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New Nitrogen-Fixers Discovered in Open Ocean

Newly discovered unicellular organisms in the open ocean could be responsible for converting large amounts of atmospheric nitrogen into a useable form, researchers say. The finding, which appears in today¿s issue of the journal Nature, has implications for scientists' understanding of the global ocean nitrogen cycle as well as for estimations of the effects of global warming.

Jonathan Zehr of the University of California, Santa Cruz, and his colleagues analyzed water samples collected in the Pacific Ocean off the coast of Hawaii, looking for nitrogen-fixers. "We initially went out thinking we were going to prove that there are no other nitrogen-fixing bacteria besides the ones we already know," Zehr says. Instead they found cyanobacteria (formerly known as blue-green algae because of their photosynthetic abilities) that expressed the nitrogenase enzyme crucial to converting gaseous nitrogen into a form that organisms can absorb. Such available nitrogen subsequently stimulates the growth of carbon-dioxide-absorbing marine algae.

Other forms of cyanobacteria have turned up in coastal waters, but the new single-celled nanoplankton are unique in two ways: they are abundant at depths more than twice those of previously identified nitrogen-fixers and they demonstrate activity in February¿a trait never before seen in ocean-dwelling nitrogen-fixers.

The new findings help explain imbalances between estimations and measurements of the amount of usable nitrogen in the oceans. "There¿s potentially as much biomass of these bacteria as of the other two known nitrogen fixers in the open ocean," Zehr says. "It¿s not as if these new things are minor components of the nitrogen-fixing process." Indeed, in a commentary accompanying the Nature report, Jed Fuhrman and Douglas Capone of the University of Southern California, Los Angeles, note that the team¿s finding is "far more than a contribution to understanding biodiversity, for it bears on a gap in our biogeochemical balance books."

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