bio sphere
How shipworms get their nitrogen fix Chemical details of the relationship between the marine animal and its retinue of bacteria are revealed.
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n wooden boats, piers, and floating spearheaded the MIMS analysis. “We studied shipworms throughout his cawood debris, there lives a peculiar, detected nitrogen as a cyanide ion—12C reer. “You take an organism, or lots of little marine bivalve called the shipand 14N, or 12C and 15N. You see, the organisms in the case of microbes, exworm. It scrapes and burrows its way ratio between 12C/14N and 12C/15N is, pose them to the label, and get the avinto wood and subsists solely on the in fact, the ratio of 14N/15N.” erage label incorporation for the whole carbon-rich cellulose. But, just like From the ratio analysis of MIMS mass. What really separates [MIMS] other animals, it needs nitrogen to data, acquired from ultrathin sections from other methods is that you can make protein. Where, pinpoint where that from its surroundings label is getting incorpoof seawater and wood, rated with subcellular does it get its nitrogen resolution.” fix? Researchers have Experts who study postulated that symrelationships between biotic nitrogen-fixing microorganisms and eubacteria work with karyotes are excited by the animals, but they the investigators’ findhaven’t been able to ings. “It’s a remarkable prove it unequivocally. story of evolutionary and Now, Claude Le Teredinibacter turnerae fix nitrogen inside the shipworm. The gland of Deshayes ecological adaptation,” chene at Harvard (red), interlamellar junctions (blue), ctenidial filaments (green), and bacteriosays Colleen Cavanaugh Medical School and cytes (outlines) are visible. (White square has no data. Adapted with permisof Harvard University Brigham and Women’s sion. Copyright 2007 American Association for the Advancement of Science.) about the shipworm’s Hospital, Daniel Distel symbiotic relationship of Ocean Genome Legacy, and colfirst examined by transmission electron with nitrogen-fixing bacteria. “There leagues have directly imaged and meamicroscopy, the investigators saw that obviously has been a lot of coevolution, sured nitrogen fixation by individual thousands of individual bacteria took and now you can start looking at the bacteria within the cells of shipworms up 15N2 in the gills of the shipworm. compounds the bacteria are giving the by a technique called multi-isotope The values of 15N/14N were a factor animal.” She points out that a better imaging MS (MIMS), a close cousin of of 17–68 more than the natural ratio understanding of the shipworm’s biolsecondary ion MS. They demonstrated in the gill regions that were densely ogy has a practical aspect, because the with subcellular resolution that the anipacked with the bacteria. The regions animals are destructive and are capable mal uses the nitrogen fixed by bacteria immediately next to the edges of indiof taking down piers and wooden boats for its metabolism (Science 2007, 317, vidual bacteria had a degree of 15N inas they riddle the wood with their 1563–1566). “It’s a very nice piece of corporation slightly more elevated than burrows. work that strongly demonstrates the that inside the bacteria, suggesting that Cavanaugh also makes the point that utility of the MIMS technology and its molecules containing fixed nitrogen the application of MIMS to the quesplace in the imaging mass spectrometry were transported to the proximity of tion of nitrogen fixation in shipworms arena,” says Richard Caprioli of Vander- the bacterial cell surface. will help other researchers approach bilt University. More intriguingly, the 15N/14N radifferent biological problems. “People The investigators exposed the shiptios were also increased in gill regions working on different processes will worm and the bacteria living in its gills and structures free of bacteria by factors see this paper and will be able to imto 15N2, a stable nitrogen isotope. Usranging from 1.4 to 11 above the natumediately tackle questions like nitrate ing MIMS, they bombarded the surral ratio. The observation indicated that uptake or other nitrogen-containing faces of individual shipworm specimens newly fixed nitrogen was transferred compounds that would be amenable to with a raster-scanned primary cesium from the bacteria into the cells of the this technique,” she says. “This sort of ion beam and created quantitative imshipworm for its own biosynthesis. visualization of the symbiosis is really ages of the distribution of 14N and “Until now, most of the [nitrogen the marriage of chemistry, physical sci15N. “N doesn’t ionize, but CN is very uptake] studies were done with stable ence, and biology.” a 2 easy to form,” explains Lechene, who isotopes in bulk,” says Distel, who has —Rajendrani Mukhopadhyay © 2 0 0 7 A m e r i ca n C h e m i ca l S o c i e t y
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