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

VIRTUALLY all microorganisms require iron for growth. The paucity of i ron in surface ocean water (approximately 0.02-1.0 nM (refs 1, 2)) has spurred a lively debate concerning iron limitation of primary product ivity3-6, yet little is known about the molecular mechanisms used by m arine microorganisms to sequester iron. Terrestrial bacteria use a sid erophore-mediated ferric uptake systems7. A siderophore is a low-molec ular-mass compound with a high affinity for ferric ion which is secret ed by microorganisms in response to low-iron environments; siderophore biosynthesis is regulated by iron levels, with repression by high iro n. Although open-ocean marine microorganisms (such as phytoplankton8 a nd bacteria9) produce siderophores, the nature of these siderophores h as not been investigated. We report here the first structure determina tion, to our knowledge, of the siderophores from an open-ocean bacteri um, alterobactin A and B from Alteromonas luteoviolacea. A. luteoviola cea is found in oligotrophic10 and coastal11 waters. Alterobactin A ha s an exceptionally high affinity constant for ferric ion. We suggest t hat at least some marine microorganisms may have developed higher-affi nity iron chelators as part of an efficient iron-uptake mechanism whic h is more effective than that of their terrestrial counterparts.