Multiple modes of iron uptake by the filamentous, siderophore‐producing cyanobacterium, Anabaena sp. PCC 7120

Summary Iron is a member of a small group of nutrients that limits aquatic primary production. Mechanisms for utilizing iron have to be efficient and adapted according to the ecological niche. In respect to iron acquisition cyanobacteria, prokaryotic oxygen evolving photosynthetic organisms can be divided into siderophore‐ and non‐siderophore‐producing strains. The results presented in this paper suggest that the situation is far more complex. To understand the bioavailability of different iron substrates and the advantages of various uptake strategies, we examined iron uptake mechanisms in the siderophore‐producing cyanobacterium Anabaena sp. PCC 7120. Comparison of the uptake of iron complexed with exogenous (desferrioxamine B, DFB) or to self‐secreted (schizokinen) siderophores by Anabaena sp. revealed that uptake of the endogenous produced siderophore complexed to iron is more efficient. In addition, Anabaena sp. is able to take up dissolved, ferric iron hydroxide species (Fe′) via a reductive mechanism. Thus, Anabaena sp. exhibits both, siderophore‐ and non‐siderophore‐mediated iron uptake. While assimilation of Fe′ and FeDFB are not induced by iron starvation, FeSchizokinen uptake rates increase with increasing iron starvation. Consequently, we suggest that Fe′ reduction and uptake is advantageous for low‐density cultures, while at higher densities siderophore uptake is preferred. Anabaena takes up endogenous Feschizokinen, ferric xenosiderophore (FeDFB) as well as dissolved ferric iron hydroxide species (Fe′), the latter two with lower rates than FeSchizokinen. Xenosiderophore and Fe′ assimilation are not induced by iron starvation, while the rates of uptake of the endogenous siderophore are. Moreover, the transport of iron complexed to endogenous and exogenous siderophores merges at the plasma membrane, while the Fe′ transport route remains unknown.