Iron-binding ligands and their role in the ocean biogeochemistry of iron

Abstract

Environmental context. It is now well accepted that iron is an essential micronutrient for phytoplankton growth in many areas of the global ocean, even though this element is present in seawater in extremely low abundance. It is also known that most of the iron in seawater is present as complexes formed with ligands of natural organic matter whose nature and origin remain largely unknown. Here we consider how these iron-complexing ligands might have evolved during geological time, what factors may have given rise to their presence and the possible roles that they play in iron biogeochemistry.Abstract. Current knowledge about the role of iron-binding organic ligands in the ocean and their role in determining the biogeochemistry of this biologically active element has been summarised. Some electrochemical measurements suggest the presence of at least two ligand types, a strong binding ligand L 1 found mainly in the mixed layer and a weaker ligand L 2 found mainly in deep water. Speciation of Fe III is dominated by L 1 in the mixed layer and L 2 in the deep ocean. There is some evidence that L 1 is siderophore-like and is specifically generated by marine microbes (i.e. heterotropic bacteria and cyanobacteria). We suggest that this is a specific biological mechanism for sequestering iron in the mixed layer that developed early in the ocean's history (Archaean period, 2500-3500 million years BP), whereas the more ubiquitous L 2 ligand only arose at the close of the Proterozoic (500-2500 million years BP) when eukaryotic organisms evolved to switch on the ocean's biological pump, allowing L 2 ligands to form from the oxidation of sinking biological particles. This development coincided with the complete oxygenation of the ocean's interior which removed the iron-binding sulfide ion and allowed maintenance of the ocean's iron inventory. These speculations are accompanied by various suggestions about avenues for future research to better understand iron biogeochemistry.

Southern Ocean phytoplankton physiology in a changing climate

“…Fe-binding organic ligands are critical for Fe biogeochemistry, improving its solubility and affecting its reactivity to support phytoplankton growth Hassler et al, 2011aHassler et al, , 2012. The distribution of Fe-binding organic ligands in the open ocean is compatible with multiple biological sources associated with Fe-stress and its recycling/remineralisation (Hunter and Boyd, 2007). However, despite their recognised importance, the production pathways, nature and binding mechanisms of insitu Fe-binding ligands are mostly unknown (Gledhill and Buck, 2012).…”

Speciation of Fe in the Eastern North Atlantic Ocean

“…This indicated that the ligands presumably coming from the phytoplankton are relatively weak, supporting the conclusion of Rijkenberg et al (2008b) that phytoplankton can modify ligand characteristics. Others stated that ligands related to phytoplankton activity belong to the relatively strong ligand group (Rue and Bruland, 1997;Cullen et al, 2006;Hunter and Boyd, 2007).…”

Saccharides enhance iron bioavailability to Southern Ocean phytoplankton

“…However, a photoactive siderophore that belongs to the L2 class has now been reported (3). Moreover, uncertainties remain about the nature of organic ligands that bind iron (12,16). Ligands too weak to fall within the analytical window of the competitive ligand exchange technique are also not detected (17); thus, their importance for iron biogeochemistry is likely to be underestimated.…”

“…Both experimental (4,14) and modeling (15) observations agree that organic complexation is necessary to maintain iron solubility and bioavailability in the ocean. In oceanic waters, two classes of organic ligands (L1 and L2) have been identified on the basis of their binding affinities for iron (12). It is widely accepted, from comparable conditional stability constants, that siderophores produced by bacterioplankton (i.e., heterotrophic and autotrophic bacteria) belong to the strongest class (L1) of organic ligands.…”

“…In marine systems, virtually all of the iron is bound to organic ligands (12,13); hence, organic complexation is the main regulator of both iron reactivity and bioavailability. Both experimental (4,14) and modeling (15) observations agree that organic complexation is necessary to maintain iron solubility and bioavailability in the ocean.…”