Iron Controls Patterns Of Nitrogen Fixation In The Atlantic

Scientists including researchers from the National Oceanography Centre, Southampton and the University of Essex have discovered that interactions between iron supply, transported through the atmosphere from deserts, and large-scale oceanic circulation control the availability of a crucial nutrient, nitrogen, in the Atlantic. Their findings have potentially important implications for understanding global climate, both past and future.

Nitrogen is an essential element for life, but in its gaseous form (N2) cannot be used by most organisms. To be useful, nitrogen has to be 'fixed' by combining it with other chemicals to form compounds such as ammonium. 'Diazotrophic' microbes such as the blue-green bacterium Trichodesmium are equipped with the enzymes needed to perform these energy-demanding transformations.

Nevertheless, over most of the ocean, a relative shortage of fixed nitrogen limits the growth of phytoplankton -- microscopic marine plants -- and their uptake of the greenhouse gas carbon dioxide through photosynthesis. Fixed nitrogen is ultimately removed by denitrification, whereby it is converted back to N2. The rate at which it is replaced depends on many factors including the availability of other crucial nutrients such as iron and phosphate needed by nitrogen-fixing microbes for growth.

Work over the last decade or so has hinted at iron availability being a significant control on oceanic nitrogen fixation, primarily because the enzyme responsible contains a lot of this element. However, other recent research had reinforced the long held view that phosphorus is the more important controlling factor. The new findings reaffirm the view that an adequate supply of iron is crucial, which, although frequently quoted, had previously been supported by little clear evidence.

"Our new data provides some of the first really conclusive evidence that iron availability is playing a significant roll in controlling the input of fixed nitrogen, at least to the Atlantic Ocean," said lead author Dr Mark Moore of the University of Southampton's School of Ocean and Earth Science based at the National Oceanography Centre.

Detailed measurements of nutrients and microbiology were made by an international team aboard the Royal Research Ship RRS Discovery during a 10,000-kilometre cruise along a meridional transect between 37°N -- 35°S in the Atlantic Ocean in 2005.

Iron-rich dust blown over the North Atlantic from the Sahara desert allows the growth of diazotrophic Trichodesmium species, which fix large amounts of nitrogen and use up excess phosphate. In contrast, diazotrophs in the South Atlantic appear more severely limited by iron availability.

These observations, backed up by a large-scale physical analysis of the Atlantic circulation, also indicated that the North Atlantic is a net source of fixed nitrogen to the world's oceans.

In addition to highlighting the importance of iron in controlling global patterns of nitrogen fixation, the potential for interaction with the overturning circulation leads to novel conclusions concerning the global nitrogen cycle.

Specifically, it appears that nitrogen fixation in the north Atlantic basin, immediately upstream of the formation region for half the global deep waters, will have a much greater influence on ocean-atmosphere partitioning of carbon dioxide than recently proposed hotspots of nitrogen fixation around the edges of denitrification zones.

"Any interactions between large-scale overturning circulation and nitrogen fixation may potentially have important consequences for understanding how the nitrogen cycle reacted to past climate change, and also how it might react in the future," says Dr Moore.

He acknowledges, however, that there is still a lot of work to be done and that many of the details concerning the supply of nutrients for nitrogen fixation remain to be worked out: "We still need to identify the sources of iron which are presumably required to support nitrogen fixation in other regions, alongside making further improvements to our understanding of how oceanic circulation supplies the required excess phosphorus."

But what is clear from the study is that past and future climate changes in both atmospheric dust transport and ocean circulation may impact the fixed nitrogen inventory and consequently have a global-scale influence on the ability of the ocean to sequester carbon dioxide.

The work was supported by the European Union Carboocean integrated project, the UK Natural Environment Research Council and the Atlantic Meridional Transect consortium.

The researchers are: C. Mark Moore (SOES/NOCS and University of Essex), Matthew Mills (Stanford University), Eric P. Achterberg (SOES/NOCS), Richard Geider (University of Essex), Julie La Roche (Leibniz-Institut für Meereswissenschaften, Kiel), Mike Lucas (University of Cape Town), Elaine L. McDonagh (NOCS), Xi Pan (NOCS), Alex Poulton (NOCS), Micha Rijkenberg (NOCS), David Suggett (University of Essex), Simon Ussher (University of Plymouth), and E. Malcolm S. Woodward (Plymouth Marine Laboratory).

Chemical Imaging Of Deep-sea Microorganisms May Help Explain Lingering Nitrogen Mystery

Researchers at the California Institute of Technology (Caltech) have identified an unexpected metabolic ability within a symbiotic community of microorganisms that may help solve a lingering mystery about the world's nitrogen-cycling budget.

A paper about their work appears in the October 16 issue of the journal Science.
The element nitrogen is a critical part of amino acids, the building blocks of proteins, and therefore essential to all life. Although nitrogen is plentiful on Earth—it comprises 78 percent of the atmosphere, by volume—the element is usually found strongly bonded to itself, in the form of the diatomic gas N2. To be biologically useful, a nitrogen atom must be released from this coupling and converted to a reduced, or "fixed," state; reduced nitrogen atoms gain an electron, which makes them chemically reactive.
Although lightning, combustion, and other nonbiological processes can reduce nitrogen, far more is generated by nitrogen-fixing microorganisms such as bacteria—in particular, photosynthetic cyanobacteria. These organisms produce the bulk of the nitrogen available to living things in the ocean.
Still, when researchers add up all of the known sources of fixed nitrogen (biological and otherwise) in the global nitrogen cycle and compare it to the sinks—where nitrogen is taken up for growth and energy—they come up short. It appears that more nitrogen is being used than is being made. The apparent nitrogen budget, in effect, does not balance. This discrepancy had led scientists to question whether the nitrogen cycle is truly out of balance, or whether the known inventories of sources and sinks are misleadingly incomplete.
Victoria J. Orphan, an assistant professor of geobiology at Caltech, along with graduate student Anne E. Dekas and postdoctoral research scholar Rachel S. Poretsky, suggest the answer is, at least in part, an incomplete catalog of the sources of fixed nitrogen.
The team studied ocean sediment samples obtained in methane cold seeps located at a depth of about 1,800 feet. The area, known as the Eel River Basin, is located approximately 20 miles off the coast of the northern California town of Eureka, on a continental margin in a region supporting high levels of natural methane seepage at the seabed.
In the laboratory, the researchers examined the methane-rich sediment and the tiny microbial conglomerations that live within. These spherical cell conglomerates, averaging about 500 cells each, consist of two types of anaerobic microorganisms living in a unique symbiotic relationship fueled by methane. The first microorganism is a bacterium that reduces the chemical sulfate into sulfide (via a process that produces the rotten-egg odor of salt marshes and mud flats) to generate energy. The second is a methane-oxidizing archaeon (the archaea are a group of nonbacterial single-celled microorganisms). Working together, these two symbionts are responsible for consuming the majority of the naturally released methane in the deep sea.
Although these symbiotic associations themselves are not new—these conglomerations were discovered about a decade ago and are found on continental margins worldwide—the Caltech scientists discovered something unexpected: the methane-consuming archaea were actively fixing nitrogen, and sharing it with their bacterial neighbors.
"This is the first time that nitrogen fixation has been documented within methane-oxidizing archaea," Dekas says.
Interestingly, although these organisms have a nitrogen-poor diet of methane gas, they live in an environment that contains reduced nitrogen—in the form of ammonium and other chemicals—which means they shouldn't need to create their own. "It's possible that they do need to because they are living in a crowded community—a tightly packed ball—that prevents some organisms from having access to the nitrogen," she says. Another possibility is that these environments do not have as much biologically available reduced nitrogen as had been thought.
To determine that the archaea were indeed fixing nitrogen, the researchers first incubated the archaeal-bacterial assemblages with a dinitrogen gas, N2, that was composed of two atoms of nitrogen-15. Nitrogen-15 is a nonradioactive isotope of nitrogen that contains one more neutron than regular nitrogen (nitrogen-14) and can be used as a tracer for the incorporation of the element.
The researchers then used a technique called fluorescent in situ hybridization (FISH) to stain the two types of organisms in the sediment, and analyzed these cells for their nitrogen-15 content using a state-of-the-art instrument called a nanometer secondary ion mass spectrometer, or nanoSIMS. The nanoSIMS, which is housed at the Caltech Center for Microanalysis, is capable of collecting chemical and isotopic data at a spatial scale of 50 to 100 nanometers, or around five to 10 times smaller than the size of a single microbial cell.
Both the archaea and, to a lesser extent, their bacterial neighbors had incorporated the nitrogen-15, which could have happened only if the N2 had been fixed by the archaea—and then shared.
"The high spatial resolution of the nanoSIMS instrument—which produces a focused beam of ions that is smaller than a single cell—allowed us to directly pinpoint which of the symbiotic cells in the consortia had assimilated the nitrogen-15–labeled N2 into their biomass," Orphan notes.
The fixation process, say the scientists, is painfully slow; the organisms themselves have ultra-slow growth rates, doubling once every three to six months. "But they are passing on some nitrogen to their neighbors, which means they are producing more than they need," despite the energy cost of doing so, Dekas says. "We don't know what benefit the archaeal organisms get from sharing it, but we do know they need the bacterial symbiont to stay alive," she adds.
"Previously, assumptions about when and where nitrogen fixation takes place made it seem unlikely that nitrogen fixation would occur in this environment, or within such energetically starved organisms," Dekas says. "These results suggest that these assumptions may need to be reevaluated, and that there could be more nitrogen-fixing organisms in other unexpected environments. Together, these previously overlooked sources of nitrogen may be an important component in the marine nitrogen inventory."
The research in the paper, "Deep-Sea Archaea Fix and Share Nitrogen in Methane-Consuming Microbial Consortia," was supported by the National Science Foundation and the Gordon and Betty Moore Foundation.
Journal reference:
Anne E. Dekas, Rachel S. Poretsky, and Victoria J. Orphan. Deep-Sea Archaea Fix and Share Nitrogen in Methane-Consuming Microbial Consortia. Science, 2009; 326 (5951): 422 DOI: 10.1126/science.1178223
Adapted from materials provided by California Institute of Technology, via EurekAlert!, a service of AAAS.

Chemical Imaging Of Deep-sea Microorganisms May Help Explain Lingering Nitrogen Mystery

Researchers at the California Institute of Technology (Caltech) have identified an unexpected metabolic ability within a symbiotic community of microorganisms that may help solve a lingering mystery about the world's nitrogen-cycling budget.

A paper about their work appears in the October 16 issue of the journal Science.

The element nitrogen is a critical part of amino acids, the building blocks of proteins, and therefore essential to all life. Although nitrogen is plentiful on Earth—it comprises 78 percent of the atmosphere, by volume—the element is usually found strongly bonded to itself, in the form of the diatomic gas N₂. To be biologically useful, a nitrogen atom must be released from this coupling and converted to a reduced, or "fixed," state; reduced nitrogen atoms gain an electron, which makes them chemically reactive.

Although lightning, combustion, and other nonbiological processes can reduce nitrogen, far more is generated by nitrogen-fixing microorganisms such as bacteria—in particular, photosynthetic cyanobacteria. These organisms produce the bulk of the nitrogen available to living things in the ocean.

Still, when researchers add up all of the known sources of fixed nitrogen (biological and otherwise) in the global nitrogen cycle and compare it to the sinks—where nitrogen is taken up for growth and energy—they come up short. It appears that more nitrogen is being used than is being made. The apparent nitrogen budget, in effect, does not balance. This discrepancy had led scientists to question whether the nitrogen cycle is truly out of balance, or whether the known inventories of sources and sinks are misleadingly incomplete.

Victoria J. Orphan, an assistant professor of geobiology at Caltech, along with graduate student Anne E. Dekas and postdoctoral research scholar Rachel S. Poretsky, suggest the answer is, at least in part, an incomplete catalog of the sources of fixed nitrogen.

The team studied ocean sediment samples obtained in methane cold seeps located at a depth of about 1,800 feet. The area, known as the Eel River Basin, is located approximately 20 miles off the coast of the northern California town of Eureka, on a continental margin in a region supporting high levels of natural methane seepage at the seabed.

In the laboratory, the researchers examined the methane-rich sediment and the tiny microbial conglomerations that live within. These spherical cell conglomerates, averaging about 500 cells each, consist of two types of anaerobic microorganisms living in a unique symbiotic relationship fueled by methane. The first microorganism is a bacterium that reduces the chemical sulfate into sulfide (via a process that produces the rotten-egg odor of salt marshes and mud flats) to generate energy. The second is a methane-oxidizing archaeon (the archaea are a group of nonbacterial single-celled microorganisms). Working together, these two symbionts are responsible for consuming the majority of the naturally released methane in the deep sea.

Although these symbiotic associations themselves are not new—these conglomerations were discovered about a decade ago and are found on continental margins worldwide—the Caltech scientists discovered something unexpected: the methane-consuming archaea were actively fixing nitrogen, and sharing it with their bacterial neighbors.

"This is the first time that nitrogen fixation has been documented within methane-oxidizing archaea," Dekas says.

Interestingly, although these organisms have a nitrogen-poor diet of methane gas, they live in an environment that contains reduced nitrogen—in the form of ammonium and other chemicals—which means they shouldn't need to create their own. "It's possible that they do need to because they are living in a crowded community—a tightly packed ball—that prevents some organisms from having access to the nitrogen," she says. Another possibility is that these environments do not have as much biologically available reduced nitrogen as had been thought.

To determine that the archaea were indeed fixing nitrogen, the researchers first incubated the archaeal-bacterial assemblages with a dinitrogen gas, N₂, that was composed of two atoms of nitrogen-15. Nitrogen-15 is a nonradioactive isotope of nitrogen that contains one more neutron than regular nitrogen (nitrogen-14) and can be used as a tracer for the incorporation of the element.

The researchers then used a technique called fluorescent in situ hybridization (FISH) to stain the two types of organisms in the sediment, and analyzed these cells for their nitrogen-15 content using a state-of-the-art instrument called a nanometer secondary ion mass spectrometer, or nanoSIMS. The nanoSIMS, which is housed at the Caltech Center for Microanalysis, is capable of collecting chemical and isotopic data at a spatial scale of 50 to 100 nanometers, or around five to 10 times smaller than the size of a single microbial cell.

Both the archaea and, to a lesser extent, their bacterial neighbors had incorporated the nitrogen-15, which could have happened only if the N₂ had been fixed by the archaea—and then shared.

"The high spatial resolution of the nanoSIMS instrument—which produces a focused beam of ions that is smaller than a single cell—allowed us to directly pinpoint which of the symbiotic cells in the consortia had assimilated the nitrogen-15–labeled N₂ into their biomass," Orphan notes.

The fixation process, say the scientists, is painfully slow; the organisms themselves have ultra-slow growth rates, doubling once every three to six months. "But they are passing on some nitrogen to their neighbors, which means they are producing more than they need," despite the energy cost of doing so, Dekas says. "We don't know what benefit the archaeal organisms get from sharing it, but we do know they need the bacterial symbiont to stay alive," she adds.

"Previously, assumptions about when and where nitrogen fixation takes place made it seem unlikely that nitrogen fixation would occur in this environment, or within such energetically starved organisms," Dekas says. "These results suggest that these assumptions may need to be reevaluated, and that there could be more nitrogen-fixing organisms in other unexpected environments. Together, these previously overlooked sources of nitrogen may be an important component in the marine nitrogen inventory."

The research in the paper, "Deep-Sea Archaea Fix and Share Nitrogen in Methane-Consuming Microbial Consortia," was supported by the National Science Foundation and the Gordon and Betty Moore Foundation.

---

Journal reference:

1. Anne E. Dekas, Rachel S. Poretsky, and Victoria J. Orphan. Deep-Sea Archaea Fix and Share Nitrogen in Methane-Consuming Microbial Consortia. Science, 2009; 326 (5951): 422 DOI: 10.1126/science.1178223

Adapted from materials provided by California Institute of Technology, via EurekAlert!, a service of AAAS.

Planet's Nitrogen Cycle Overturned By 'Tiny Ammonia Eater Of The Seas'

It's not every day you find clues to the planet's inner workings in aquarium scum. But that's what happened a few years ago when University of Washington researchers cultured a tiny organism from the bottom of a Seattle Aquarium tank and found it can digest ammonia, a key environmental function. New results show this minute organism and its brethren play a more central role in the planet's ecology than previously suspected.

The findings, published online September 30 in the journal Nature, show that these microorganisms, members of ancient lineage called archaea, beat out all other marine life in the race for ammonia. Ecologists now assume that ammonia in the upper ocean will first be gobbled up by phytoplankton to make new cells, leaving very little ammonia for microbes to turn into nitrate.

"Our data suggests that it's the other way around," said co-author Willm Martens-Habbena, a UW postdoctoral researcher. "Archaea are capable of stealing the ammonia from other organisms and turning it into nitrate. Then it's the phytoplankton that take up that nitrate once again."

Ammonia is a waste product that can be toxic to animals. But plants, including phytoplankton, prize ammonia as the most energy-efficient way to build new cells.

The new paper also shows that archaea can scavenge nitrogen-containing ammonia in the most barren environments of the deep sea, solving a long-running mystery of how the microorganisms can survive in that environment. Archaea therefore not only play a role, but are central to the planetary nitrogen cycles on which all life depends.

"Bacterial nitrifiers were discovered in the late 19th century. One century later this other group of nitrifiers is discovered that is not a minor population, it turns out to be the major population," said co-author David Stahl, a UW professor with appointments in the departments of civil and environmental engineering and microbiology. "We have to revise our basic understanding of the nitrogen cycle."

In the tree of life, archaea occupy their own branch. Archaea were discovered only about 30 years ago and were first thought to exist only in extreme environments, such as hot springs or hydrothermal vents. They are now known to be more widespread.

In the early 1990s scientists collecting seawater found strands of genetic material that suggested at least 20 percent of the ocean's microbes are archaea, and circumstantial evidence suggested they might live off ammonia. Stahl's group in 2005 was the first to isolate the organism, which they got from a tropical tank in the Seattle Aquarium, and demonstrate that it can, in fact, grow by oxidizing ammonia. His lab and others have since found the organism in many marine environments, including Puget Sound and the North Sea. The microbe is likely ubiquitous on land and in the seas, they say.

The new experiments show that the organism can survive on a mere whiff of ammonia – 10 nanomolar concentration, equivalent to a teaspoon of ammonia salt in 10 million gallons of water. In the deep ocean there is no light and little carbon, so this trace amount of ammonia is the organism's only source of energy.

"What Willm's work has shown is that these archaea can grow at the vanishingly low concentrations of ammonia found in the ocean," Stahl said. "Until we made the measurements, no one thought it would be possible that an organism could live on these trace amounts of ammonia as a primary energy source."

That finding has two important implications for ocean ecosystems. Scientists knew that something was turning ammonia into nitrate in the deep ocean, but could not fathom what organism might be responsible. Now it appears archaea are those mysterious organisms.

And in the sun-dappled upper ocean waters, it appears that archaea can out-compete phytoplankton for ammonia. The same may be true in soil environments, the researchers say.

The archaea in question are small even by the standards of single-celled organisms. At 0.2 micrometers across, about 8 millionths of an inch, the only life forms smaller are viruses. Martens-Habbena speculates that archaea's size could explain how they are able to survive on such a scant energy supply. The strain used in these experiments is named Nitrosopumilus maritimus, which means "tiny ammonia-oxidizer of the sea."

A better understanding of archaea's lifestyle and role in nitrogen cycles not only would rewrite ecology textbooks. It could also have practical applications, such as devising natural ways to boost a soil's nitrogen content without needing to use chemical fertilizers, or designing sewage treatment plants that employ microbes to remove nitrogenous waste more efficiently, or understanding which microbes produce global-warming gases such as nitrous oxide.

The new findings will also affect the equations used in global climate models, researchers say. Computer models use global cycles of nitrogen and other chemicals to estimate how much carbon dioxide the oceans will absorb and ultimately sink to the bottom of the sea. The new findings suggest that most of the nitrate in the surface water comes from recycling of biomass, and not from the deep water as currently assumed.

"Our data suggest that the carbon pump is weaker than currently assumed, so current climate models may overestimate how much carbon can be absorbed by the oceans," Martens Habbena said.

Other co-authors are the UW's Paul Berube, Hidetoshi Urakawa and Jose de la Torre. The research was funded by the National Science Foundation.

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Adapted from materials provided by University of Washington.

Denitrification, Its Importance Once Diluted, May Be Back On Top

After more than a decade of inquiry, a Princeton-led team of scientists has turned the tables on a long-standing controversy to re-establish an old truth about nitrogen mixing in the oceans.

For decades, scientists thought they had a handle on the workings of an intricate natural mechanism known as the nitrogen cycle, essential to maintaining life on Earth. This process, one of nature's most elegant sleights-of-hand, shuttles nitrogen from the soils to the oceans to the atmosphere and back.
A key part of that cycle, researchers once thought, was a process known as denitrification. In low-oxygen -- or anaerobic -- conditions seen in large stretches of ocean sediments and in a few important regions of the open ocean, bacteria act as "denitrifyers," performing the crucial task of gobbling up nitrates and converting them to nitrogen gases, which complete the cycle by flowing back to the atmosphere.
In 1995, a group of Dutch scientists who had been studying the cycling of nitrogen through wastewater treatment plants came up with a startling conclusion. A new process, which they called anaerobic oxidation or "anammox" and that involved different bacteria, was the real player in removing nitrogen in low-oxygen environments, they said. They found the process worked to break down materials in sewage, and they confirmed that the mechanism also was operating in low-oxygen marine environments. They went so far as to suggest that the nitrogen cycle for oceans needed to be revised, as denitrification, according to their inquiry, did not play the major role that had been thought.
The notion was controversial and did not sit well with some scientists.
Now, a research team, led by Bess Ward, the William J. Sinclair Professor of Geosciences at Princeton University, writing in the Sept. 3 issue of Nature, is presenting data that could re-establish denitrification as the main actor in returning nitrogen to the air. After traveling through some of the key low-oxygen sites of the world's oceans, the team has found the telltale chemical signatures proving that denitrification and not anammox is the pivotal process at work most of the time.
"In our paper, we report that in the world's largest anoxic marine ecosystem -- the low-oxygen waters of the Arabian Sea -- denitrification rather than anammox is the dominant process," said Ward, who is also chair of the Department of Geosciences at Princeton. "If denitrification is important in the Arabian Sea, then it is important on a global scale, and the nitrogen cycle must be evaluated in that light."
The work, according to a leading expert in the marine nitrogen cycle, confirms his own observations of seawater processes showing that denitrification is key and indicates that the current mainstream view in science may be based on a false impression. "My suspicions that future work would, once again, demonstrate the importance of conventional denitrification have now been confirmed," said Louis A. Codispoti, an oceanographer and research professor at Horn Point Laboratory, part of the University of Maryland in Cambridge, who was not involved with the research.
The researchers who discovered the anammox process nearly 15 years ago, led by Gifjs Kuenen, then at the Delft University of Technology in the Netherlands, moved beyond the original discovery in wastewater treatment plants and found the reaction was also at work in removing nitrogen in a few regions of the ocean known as "oxygen minimum zones." Zeroing in on a low-oxygen zone off the coast of Peru, the work of Dutch, Danish and German scientists found that anammox reactions, rather than denitrification, were operating there.
"That was astounding," Ward remembered.
Thinking there may be a problem with the methodology or that scientists didn't understand the nitrogen cycle as much as they thought they did, she began to devise experiments to seek answers.
Working with other members of her team over the next decade, they learned the methods of the European experts and started to plan to replicate the studies. In 2005, they confirmed that bacteria supporting the anammox reaction dominated the removal of nitrogen in a low-oxygen region off the Peru coast. But when they took samples of water in the Arabian Sea, they found just the opposite -- denitrification was a major force there. The European researchers had found anammox in the Peru system but had never reported on the Arabian Sea.
The notion that microbial processes can vary in low-oxygen zones around the world is startling and important to know, the researchers said.
"We care because nitrogen is a key limiting nutrient to primary productivity," said Jeremy Rich, a former postdoctoral fellow in Ward's lab and now an assistant professor of environmental studies at Brown University, who contributed to the study. "We already knew these zones removed nitrogen, but now that we know the actual processes taking place, we'll be in a much better position to predict how these zones change. And how these zones change will in turn influence primary productivity."
The findings have forced the scientists to re-evaluate what they already knew.
"This made us think -- this means the Arabian Sea is somehow different from the Peru system," Ward said. "Previously, we thought they were the same. Clearly, something was different and that, in and of itself, is an important insight. And, clearly, denitrification is important -- you cannot rewrite the nitrogen cycle."
Because the Arabian Sea is the world's largest anoxic marine ecosystem, that body's most dominant process is almost certainly the primary way for nitrogen to be removed from the world's oceans.
To confirm the conclusions, the team designed a new way of sampling and identifying chemicals and repeated the experiments. The results were the same.
The nitrogen cycle is one of the most important nutrient cycles in nature, providing a transformative process in which nitrogen is taken from the atmosphere and converted into a form that can be consumed by plants. Nitrogen makes up about 80 percent of the earth's atmosphere. It is used by living organisms to produce a number of complex organic molecules, including DNA.
Processing or fixation is necessary to convert gaseous nitrogen into forms usable by living organisms. Most is done by bacteria that possess a nitrogenase enzyme that combines gaseous nitrogen with hydrogen to produce ammonia, which is then converted by the bacteria to make their own organic compounds.
In low-oxygen conditions, denitrification by bacteria occurs when nitrates are converted to nitrogen gases like nitrous oxide and returned to the atmosphere. In the anammox process, nitrates are reduced to nitrites and then combine with ammonium before returning to the atmosphere.
In addition to Ward and Rich, other authors on the paper include: Silvia Bulow, a graduate student, and Amal Jayakumar, a senior professional specialist, in Princeton's Department of Geosciences; Allan Devol, research professor of oceanography, and Bonnie Chang, a graduate student, at the University of Washington; and Hema Naik, a scientist, and Anil Pratihary, a graduate student, at the National Institute of Oceanography in India.
The research was funded by the National Science Foundation.
Adapted from materials provided by Princeton University.

Atmosphere Threatened By Nitrogen Pollutants Entering Ocean

A large quantity of nitrogen compounds -- emitted into the atmosphere by humans through the burning of fossil fuels and the use of nitrogen fertilizers -- enters the oceans and may lead to the removal of some carbon dioxide from the atmosphere, concluded a team of international scientists led by Texas A&M University Distinguished Professor of Oceanography and Atmospheric Sciences Robert Duce.

The team of 30 experts from institutions around the world presented its conclusions in the current issue of the journal Science.
Human-caused atmospheric nitrogen compounds are carried by wind and deposited into the ocean, where they act as a fertilizer and lead to increased production of marine plant life. The increase in plant life causes more carbon dioxide to be drawn from the atmosphere into the ocean. This process results in the removal of about 10 percent of the human-caused carbon dioxide in the atmosphere, thus potentially reducing the climate warming potential, according to the team's paper.
However, some of the nitrogen deposited in the ocean is re-processed to form another nitrogen compound called nitrous oxide, which is then released back into the atmosphere from the ocean. Nitrous oxide is a powerful greenhouse gas itself -- about 300 times more powerful per molecule than carbon dioxide -- thus cancelling out about two-thirds of the apparent gain from the carbon dioxide removal, Duce explained. "But of course, the whole system is so complex that we're still rather unsure about what some of the other impacts might be within the ocean," he said.
In most areas of the ocean, nitrogen is the nutrient that limits the production of plant life, Duce said. So when all of the nitrogen in an area of the surface ocean is used up, no more plant life forms in that area. The team found that human-caused nitrogen deposits account for up to one-third of the external input of nitrogen into the ocean, and this increase in nitrogen available for the production of plant life causes more plants to form, Duce explained.
Oceanic plant life is produced from marine carbon (bicarbonate) in the ocean, and that amount of bicarbonate is in equilibrium with the carbon dioxide in the atmosphere. So when more bicarbonate is used up to produce marine plant life, it disrupts the equilibrium, and carbon dioxide is drawn down to the ocean from the atmosphere to restore the balance, Duce explained.
Thus, the human-caused nitrogen fertilization of the ocean removes some of the most important greenhouse gas -- carbon dioxide -- from the atmosphere, Duce said. This gain, however, is offset by the nitrogen compound, nitrous oxide, that also forms in the ocean due to the nitrogen fertilization and is re-emitted into the atmosphere as a powerful greenhouse gas, he added.
"If you don't consider the impact of human-caused nitrogen when trying to model climate change, you're missing a possibly significant part of the overall carbon cycle as well as the nitrogen cycle," Duce said. "So nitrogen deposition is potentially a very important factor in the climate change issue."
According to the team's calculations, about 54 million tons of nitrogen produced from human activities entered the ocean from the atmosphere in the year 2000. The team also found that the current nitrogen emissions are about 10 times what they were in 1860, Duce said. He added that the amount of nitrogen entering the atmosphere is expected to rise in the coming decades with the increase in demand for energy and fertilizers, and the team estimates that by the year 2030, human-caused nitrogen emissions will have risen to around 62 million tons per year.
"Clearly, there is much that we do not know about the extent and timescale of the impacts of this nitrogen deposition on the oceans and the subsequent feedbacks to the climate system," Duce said. "The implications are complex and interactive, and this is a very important issue that policy makers need to address and that scientists trying to model and understand the future of climate and climate change need to take into consideration."
Adapted from materials provided by Texas A&M University.

Anammox Bacteria Produce Nitrogen Gas In Oceans' Snackbar

Thirty to fifty percent of the global conversion of nutrients to nitrogen gas occurs in these areas. In ‘The Proceedings of the National Academy of Science of the United States (PNAS)’ of Tuesday 19 April, researchers from Bremen and Nijmegen state that this conversion is not carried out by denitrifying bacteria, as was believed for decades, but by anammox bacteria. Nitrogen compounds act as fertilizers and are the ecological basis for any lifeform on earth as these compounds limit the overall growth rate.

The researchers discovered this type of bacteria for the first time a few years ago in the oxygen poor Black Sea and now also in the open ocean. This discovery has major consequences for our understanding of the global nitrogen cycle. The Benguela current system leads to upwelling of nutrient-rich cold water off the coast of Namibia and acts as a kind of snackbar in the tropical ocean, which is visited by many animals including giant whales. The newly discovered anammox bacteria remove ammonium from the ocean, which as a result can not be taken up anymore by other organisms. Algae and cyanobacteria only partly succeed in fixing the released nitrogen gas to form new nutrients that can be fed into the nutrient cycle again.
In the article in PNAS, researchers from the Max Planck Institute for Marine Microbiology (Bremen, Germany) and the Institute of Water and Wetland Research (IWWR) of the Radboud University of Nijmegen show for the first time that anammox bacteria are present in the Atlantic ocean in oxygen poor waters at ~100 m depth. The number and activity of anammox bacteria present at this depth is sufficient to remove the ammonium that rises up from deeper waters and the seafloor. A unique combination of microbiological methods involving high resolution nutrient profiles, experiments with stable isotope labeled nutrients, depth profiles of unique membrane lipids (ladder molecules), fluorescence microscopy and DNA analyses of water samples was used to proof the abundance of anammox bacteria in the Atlantic Ocean. The discovery of anammox bacteria in the open ocean has major consequences for our understanding of the global nitrogen cycle. Models of global nitrogen budgets, which play an important role in long term climate predictions, will have to be revised. The discovered nitrogen loss has also consequences for the carbon cycle in marine ecosystems.Denitrification.The measurements refute the dominating theory that oceanic nitrogen loss results from the activity of bacteria that convert nitrate (via nitrite) with organic matter to nitrogen gas in the absence of oxygen (denitrification). In fact, the researchers recently discovered that anammox bacteria can use organic matter to convert nitrate into nitrite (much better than denitrifying bacteria). This new finding even increases the importance of anammox bacteria in the ocean. New speciesThe anammox bacteria discovered in the Atlantic Ocean are closely related to the bacterial species Scalindua sorokinii, which was recently discovered in the Black Sea (Kuypers et al., 2003 Nature 8 April). Like their relatives in the Black Sea, the Namibian anammox bacteria contain unique ladder molecules (Damsté et al., Nature 17 October 2002) in the membrane surrounding a special prokaryotic organel in which ammonium is converted to nitrogen gas. These ladder molecules are ether bound in the membrane. This property was believed to be restricted to the Archaea, the ‘ancient’ bacteria. Waste water treatmentThe anammox process, in which ammonium is oxidized with nitrite to nitrogen gas, is a promissing alternative to current methods of nutrient nitrogen removal from waste water. The running costs of a waste water treatment system using the anammox process are only 10% of the costs of current treatment systems using conventional methods. Additionaly, the anammox process could reduce the emission of the greenhouse gas CO2 during waste water treatment by 88%. Because of these advantages the first large scale waste water treatment system was recently installed in Rotterdam (The Netherlands).Title of publication and authors:Massive nitrogen loss from the Benguela upwelling system through anaerobic ammonium oxidation Marcel M.M. Kuypers*, Gaute Lavik*, Dagmar Wöbken*, Markus Schmid, Bernhard M. Fuchs*, Rudolf Amann*, Bo Barker Jørgensen* & Mike S.M. Jetten *Max-Planck-Institute for Marine Microbiology, Celsiusstraße 1, 28359 Bremen, Germany Department of Microbiology, IWWR Radboud University Nijmegen, Toernooiveld 1, 6525 ED Nijmegen, The Netherlands
Adapted from materials provided by Max Planck Society.

Ocean Nitrogen Fixation: New Findings Blow A Decade Of Assumptions Out Of The Water

The Atlantic Ocean doesn't receive the mother lode of fixed nitrogen, the building block of life, after all. Instead, comparing fathom for fathom, the Pacific and Indian oceans experience twice the amount of nitrogen fixing as the Atlantic, say researchers in the Jan. 11 issue of Nature. The title of an accompanying News and Views piece says it all, "Looking for N2 Fixation in all the Wrong Places." It's important to have a global picture of where nitrogen fixation is occurring -- that is where nitrogen gas is being converted into substances like nitrate that are usable by life -- in order to understand the environmental controls on nitrogen fixation and its likely response to climate change in the past and in the future, says Curtis Deutsch, a University of Washington research assistant and lead author of a paper in the Jan. 11 issue of Nature. The new research, for example, indicates that the inventory of nitrogen in the oceans is likely to be less subject to major fluctuations than had been assumed. Because it has been thought that nitrogen fixation is limited without enough iron, the conventional wisdom for the past decade dictated that the Atlantic Ocean would be the prime site for fixing nitrogen. That's because compared to the other low-latitude oceans, the Atlantic is peppered with iron-laden dust blowing off the African continent. Winds can't carry such dust all the way across the Pacific Ocean because it is so vast. Iron may still be a limiting factor in nitrogen fixation, but if it is, then the Pacific and Indian oceans are getting iron from some source other than atmospheric dust, Deutsch says. The new research also means places where nitrogen is being fixed by certain microorganisms are in close proximity to where it is being pulled back apart into its gaseous state by a different kind of micoorganism, he says. Nitrogen gas, N2, is unusable by life. It has to be fixed, that is, latched onto other chemicals to form compounds such as nitrate, NO3. Only then can it be used to build amino acids and proteins essential to all life. Eventually the fixed nitrogen is returned to its gaseous state, a process called denitrification. Scientists have known for several decades that denitrification occurs in the deep, low-oxygen waters of the Pacific and Indian oceans. If the Atlantic was the site of a lot of nitrogen fixation, that would have put the two processes half a world away from each other. Scientists had estimated that, at those distances, it could take 1,000 years to re-balance the ocean's nitrogen cycle if large-scale changes were to occur in either nitrogen fixation or denitrification -- if climate change altered ocean temperatures and the rates of the two processes, for instance. The new findings show the processes are happening within a few hundred miles of each other so the balance could be reached within a decade, the authors estimate. Deutsch compares the old assumption to a house where the thermostat is many rooms away from a window that has swung open, letting in cold air. The house could get quite chilly before the draft reaches the thermostat and the furnace turns on. But if the thermostat is in the same room as the window, the furnace will turn on and even out the temperature much faster. In his research Deutsch used a novel analysis of surface nutrients in the world's oceans that relied on several decades of existing large-scale data on nitrogen-to-phosphorous ratios, phosphorous also playing a major role in primary production. His work has been supported by a NASA Earth System Science Fellowship and the UW Program on Climate Change. "There has been a great deal of controversy in the literature as to whether fixed nitrogen in the ocean remains constant with time or fluctuates widely," says Jorge Sarmiento, professor of geosciences at Princeton University and one of the co-authors. "This study is a major advance for those of us who have been arguing that it is relatively stable." Other authors are Daniel Sigman, a professor at Princeton; Nicolas Gruber, a professor at ETH Zurich, Switzerland, and associate professor with the University of California, Los Angeles; and John Dunne, with the National Oceanic and Atmospheric Administration's Geophysical Fluid Dynamics Laboratory at Princeton. Funding for this work also came from the U.S. Department of Energy, NOAA and the National Science Foundation.Note: This story has been adapted from a news release issued by University of Washington.