The organic complexation of iron in the marine environment: a review

Abstract

Iron (Fe) is an essential micronutrient for marine organisms, and it is now well established that low Fe availability controls phytoplankton productivity, community structure, and ecosystem functioning in vast regions of the global ocean. The biogeochemical cycle of Fe involves complex interactions between lithogenic inputs (atmospheric, continental, or hydrothermal), dissolution, precipitation, scavenging, biological uptake, remineralization, and sedimentation processes. Each of these aspects of Fe biogeochemical cycling is likely influenced by organic Fe-binding ligands, which complex more than 99% of dissolved Fe. In this review we consider recent advances in our knowledge of Fe complexation in the marine environment and their implications for the biogeochemistry of Fe in the ocean. We also highlight the importance of constraining the dissolved Fe concentration value used in interpreting voltammetric titration data for the determination of Fe speciation. Within the published Fe speciation data, there appear to be important temporal and spatial variations in Fe-binding ligand concentrations and their conditional stability constants in the marine environment. Excess ligand concentrations, particularly in the truly soluble size fraction, seem to be consistently higher in the upper water column, and especially in Fe-limited, but productive, waters. Evidence is accumulating for an association of Fe with both small, well-defined ligands, such as siderophores, as well as with larger, macromolecular complexes like humic substances, exopolymeric substances, and transparent exopolymers. The diverse size spectrum and chemical nature of Fe ligand complexes corresponds to a change in kinetic inertness which will have a consequent impact on biological availability. However, much work is still to be done in coupling voltammetry, mass spectrometry techniques, and process studies to better characterize the nature and cycling of Fe-binding ligands in the marine environment.

Keywords: colloids; exopolymeric substances; humic substances; ligands; nanoparticles; seawater; siderophores; speciation.

Figure 1

(A) Annualized average nitrate (μM) and (B) composite chlorophyll a (mg L⁻¹) distributions observed in surface waters in the global ocean. The nitrate distribution was obtained using data from the World Ocean Atlas 2009 (http://www.nodc.noaa.gov/OC5/WOA09/pr_woa09.html), while the chlorophyll a distribution represents the 2009 Aqua MODIS chlorophyll composite (http://oceancolor.gsfc.nasa.gov/cgi/l3).

Figure 2

Comparison of data obtained for (A) the ligand concentration [L_i], (B) conditional stability constant $\text{log}{K}_{\text{Fe}{\text{L}}_i,\text{Fe’}}^{\text{cond}},$ and (C) excess ligand concentration [eL_i] determined using either total Fe concentrations or the lower dissolved or reactive Fe concentrations. Closed symbols represent data from Gledhill et al. (1998), where Fe_R was used in the comparison, and open symbols represent data from Thuroczy et al. (2010), where dFe was used in the comparison. (D) Plot of the dependence of the change in $\text{log}{K}_{\text{Fe}{\text{L}}_i,\text{Fe’}}^{\text{cond}}$ on the difference between Fe_T and the lower Fe concentration. Solid lines represent a 1:1 relationship.

Figure 3

Structures of fully characterized siderophores that have been identified in seawater or natural seawater incubations. (A) Ferrioxamine B, (B) Ferrioxamine D₂, (C) Ferrioxamine E, (D) Ferrioxamine G, and (E) Amphibactins D (R = C₁₃H₂₇) and E (R = C₁₅H₂₉).

Figure 4

Schematic figure illustrating potential components of the dFe pool so far identified in seawater. Decreasing kinetic lability of Fe within the components is represented by deeper orange background shading.