammetry (CLE-ACSV) using multiple analytical windows (MAWs) revealed three classes of iron-binding ligands present throughout the water column (L 1 −L 3 ), whose distributions closely matched those of dissolved iron and nitrate. Despite significant biogeochemical gradients, ligand profiles were similar between stations, with surface minima in strong ligands (L 1 and L 2 ), and relatively constant concentrations of weaker ligands (L 3 ) down to 500 m. A phytoplankton grow-out incubation, initiated from an iron-limited water mass, showed dynamic temporal cycling of iron-binding ligands. A biological iron model was able to capture the patterns of the strong ligands in the grow-out incubation relatively well with only the microbial community as a biological source. An experiment focused on remineralization of particulate organic matter showed production of both strong and weak iron-binding ligands by the heterotrophic community, supporting a mechanism for in-situ production of both strong and weak iron-binding ligands in the subsurface water column. Photochemical experiments showed a variable influence of sunlight on the degradation of natural iron-binding ligands, providing some evidence to explain differences in surface ligand concentrations between stations. Patterns in ligand distributions between profiles and in the incubation experiments were primarily related to macronutrient concentrations, suggesting microbial remineralization processes might dominate on longer time-scales over short-term changes associated with photochemistry or phytoplankton growth.

The Size Fractionation and Speciation of Iron in the Longqi Hydrothermal Plumes on the Southwest Indian Ridge

“…Most publications reported a single class of ligands in hydrothermal plumes (Bennett et al, ; Hawkes, Connelly, et al, ; Kleint et al, ) and seawater (Boye et al, , ; Bundy et al, ; Gerringa et al, , ; Hunter & Boyd, ; Ibisanmi et al, ; Kondo et al, ; van den Berg, , ) although more than two ligand classes have also been seen in coastal and open ocean water (Buck et al, , ; Buck & Bruland, ; Bundy et al, , , ; Cullen et al, ; Fitzsimmons, Bundy, et al, ; Hogle et al, ). Here we use one ligand model considering it can provide a better fit to the measured data than two ligand model (see supporting information).…”

Toward a Regional Classification to Provide a More Inclusive Examination of the Ocean Biogeochemistry of Iron-Binding Ligands

“…Generally, the weaker ligands found at depth are thought to result from remineralization by heterotrophic bacteria (Hunter and Boyd, 2007;Buck et al, 2015). The production of L 2 as well as L 1 ligands in remineralization experiments Bundy et al, 2016;Velasquez et al, 2016), clearly highlights a dynamic environment which is probably linked with bacterial colonization of sinking particles. Photochemistry is responsible for the production of weaker ligands and/or the degradation of iron binding ligands (e.g., Barbeau et al, 2001;Butler and Theisen, 2010;Gledhill and Buck, 2012), thus representing a loss pathway of strong ligands that can predominate in the upper few meters of surface waters (Figure 1) where they are likely to represent an important transient source of labile iron (Croot and Heller, 2012) to support phytoplankton growth.…”

“…The concentration of Fe-binding ligands tends to be greater and shows most variability in surface waters, with peaks often found co-located with the subsurface chlorophyll maximum implying both a strong relationship with biological activity (Gledhill and Buck, 2012;Gerringa et al, 2015;Bundy et al, 2016) and rapid ligand turnover comparable to that reported for labile DOC (Hansell, 2013). Ligand turnover rates may potentially be derived from variability of organic ligand concentrations in depth profiles or seasonal studies, but in most cases, there are insufficient data to constrain turnover estimates.…”

The biogeochemical cycling of iron, copper, nickel, cadmium, manganese, cobalt, lead, and scandium in a California Current experimental study

“…Field incubation studies are a useful tool for probing the response of natural planktonic communities to changes in micronutrient availability and the processes controlling metal bioavailability. Grow out experiments tracking changes in Fe speciation report strong (i.e., L 1 ‐type) ligand production under Fe‐limiting conditions, hypothesized to be tied to elevated nitrate: dissolved Fe (dFe) ratios (Buck et al ; King et al ; Bundy et al ). Other incubations have shown that siderophore (also an L 1 ‐type ligand) bound Fe is bioavailable to planktonic communities (Maldonado and Price ; Maldonado and Price ), providing a possible mechanism to explain the observed speciation changes.…”

“…The organic ligand pool detected using conventional methods includes strong ligands, which are characteristic of siderophores, but also other weaker ligand classes (i.e., L 2 to L 4 ‐type) that likely represent other metal‐reactive portions of the dissolved organic carbon pool such as polysaccharides, humic substances, and high molecular weight compounds (Gledhill and Buck ). Weak ligand production has also been observed in incubations (Bundy et al ) and attributed to remineralization by bacteria (Boyd et al ), photochemical degradation of stronger L 1 ligands (Barbeau et al ), and viral cell lysis (Poorvin et al ), although the magnitude of each source contribution is still unclear. Indeed, viruses themselves have recently been proposed to constitute a colloidal‐sized Fe‐binding ligand (Bonnain et al ).…”