A SIDEROPHORE FROM A MARINE BACTERIUM WITH AN EXCEPTIONAL FERRIC ION AFFINITY CONSTANT

VIRTUALLY all microorganisms require iron for growth. The paucity of iron in surface ocean water (approximately 0.02-1.0 nM (refs 1, 2)) has spurred a lively debate concerning iron limitation of primary productivity3-6, yet little is known about the molecular mechanisms used by marine microorganisms to sequester iron. Terrestrial bacteria use a siderophore-mediated ferric uptake systems7. A siderophore is a low-molecular-mass compound with a high affinity for ferric ion which is secreted by microorganisms in response to low-iron environments; siderophore biosynthesis is regulated by iron levels, with repression by high iron. Although open-ocean marine microorganisms (such as phytoplankton8 and bacteria9) produce siderophores, the nature of these siderophores has not been investigated. We report here the first structure determination, to our knowledge, of the siderophores from an open-ocean bacterium, alterobactin A and B from Alteromonas luteoviolacea. A. luteoviolacea is found in oligotrophic10 and coastal11 waters. Alterobactin A has an exceptionally high affinity constant for ferric ion. We suggest that at least some marine microorganisms may have developed higher-affinity iron chelators as part of an efficient iron-uptake mechanism which is more effective than that of their terrestrial counterparts.