The role of bacterial and algal exopolymeric substances in iron chemistry

It is widely accepted that the complexation of iron (Fe) with organic compounds is the primary factor that regulates Fe reactivity and its bioavailability to phytoplankton in the open ocean. Despite considerable efforts to unravel the provenance of the many organic ligands present in the ‘ligand soup’ and their contribution to Fe chemistry, much of this pool remains largely unresolved. Bacteria and phytoplankton are known to release exopolymeric substances (EPS) for a variety of functions and it is known that this material has metal binding properties. However, the contribution that bacterial and algal EPS makes to Fe biogeochemistry is not well documented. This study revealed that both bacterial and algal EPS contain functional components known to bind Fe (uronic acid, saccharides) and details the molecular weight distribution of the EPS. It is also demonstrated that components of the EPS have a high affinity for Fe-binding, in some cases similar to that of bacterial siderophores (~ K_Fe′L 10¹²) and that this material greatly enhances Fe solubility (and, possibly, Fe oxyhydroxide reactivity via prevention of aggregation) in seawater. However, EPS may also accelerate Fe(II) oxidation and thus Fe(II) removal from the system. Our findings indicate that, in remote ocean regions, bacterial and algal EPS could play a significant role in the biogeochemical cycling of Fe and their contribution should be considered to further our understanding of the dynamics of Fe-limited oceans.