FIGURE 5.

Iron sources available to marine phytoplankton and employed uptake pathways. Particulate iron from sources including atmospheric dust, glaciers, coastal sediments, hydrothermal vents is dissolved by the photochemical reactions, complexation and the microbial ferrous wheel (Kirchman, 1996). The enormous complexity of the mechanisms and species behind the biological iron recycling is only recently being fully acknowledged, involving organisms as diverse as viruses, both heterotrophic and photosynthetic bacteria and protists, mesozooplankton (Boyd and Ellwood, 2010). Different species of iron are available to phytoplankton based on marine chemistry, i.e., ferric and ferrous ions; in inorganic form or chelated to organic ligands. It appears that the main strategy of cyanobacterial iron acquisition is the reductive iron uptake that can be combined with siderophore-mediated pathway. The common iron uptake mechanism employed by eukaryotic phytoplankton is mediated by phytotransferrin, while some species additionally use reductive iron uptake and (at least) some diatoms are able to acquire xenosiderophores. Phycosphere represents a unique environment in the close proximity to algal cell surface, where iron availability may be significantly increased due to bacterial iron metabolism.