Structure and membrane affinity of a suite of amphiphilic siderophores produced by a marine bacterium

Abstract

Iron concentrations in the ocean are low enough to limit the growth of marine microorganisms, which raises questions about the molecular mechanisms these organisms use to acquire iron. Marine bacteria have been shown to produce siderophores to facilitate iron(III) uptake. We describe the structures of a suite of amphiphilic siderophores, named the amphibactins, which are produced by a nearshore isolate, ␥ Proteobacterium, Vibrio sp. R-10. Each amphibactin has the same Tris-hydroxamatecontaining peptidic headgroup composed of three ornithine residues and one serine residue but differs in the acyl appendage, which ranges from C-14 to C-18 and varies in the degree of saturation and hydroxylation. Although amphiphilic siderophores are relatively rare, cell-associated amphiphilic siderophores are even less common. We find that the amphibactins are cellassociated siderophores. As a result of the variation in the nature of the fatty acid appendage and the cellular location of the amphibactins, the membrane partitioning of these siderophores was investigated. The physiological mixture of amphibactins had a range of membrane affinities (3.8 ؋ 10 3 to 8.3 ؋ 10 2 M ؊1 ) that are larger overall than other amphiphilic siderophores, likely accounting for their cell association. This cell association is likely an important defense against siderophore diffusion in the oceanic environment. The phylogenetic affiliation of Vibrio sp. R-10 is discussed, as well as the observed predominance of amphiphilic siderophores produced by marine bacteria in contrast to those produced by terrestrial bacteria.

Amphiphilic siderophore production by oil-associating microbes

“…Interestingly, no amphibactins with hydroxylated fatty acid tails were isolated as has been previously seen with other amphibactin-producing bacteria. 17,37 Small amounts of 50 amphibactins with shorter tail lengths were found in the culture supernatant of both Vibrio spp. (data not shown); however, the majority of siderophores were extracted from the pellet.…”

Physiological, Ecological, and Phylogenetic Characterization of Stappia , a Marine CO-Oxidizing Bacterial Genus

“…Growth responses to salinities ranging from 0 to 45 ppt were determined for strains GA15, HI, M4, and MIO by growing the isolates in 250-ml Erlenmeyer flasks containing 50 ml basal salts medium (20) in artificial seawater supplemented with 0.05% yeast extract; salinities were adjusted to 5, 15, 25, 35, and 45 ppt. Nonmarine media were created using deionized water.…”

“…Additional Stappia strains were obtained from seawater and macroalgae by incubating 20 to 100 ml basal salts medium (PYE) (20) containing 0.01 to 0.05% yeast extract and 25 mM pyruvate with 1 ml of surf water samples from Ka Lae, Hawaii, or 1 to 5 g fresh weight of macroalgae: Gracilaria sp., Ulva sp., Caulerpa sp. or an unidentified, tube-shaped green alga, Ascophyllum nodosum, or Ulva lactuca (the latter was from the Damariscotta River, Walpole, ME).…”

“…Results of subsequent studies have shown that Stappia-like isolates can account for a significant percentage of cultivable ␣-Proteobacteria containing dioxygenase genes (3). Stappia or Stappia-like isolates have been reported to produce sodium channel-blocking proteins (5,34) and a rhizobactin-like siderophore (20). Stappia strains are thus functionally versatile, occupy several ecological niches, and participate in biogeochemical cycles and processes that are important on microscales to global scales (e.g., CO oxidation and denitrification).…”

Biological inorganic chemistry at the beginning of the 21st century

“…Much of the chemistry of natural waters by necessity involves metal complex equilibria (153), with the solubilization of ferric iron long a matter of intense interest. In this issue, Alison Butler reports a new class of amphiphilic siderophores, called amphibactins, which associate with bacterial cells, presumably facilitating the mobilization of iron from sea water (154).…”