Iron dust helps oceans to fix nitrogen
2 November 2009, by Sara Coelho
Winds blowing from the Sahara Desert in Africa to the Atlantic Ocean carry more that just sand: new research finds that dust storms are a major source of iron to the ocean and control the cycle of nitrogen, an important nutrient for marine creatures.
A Saharan dust storm blowing dust out across the Atlantic.
'Our new data provides some of the first conclusive evidence that iron availability is playing a significant role in controlling the input of fixed nitrogen, at least to the Atlantic Ocean,' says Mark Moore, a researcher at the National Oceanography Centre, Southampton.
Nitrogen makes up 75% of the atmosphere and is an essential nutrient for plants and animals, but most organisms cannot use its common gaseous form (N2). Some marine blue-green bacteria are capable of absorbing nitrogen from the atmosphere, turning it into ammonium and nitrates - forms readily available to other marine creatures.
Bacteria colony, Trichodesmium.
To fix the nitrogen, the bacteria use an enzyme called nitrogenase. Nitrogenase contains iron and 'it has been argued that the amount of iron available is a limiting factor in the production of the enzyme and nitrogen fixation,' says Moore, who started working on this problem with Richard Geider at the University of Essex. In contrast, other authors have suggested that the nutrient phosphorous is more important in regulating the growth of the nitrogen fixing bacteria.
'If iron is important, then we might expect that the rate of nitrogen fixation would be higher in areas close to an iron source,' says Moore. The major iron input to the tropical Atlantic is the dust blown on the wind from the Sahara Desert to the ocean surface.
Moore and colleagues tested the idea during a six-week scientific cruise on board the RRS Discovery in 2005. The ship sailed along a line between the UK and South Africa, about 10,000km long. The team collected samples along the way to find out which factors control the fixation of nitrogen gas by marine micro-organisms.
The international team of scientists measured the amount of phosphorous, fixed nitrogen and iron dissolved in the ocean water. 'We also analysed aluminium, which can be used as a marker for desert dust,' says Moore.
They also counted the numbers of nitrogen-fixing organisms, especially the cyanobacteria Trichodesmium,sometimes referred to as 'sawdust of the sea' by sailors, and measured how fast the bacteria fixed nitrogen.
The team found that the amount of dissolved nitrogen was similar throughout the Atlantic Ocean. But the North Atlantic had significantly more iron and less phosphorous than the Southern Atlantic.
The results, published online yesterday in Nature Geoscience, show that the bacteria take up more nitrogen gas in the North, near the Sahara Desert, where the iron content is high and that the nitrogen-fixation rate does not appear to depend on phosphorous.
This is because it is easier to produce the nitrogenase enzyme if there is an ample supply of iron. In the South Atlantic, where iron is scarce, there is less nitrogenase and bacteria fix less nitrogen gas. Lower numbers of nitrogen fixing bacteria mean that more phosphorous is available in the southern Atlantic waters.
This could be an important conclusion for understanding past and future climate changes, as the cycle of oceanic nitrogen is crucial for marine life and ultimately for the storage of carbon in the oceans.
'Any interactions between large-scale 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 Moore.
Moore, C. M., et al. Large-scale distribution of Atlantic nitrogen fixation controlled by iron supply. Nature Geoscience, published online 1 November 2009.
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