A siderophore from a marine bacterium with an exceptional ferric ion affinity constant

Abstract

VIRTUALLY all microorganisms require iron for growth. The paucity of iron in surface ocean water (∼0.02−1.0 nM (refs 1, 2)) has spurred a lively debate concerning iron limitation of primary productivity^3–6, yet little is known about the molecular mechanisms used by marine microorganisms to sequester iron. Terrestrial bacteria use a siderophore-mediated ferric uptake system⁷. A sidero-phore 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. luteoviol-acea 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.