Table 1.

Iron carrier classification.

Type of Siderophore	Characteristic Functional Group	Characteristic	Siderophore-Producing Microorganism
Hydroxamic salt-type		The most common in nature, the structure is more complex; it is more hydrophilic and prone to photooxidation	Pseudomonas fluorescens [26] Aspergillus nidulans [29]
Catechol salt-type		Strong lipophilicity and high affinity with Fe; strong resistance to environmental pH changes	Escherichia coli [28] Klebsiella pneumoniae [28]
Carboxylate-type		Potential ligands for the ocean iron cycle; some of them are photoactive	Rhizobium meliloti [30] Staphylococcus aureus [30]
Mixed-type	Mixed functional groups	It presents different characteristics according to different functional groups	Rhodococcus erythropolis [28] Escherichia coli [28]