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Nitrogen Fixation in Corals
Lesser, M.P., Mazel, C.H., Gorbunov, M.Y. and Falkowski, P.G.  2004.  Discovery of symbiotic nitrogen-fixing cyanobacteria in corals.  Science 305: 997-1000.

What was done
In their quest to detect, identify and determine the function of certain bacteria that are associated with scleractinian corals, the authors of this important paper employed a number of techniques in their study of the Caribbean coral Montastraea cavernosa, including the measurement of in vivo excitation/emission spectra, fluorescence lifetime analyses, epifluorescence microscopy, transmission electron microscopy, immunogold probing and ribosomal DNA sequencing.

What was learned
Lesser et al. identified "unicellular, nonheterocystis, symbiotic cyanobacteria within the host cells of the coral" that "coexist with the symbiotic dinoflagellates (zooxanthellae) of the coral and express the nitrogen-fixing enzyme nitrogenase."  They say their results "clearly suggest that endosymbiotic cyanobacteria capable of fixing nitrogen are present in M. cavernosa and form a stable long-term association within host cells," noting that "this symbiont could potentially be a source of the limiting element nitrogen for the symbiosis through the release of fixed nitrogen products to the coral host."

What it means
It is beginning to appear that corals are not as much at the mercy of their environment as climate alarmists have long made them out to be.  Following on the heels of the discovery that corals can "shuffle" their zooxanthellae symbionts to accommodate more warmth-tolerant genotypes in response to episodic high water temperatures -- and thereby better cope with subsequent anomalous warm events [see our Editorial of 18 Aug 2004, which discusses the work of Baker et al. (2004) and Rowan (2004)] -- we now find that corals may not be totally dependent upon environmental sources of nitrogen for meeting the needs of their symbiotic association.  Perhaps that is why the tenacious scleractinians have been around since as long ago as the Triassic (Veron, 1995).

Baker, A.C., Starger, C.J., McClanahan, T.R. and Glynn, P.W.  2004.  Corals' adaptive response to climate change.  Nature 430: 741.

Rowan, R. 2004. Thermal adaptation in reef coral symbionts.  Nature 430: 742.

Veron, J.E.N.  1995.  Corals in Space and Time.  Cornell University Press, Ithaca, New York, USA.

Reviewed 1 September 2004