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.
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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.
