Scientists discover new source of natural fertilizer in oceans

August 08, 2001

SANTA CRUZ, CA--New findings suggest that the deep ocean is teeming with organisms that produce essential natural fertilizers. A research team led by Jonathan P. Zehr, a professor of ocean sciences at the University of California, Santa Cruz, has discovered a previously unknown type of photosynthetic bacteria that fixes nitrogen, converting nitrogen from the atmosphere into a form other organisms can use.

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."

The amount of nitrogen fixation in the open ocean isn't just a matter for academic curiosity. It has implications for global warming because nitrogen stimulates the growth of marine algae, which absorb carbon dioxide from the atmosphere.

Scientists estimate the amount of nitrogen in the ocean by comparing samples from deep water and surface water, and then considering how they mix together, Zehr said. Recent calculations indicate that there is more nitrogen fixation in the surface water than scientists previously thought.

"There seems to be a lot more nitrogen than we can account for," Zehr said. "It's like making a big budget, but the budget doesn't balance."

The unexpectedly large amount of nitrogen in the open ocean set Zehr looking for its sources. Over the past 12 years, Zehr has uncovered evidence of dozens of nitrogen-fixing bacteria by looking not for the organisms themselves but for their DNA fingerprints--specifically, for a gene encoding the protein responsible for nitrogen fixation.

"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 said. "When we found these genes, we were very surprised."

The nitrogen-fixing marine Synechocystis is the first of these organisms Zehr's team has succeeded in cultivating in the laboratory. It was isolated from samples collected at a long-term monitoring site near Hawaii where Zehr works with David Karl of the University of Hawaii in Honolulu. To cultivate the bacteria, Zehr's collaborator John Waterbury of the Woods Hole Oceanographic Institution subjected a soup of organisms to an environment containing all the factors needed for growth except nitrogen compounds. Deprived of usable nitrogen, they reasoned, all the organisms would die out, except those that could fix nitrogen from the atmosphere.

"We gave the organisms we were interested in an ecological edge," Zehr said.

What emerged was a strain of nitrogen-fixing cyanobacteria that are about 100 times as large as typical photosynthetic ocean bacteria. "They're like basketballs, compared to the pins of light we're used to looking at," Zehr said.

The relatively large size of these cyanobacteria, together with their abundance, suggests that they make a significant contribution to nitrogen fixation in the ocean. "There's potentially as much biomass of these bacteria as of the other two known nitrogen fixers in the open ocean," Zehr said. "It's not as if these new things are minor components of the nitrogen-fixing process."

The new bacteria are abundant as deep as 100 to 200 meters below the surface, compared to 50 meters for the most productive known nitrogen fixer. And unlike the previously known nitrogen fixers, which are only active in warm seasons, the new bacteria have shown activity in February, Zehr said. "They are likely to be there, fixing nitrogen, over wider scales than was previously thought," he said.
Zehr's coauthors, in addition to Waterbury and Karl, are Patricia J. Turner, Enoma Omoregie, and Grieg F. Steward of UCSC; Joseph. P. Montoya of the Georgia Institute of Technology in Atlanta; and Andrew Hansen of the University of Hawaii.

University of California - Santa Cruz

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