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ETH - Eidgenoessische Technische Hochschule Zuerich - Swiss Federal Institute of Technology Zurich
Section: Science Life
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Published: 18.01.2007, 06:00
Modified: 17.01.2007, 22:40
Nitrogen fixation by marine organisms
A big contribution from the really small

Nitrogen-fixing micro-organisms supply the ocean with biologically available nitrogen and are thus not only an important jigsaw in the global nitrogen cycle, but are also of great importance to the entire carbon cycle. This is because the carbon cycle is closely linked to that of nitrogen.

Peter Rüegg

Not much happens in the sea without nitrogen. Put another way, the productivity of an ocean greatly depends on how much biologically available nitrogen, nitrate for example, is available to green plants in the surface water. The research community previously assumed that nitrate was to be found on the surface mainly due to water rising from the depths of the ocean, allowing organisms to convert the nitrogen contained in nitrates into important building blocks such as amino acids or DNA. (Previous sentence correctly interpreted?) When an organism dies, its biomass sinks down into the depths where its nitrogen is converted into nitrate via numerous degradation processes until one day the nitrate is carried back up again with an ascending flow.

The simplified picture is obsolete

In a new study published in “Nature” on Thursday 11 January 2007 (1), researchers, including ETH Professor Nicolas Gruber from the Institute of Biogeochemistry and Pollutant Dynamics, showed that this simplified picture needs to be revised. Marine micro-organisms introduce considerable amounts of biologically available nitrogen into the ocean. These micro-organisms, for example the cyanobacterium Trichodesmium, fix elemental nitrogen (N2) dissolved in the water and convert it into nitrate. This fertilizes the surface water and increases the productivity of the oceans. The entire nutrient chain benefits from this and influences not least the climate. This is because a productive ocean fixes more carbon and thus reduces the level of the greenhouse gas carbon dioxide in the atmosphere..

The first global overview

In the study, the researchers show that nitrogen-fixing micro-organisms occur mainly in the warm seas of the sub-tropics and tropics, for example in large parts of the Pacific, the Arabian Sea or off the west coast of Africa. This first global description of the marine distribution of nitrogen fixation became possible because researchers combined knowledge based on nutrient data with model simulations.

The scientists also realised that regions with a high level of nitrogen fixation at the ocean surface coincide with regions in the depth of the seas where organic nitrogen is degraded to elemental nitrogen under oxygen-depleted conditions. Therefore this microbial denitrification process deprives the ocean of biologically available nitrogen because, except for the nitrogen-fixing organisms, marine algae cannot utilise elemental nitrogen.

Denitrification and nitrogen fixation coupled together

Therefore the equilibrium between the input of nitrogen by the fixers and the loss resulting from the denitrifiers in the ocean is very important to the marine nitrogen cycle and the productivity of the oceans. Gruber says that “If denitrification was strong and fixation weak, the oceans would lose nitrogen and their productivity would decrease.”


The cyanobacterium Trichodesmium is one of the most important nitrogen fixing organisms in the world’s oceans (Photo: B. Bergman / Stockholm University). large

However, the close spatial association between the two processes seems to be evidence against imbalances of this kind. As a result of denitrification water loses nitrate and rises to the strongly illuminated surface where the cyanobacteria live and fix the nitrogen again. In this manner they return the lost nitrogen to the sea again relatively quickly. This strong coupling between the two processes stabilises the marine nitrogen cycle and so too its biological productivity.

The Atlantic is less productive than the Pacific

That cyanobacteria could make such an important contribution to supplying the seas with biologically available nitrogen was only recently recognised. (2) Earlier estimates were based on a proportion that was ten times less.(ten times less than … (what)?) Gruber stresses that the researchers were astonished that nitrogen fixation plays such a large role especially in the Pacific. For example micro-organisms fix about twice as much nitrogen in the Pacific as in the Atlantic.

The new study also shows that iron has less effect (on plant life?) than was assumed, even though nitrogen-fixing organisms need a large amount of iron. For example, this element enters the oceans as a result of sandstorms. The metal is very rare in seawater and scarcely occurs at all in the Pacific, where the nitrogen-fixing micro-organisms are particularly active. “On the other hand, the available phosphate and its concentration relative to nitrate is much more important,” says the ETH Professor. The nitrogen-fixers find their optimum ecological niche where there is a large amount of phosphate and little nitrate. Like nitrogen, phosphate is also a deficiency element and is important for plant growth.

Feedback is possible

These results have consequences for the world’s climate because the marine nitrogen and carbon cycles are closely coupled. If the sea is deficient in nitrogen, productivity is minimized and less carbon is bound in the sea. However, a high level of biological activity in the water forms more biomass, thus abstracting carbon from the atmosphere. This carbon in the form of excreta or dead animals and plants ultimately drops down into the depths of the oceans where it is slowly degraded to CO2 again, but remains withdrawn from the atmosphere for a long time. However, the warming up of the seas as a consequence of the rising concentration of CO2 in the atmosphere could reduce productivity. Gruber fears that “This could lead to a positive feedback effect for atmospheric CO2.”

(1) Deutsch, C. et al (2007): Spatial coupling of nitrogen inputs and losses in the ocean. Nature Vol. 446, 11 January 2007.
(2) Gruber, N. (2005): A bigger nitrogen fix. Nature Vol. 436, 11 August 2005, 786-787.

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