Iron acquisition by heterotrophic marine bacteria

Recent studies demonstrate that the dissolved iron in seawater is bound to strong organic complexes that have stability constants comparable to those of microbial iron chelates (siderophores). We investigated iron acquisition by 7 strains of heterotrophic marine bacteria using siderophores as a model for the natural iron-binding ligands. Siderophores were detected in the supernatants of 4 strains. All strains utilized iron bound to siderophores regardless of whether they produced their own. The majority took up iron bound to the fungal siderophore desferrioxamine B (dfoB). Over half also utilized iron bound to strain Neptune's siderophore, nep-L, while iron bound to pwf-L was available solely to the producing strain, Pwf3. Uptake rates of Fe-siderophores were similar among iron-limited strains and among ligands. Transport of Fe-dfoB in Neptune was enhanced 20 times by iron limitation. The half-saturation constant of Fe-dfoB transport was 15 nM, the lowest reported for Fe-siderophore transport in microorganisms. In contrast, uptake of inorganic iron (Fe' ) by iron-limited Neptune did not saturate at the highest concentration tested and was not upregulated under iron stress. This suggests that Fe ' uptake occurs by simple diffusion through the outer membrane.Strain Lmg1, the sole catechol producer, did not take up iron bound to exogenous siderophores (dfoB, pwf-L, or nep-L). However, it utilized iron bound to its own ligand and, possibly, iron bound to the synthetic chelator EDTA. Transport of Fe' by iron-limited Lmg1 was 10 times higher than in the other strains and was upregulated 46 times in low iron conditions. The results suggest iron transport in Lmg 1 may be mediated by surface-associated catechol siderophores that scavenge inorganic ferric species as well as iron bound to weaker complexes, such as EDTA. This study elucidates the importance of siderophores in iron transport by heterotrophic marine bacteria. (Abstract shortened by UMI.