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

Abstract

SummaryIron 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.

Multiplicity and specificity of siderophore uptake in the cyanobacterium Anabaena sp. PCC 7120

“…While the α-carboxylate hydroxamate siderophore aerobactin is structurally similar to schizokinen, the tris-hydroxamate DFB is less structurally related (Harris et al 1979;Dhungana et al 2001). It was demonstrated that Fe-DFB is not taken up by SchT, which is consistent with the structural differences of DFB and schizokinen and that uptake across the plasma membrane is mediated by the Fhu system similarly to Fe-schizokinen (Rudolf et al 2015). medium and the cell concentration was adjusted to a final OD 750 of 0.05.…”

“…For uptake experiments cells were treated and collected as described (Rudolf et al 2015). Cells were grown for 3 days in YBG11+10 µM Fe-EDTA as Fe-EDTA cannot be incorporated by cyanobacteria and buffers a constant concentration of bioavailable Fe′ (Kranzler et al 2011).…”

“…in a siderophore-independent manner, which involves a reduction step (Rudolf et al 2015). It is known that reductive iron uptake of both Fe′ and ferrisiderophores is used by several other cyanobacterial species to sequester iron (Kranzler et al 2011;Lis et al 2014Lis et al , 2015.…”

“…the TonB3-ExbB3-ExbD3 system facilitates the uptake of Fe-schizokinen through the outer membrane (Stevanovic et al 2012). Once in the periplasm, Fe-schizokinen is recognized by a periplasmic transfer protein and transported across the inner membrane via the ABC-type transporter, FhuBCD (Stevanovic et al 2012;Rudolf et al 2015).…”

“…Anabaena sp. has also been shown to take up the xenosiderophores Fe-DFB (desferrioxamine B) and Fe-aerobactin (Goldman et al 1983;Rudolf et al 2015) but the transporters involved herein have not been identified yet. While the α-carboxylate hydroxamate siderophore aerobactin is structurally similar to schizokinen, the tris-hydroxamate DFB is less structurally related (Harris et al 1979;Dhungana et al 2001).…”