In the Southern Ocean, phytoplankton growth is largely limited by the lack of iron, affecting the biogeochemical cycling not only of iron itself but also of other elements, including nutrients and carbon. It is now recognized that iron limitation affects carbon cycling globally and thus plays a role in Earth's climate regulation. The bioavailable fraction of iron is the fraction that can effectively interact with phytoplankton to support their iron-dependent metabolic reactions and growth. As such, it is the bioavailable iron pool that shapes phytoplankton communities in most of the Southern Ocean. Despite numerous studies, parameters controlling iron bioavailability to phytoplankton are still poorly understood, probably due to an extremely complex and dynamic interplay between iron chemistry and biology in surface waters. Iron bioavailability depends on chemical and physical speciation and the different uptake strategies of the phyto- and bacterio-plankton communities. In the Southern Ocean, 99% of the dissolved iron is complexed by organic ligands, which likely controls its bioavailability. Furthermore, microorganisms also exert feedback on iron chemistry, for instance, by releasing organic iron-binding ligands through production, cell lysis, or degradation of fecal pellets, as well as by reducing iron at the cell surface. Regeneration of iron, through grazing as well as bacterial and viral activities, is another pathway that supplies iron to phytoplankton communities. Field investigations of iron speciation in the Southern Ocean are discussed in conjunction with laboratory assessments of iron speciation and bioavailability using natural assemblages and strains isolated from the Southern Ocean. Methods to measure iron bioavailability and recent developments in mathematical models are also presented.
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