Figure 3.

Iron uptake mechanisms in Gram-negative and Gram-positive bacteria. The Feo system is the main ferrous iron uptake mechanism used by Gram-negative bacteria, including FeoA, FeoB, and FeoC proteins. Most marine bacteria use siderophores for ferric iron uptake. Siderophores, secreted into the environment after their synthesis, form Fe³⁺ complexes, which are internalized through specific receptors. In Gram-negative bacteria, siderophore-ferric ion complexes are transported via outer membrane receptors (e.g., FepA, FecA, FhuA), interacting with the TonB-ExbB-ExbD complex for energy-dependent uptake and periplasmic binding proteins like FhuD. Gram-positive bacteria, lacking outer membranes, use simpler mechanisms involving extracellular membrane-anchored siderophore-binding proteins and ABC permeases. Once inside the cell, ferric iron is reduced to ferrous iron, while siderophores and membrane receptors can be reused or degraded. Created with graphical elements available on BioRender.com.