Top Papers in Environmental Science, Second Runner-Up (Tie): Bacterial weapons in the war on iron in the seas
10.1021/es900339v
metal ion. The bacteria can then take up these complexes to utilize the iron. Gledhill thought it likely that marine bacteria would use a similar mechanism. As for the phytoplankton and how they made use A. DEVEZ
“Hydroxamate Siderophores: Occurrence and Importance in the Atlantic Ocean” by Edward Mawji, Martha Gledhill, James A. Milton, and Eric P. Achterberg, University of Southampton (U.K.); Glen A. Tarran, Plymouth Marine Laboratory (U.K.); Simon Ussher, University of Plymouth (U.K.); Anu Thompson and George A. Wolff, University of Liverpool (U.K.); Paul J. Worsfold, University of Plymouth (U.K.), 2008, 42 (23), 8675-8686; DOI 10.1021/es801884r. “Give me a half tanker of iron,” joked oceanographer John Martin in 1989, “and I will give you an Ice Age.” Martin wanted to provoke a reaction when he made this blunt comment as a way of summing up his hypothesis that biological growth in the ocean was limited by iron. Seeding a section of the ocean with iron would cause phytoplankton to flourish, he hypothesized. The phytoplankton would act like a CO2 pump by incorporating CO2 into their bodies. Then, after the phytoplankton died, the CO2 would sink along with them to the depths of the ocean. This would remove the greenhouse gas from the atmosphere and hence could greatly reduce global warming. Fifteen years ago, Martin’s ideas inspired Martha Gledhill, then a young Ph.D. student at the University of Liverpool (U.K.), to begin the research that would lead to one of ES&T’s top papers of 2008. She wanted to study the chemical speciation of iron in the surface ocean because that speciation was likely to play a big role in the iron’s availability. Gledhill, now a marine biogeochemist at the University of Southampton (U.K.), knew that terrestrial bacteria can scavenge the iron they need. They secrete siderophores, compounds that dissolve iron by forming soluble compounds, or complexes, with the
In the summer of 2005, Martha Gledhill and colleagues spent more than a month sailing in the middle of the Atlantic Ocean in search of siderophores.
of the iron, that was a problem for later. Gledhill’s plan to attack the problem was logical. Take seawater into the lab, add iron, and then see what happens. The experiments worked to a point, she says. “We added the iron, waited, and then measured. What we measured was less than what we put in, so we knew the iron was being complexed, we just couldn’t identify the complexes with the equipment we had at the time.” When she started her research, Gledhill says, she had no idea that it would take 15 years to identify the complexes, and that in doing so, she would become an indirect observer of a long-standing war between very tiny opponents. “The project flopped,” says organic chemist George Wolff at the University of Liverpool, one of
2009 American Chemical Society
Published on Web 02/18/2009
Gledhill’s former Ph.D. advisers and a coauthor of the paper. “But Martha never gave up. She is like a terrier dog when she gets her teeth into something,” Wolff says. After receiving her Ph.D., Gledhill tried to use LC/MS to identify the complexes, but the spectra were so busy that it was impossible. Disappointed but determined, she hit on the idea of using gallium as a proxy for iron. In the lab, she flooded seawater samples with gallium. Gallium displaces iron, making the complexes easier to distinguish, but too many interference peaks made the data too noisy. Subsequently, Gledhill moved to Southampton, where she had access to an inductively coupled plasma mass spectrometer. Because gallium is relatively simple to measure with this instrument, the metal made an excellent marker for tracking down the complexes. By this time, Gledhill says, she was feeling pessimistic about the chances of good results, but the technique worked, and she was able to identify many different siderophores. Indeed, after her 2005 cruise to collect ocean water and look for siderophores in the Atlantic Ocean, Gledhill was able to identify two kinds of siderophores, although she suspects there are many more. As for the big picture, there’s a battle going on in the ocean for scarce iron, she says. “There is a little war between different organisms to get what iron is in the ocean. Siderophores are part of the armory of weapons available to bacteria. If all the iron is complexed to siderophores, then the phytoplankton don’t get any. We know the plankton are doing something to get iron, but we don’t know what it is,” she says. Looks like a nice project for the next 15 years. —REBECCA RENNER
April 1, 2009 / ENVIRONMENTAL SCIENCE & TECHNOLOGY 9 2199
