California Sea Grant College Program

Siderophores are low molecular weight, ferric ion specific chelating agents produced by microorganisms grown under low iron conditions. The role of these compounds is to scavenge iron from the environment and make this essential mineral available to the microbial cells. Iron levels in swface ocean water range from 20 pM to 1 nM. These low iron conditions have been suggested to limit both microbial growth and biomass in certain ocean regions. Many marine microorganisms have been found to use siderophore-mediated iron acquisition systems, similar to their terrestrial counterparts. However, only a few marine siderophore structures have been determined.

The marinobactins, a series of secreted siderophores, have been isolated from the marine bacterium DS40M6. The structures of the marinobactins have been determined using amino acid analysis, mass spectrometry, ozonolysis, and NMR spectroscopy. All marinobactins share the same hexapeptide, connected to different fatty acid tails. The six amino acid residues from the C-terminus are L-N-OAc,N-OR-ornithine, L-serine, D-N-OAc,N-OH-omithine, D-serine, L-diaminobutyric acid, and D-threo-B-OH-aspartate. The side-chain -NH2 and -COOH groups of L-diaminobutyric acid and D-threo-B-OH-aspartate form a unique nine-membered ring via condensation.The N-terminal amine of D-threo-B-OH-aspartate is connected to a fatty acid chain by an amide bond. The fatty acids for marinobactins A, 8, C, D1, D2, and E are lauric acid, cis-w7-tetradecenoic acid, myristic acid, cis-w7- hexadecenoic acid, cis-w9-hexadecenoic acid, and palmitic acid, respectively. The relative abundance of each marinobactin is affected by culture conditions. The marinobactins coordinate Fe(III) with 1:1 stoichiometry and have a proton independent stability constant of approximately 1033. They form molecular assemblies in aqueous solution at a very low concentration. The marinobactins can enhance the growth of the DS40M6 bacterium. However, to other bacteria, such as DS40M3 (Deleya aquamarina) and E. coli, the marinobactins behave as iron competitors and inhibit cell growth.

Marinobacter hydrocarbonoclasticus is an oil-degrading marine bacterium, from whose culture medium two siderophores, petrobactins A and B, have been extracted. Both siderophores have been found to contain catechols. The structure of petrobactin A, a citrate-based bis-catecholate siderophore, has been fully elucidated.