Research: Marine Bacteria Foster Iron Cycling

Researchers reported findings that "have significant implications for the cycling of iron in the oceans" in an article published in Nature last month. Their experiments show that iron bound to siderophores (small molecules or ligands that bind iron) produced by bacteria and other micro-organisms reacts to light.

This photoreactivity "is an important new concept in our understanding of how siderophores function in biological iron acquisition," the authors state.

The authors of "Photochemical Cycling of Iron in the Surface Ocean Mediated by Microbial Iron(III)-Binding Ligands" are all affiliated with the University of California. First author Katherine Barbeau, a postdoctoral fellow in the laboratory of UCSB chemist Alison Butler, joined the faculty this fall at San Diego's Scripps Institution of Oceanography. The other two authors are Eden Rue, a postdoctoral fellow, and oceanographer Ken Bruland, both affiliated with the Institute of Marine Sciences at UC Santa Cruz.

Butler will present her team's marine iron studies to the Nobel centennial symposium on Wednesday, Oct. 24, at 9 a.m. in UCLA's Korn Convocation Hall. Nobel Laureate and UCSB physicist Alan Heeger also will speak at the same session.

Bacteria, with the exception of only two known species, need iron to carry on enzymatic processes essential to their existence. In many areas of the ocean, other substances that marine bacteria need are plentiful, so iron is the limiting nutrient for their growth. Starve bacteria of iron and many types will produce small ligands that bind iron and are therefore called "siderophores" (Greek for "iron-loving"). Membranes on the surface of bacteria have receptors for the iron-bound ligands.

The bond of ironÑas iron(III)Ñto oxygen in siderophores is among the strongest molecular configurations of iron and oxygen. Those siderophores studied by Butler's group are amphiphilic peptides, meaning that the peptide head of the two-sided molecule is attracted to water and the tail to fatty substances.

"The question we asked," said Butler, "is 'Are these siderophore-iron complexes photochemically reactive, and if so, how does that affect iron availability to marine organisms?'" Subsequent research demonstrated that exposure to light not only results in the temporary reduction of iron(III) to iron(II), but in the dropping of the fatty-acid-loving tail of the siderophore ligand, leaving the peptide head bound to iron. "Possibly the dropping of the tail," said Butler, "coincident with the reduction of iron(III) to the more readily usable iron(II), enables the bacteria to acquire more easily the essential iron."

To test this hypothesis Barbeau acquired a sample of ocean water off Bermuda in the mid-North Atlantic. Said Barbeau, "We wanted to get our water with micro-organisms from the open ocean, where levels of dissolved organic carbon and iron are relatively low." Results clearly demonstrated that the effects of light on the iron-ligand complexes increased the biological availability of iron.

"The way iron cycles in the oceans is important because it affects the carbon cycle," said Bruland. "Understanding the uptake of this scarce micronutrient will help provide more insight into how these microscopic plants and bacteria cope in oceanic environments. For example, the photochemical cycling described in this paper can make what would have been an unavailable form of iron into an available form for some micro-organisms."

In addition to being a chemistry professor at Santa Barbara, Butler is associate dean for bioengineering in the College of Engineering, and a participant in the California NanoSystems Institute. Her research group previously showed that the ligands that bind iron can form vesicles about 100 nanometers in diameterÑessentially a fat sheath lined on the inside and outside with the water-loving head groups. Butler has envisioned possibilities from the combined research findings.

"If you have a vesicle," she said, "you can trap something in it. Now we know that if you shine light on these ligands, they lose their fatty acid tails. That leaves us with tantalizing possibilities, [one of which] is the possibility of using these ligands as a drug delivery system within the body."