The question has stumped oceanographers for decades: How is dissolved nitrogen gas transformed into a kind that can be used by living things? Researchers have now discovered that free-floating bacteria play a major role. These bacteria may "fix" as much nitrogen as all previously known ocean organisms combined.
All organisms need nitrogen, but most can only use certain forms of the compound, known collectively as "fixed" nitrogen because the nitrogen is affixed to other elements in molecules such as ammonia. Dissolved nitrogen gas is abundant in the ocean and fixed nitrogen is scarce, so scientists predict that ocean creatures that fix their own nitrogen gas should be wildly successful. Indeed, marine surveys show that some unknown sea creatures fix large quantities of nitrogen. But so far, scientists have discovered few of the responsible parties. The biggest customer by a large margin, the colonial bacteria Trichodesmium, accounts for only one-tenth of known nitrogen fixation.
Several years ago, Jon Zehr and his colleagues at the University of California, Santa Cruz, discovered fixation genes in seawater that looked like they came from single-celled bacteria. Now the group has identified these bacteria and observed them fixing nitrogen in the laboratory and the ocean. The nitrogen-eaters belong to two families of photosynthetic cyanobacteria and a distantly related proteobacteria group, the researchers report in the 9 August issue of Nature. Based on the abundance of these cyanobacteria and the rate at which they gobble down radioactive nitrogen in the laboratory, the researchers estimate they account for about 10% of nitrogen fixation in the ocean.
The finding "changes everything" about scientists' understanding of the nitrogen cycle, says biological oceanographer Tracy Villareal of the University of Texas, Austin. But he points out the vast majority of ocean nitrogen fixation is still unaccounted for--a problem "that needs to be fixed."