Although nitrogen accounts for nearly 80 percent of the Earth's atmosphere, most organisms can use it only when it is "fixed" to other elements, to make compounds like ammonia or nitrate. As a component of proteins, nitrogen is essential to all known forms of life.
Zehr and his coworkers found the nitrogen-fixing bacteria, which they have grown in the laboratory, in water samples collected from the Pacific Ocean near Hawaii. The organisms appear to belong to the genus Synechocystis, a group of cyanobacteria (photosynthetic bacteria formerly known as blue-green algae) that includes both marine and freshwater species. The newly discovered nitrogen fixers appear to be active at greater depths and over longer time periods than other marine cyanobacteria known to fix nitrogen in the open ocean.
The researchers are publishing their findings in the August 9 issue of the journal Nature. Zehr has also found evidence that many additional kinds of nitrogen-fixing bacteria live in the open ocean.
On land, nitrogen-fixing bacteria are a known quantity, residing in the roots of legumes like peas and beans. But in the ocean, they are something of a mystery. While many nitrogen-fixing cyanobacteria have been found in coastal waters, very few are known to occur in the open ocean.
"It appears that there is much more nitrogen fixation than we know about," Zehr said. "In the open ocean, there are only one or two organisms known to fix nitrogen. They probably can't account for all the nitrogen getting fixed."