Nitrogen-fixing Nanoplankton

Tiny marine bacteria surprise scientists with their importance in global nitrogen cycle

By Eva Emerson
September 2004

Small, single-celled microbes play a significant role in the natural fertilization of the upper ocean, according to new research by biological oceanographer Douglas G. Capone of USC College of Letters, Arts & Sciences and his colleagues.

Writing in the Aug. 26 issue of the journal Nature, the team reports that a group of single-celled bacteria including some photosynthetic cyanobacteria, fix nitrogen — convert gaseous nitrogen into a form useful for other living things — at a rate many times higher than shown in earlier studies.

The team also found that these diminutive bacteria and cyanobacteria, which measure less than 10 microns across, are more widely distributed and abundant than scientists had thought. The nano-sized phytoplankton thrive in the expansive blue water zones of the tropical and subtropical Pacific Ocean where scarce nutrients limit the growth of most organisms.

“We thought that the nanoplankton were involved in fixing nitrogen, but we were surprised to find out just how important they are,” said Capone, the Wrigley Chair in Environmental Science and a professor of biological sciences. “People suspected it, but no one has previously been able to show it quantitatively.”

In samples taken from the northern Pacific Ocean, the scientists used a combination of genetic and high-sensitivity chemical tracer methods to show that the microbes could produce approximately 7 milligrams of fixed nitrogen per square meter of ocean per day.

The addition of this amount of fixed nitrogen to the Pacific alone would provide a substantial boost to marine life, supplying the key nutrient for new biological growth in the ocean equal to about 10 percent of the total global marine biomass.

“These unicells are the largest single source of nitrogen entering the water in broad areas of the ocean,” said Joe Montoya, a biologist at the Georgia Institute of Technology and the study’s first author.

Beyond confirming the role of the nanoplankton in the marine nitrogen cycle, the new finding has broader implications for understanding the movement of carbon dioxide between the oceans and the atmosphere.

“This is an important finding that helps us understand the ocean’s nitrogen cycle,” said James Yoder, director of the National Science Foundation’s ocean sciences division, which funded the research. “The nitrogen cycle is one of the keys to understanding the role of ocean biology in the global carbon cycle.”

Understanding the complex dynamics of both cycles has gained new importance and urgency in the face of climate change. Carbon dioxide gas helps insulate the Earth through the greenhouse effect, but excessive emissions of the gas from vehicles and industry have contributed to global warming.

Balancing the Nitrogen Budget
For many years, geochemists modeling the movement of nitrogen through the marine biosphere realized their budgets did not add up — physical processes alone accounted for only about half of the fixed nitrogen available in the ocean.

Capone was among the first marine scientists to reveal the critical role biological organisms play in the marine nitrogen cycle.

He is perhaps most well known for his work on another group of nitrogen-fixing marine bacteria — the larger, colonial cyanobacteria Trichodesmium, which up to now, has been considered the dominant marine nitrogen-fixer.

“Because they’re colonial, and they live in clumps, it’s been easier to collect and observe Trichodesmium,” said Capone, who is also a faculty member at the USC Wrigley Institute for Environmental Studies.

Only recently have scientists had the tools to look as closely at the smaller forms of photosynthetic cyanobacteria and bacteria that comprise the nanoplankton, he said.

In 2001, Jonathan Zehr, a UC Santa Cruz biologist and a co-author on the current paper, discovered that the nanoplankton actively express the gene for the nitrogenase enzyme, which catalyzes the reaction turning atmospheric N2 gas into nitrates.

But it wasn’t clear until the current study whether these tiny microbes were fixing enough nitrogen to actually impact the environment.

On research cruises between Hawaii and California and near Australia, the team collected microbes from the ocean and incubated them in tanks with enriched levels of the heavy nitrogen isotope N-15. In nature, N-15 represents less than a half a percent of all nitrogen atoms; N-14 more than 99 percent. Researchers then chemically analyzed the samples with a mass spectrometer.

“It turns out that the nanoplankton are taking up significant amounts of nitrogen gas — equal to or even greater than the amounts fixed by the larger Trichodesmium,” Capone said.

“The low-nutrient, blue water zones turn out to be great ecological niches for these nitrogen fixers,” Capone said. “The nanoplankton can quickly become dominant in the blue zones, growing to capacity until they hit another limit” — a lack of phosphorus or another essential mineral, for example.

On research cruises scheduled for 2006 and 2007, the team plans to continue its investigations in the Pacific, as well as in the north Atlantic and the south Pacific. In addition to collecting more detailed nitrogen fixation rate measurements, the researchers will conduct manipulation experiments to determine how levels of phosphorus, iron and other environmental factors affect the abundance, distribution and activity of the nanoplankton.

“We’re interested in figuring out who fixes gaseous nitrogen and where it goes in the marine biosphere,” said Capone, who thinks that there are probably many more nitrogen-fixing marine organisms awaiting discovery.

“Eventually, we want to know how biologically driven nitrogen-fixation rates may ultimately impact the ocean’s carbon dioxide absorption rate, and how that affects the total levels of carbon dioxide in the atmosphere.

“The closer we look at the oceans, the more important the tiniest organisms appear to be,” Capone said.

Joseph Montoya, Carolyn M. Holl, Jonathan P. Zehr, Andrew Hansen, Tracy A. Villareal, and Douglas G. Capone. “High Rates of N2-Fixation by Unicellular Diazotrophs in the Oligotrophic Pacific.” Nature 430, 1027 – 1032 (26 August 2004).