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Nitrogen fixation and phytoplankton blooms in the southwest Indian Ocean

Nitrogen fixation and phytoplankton blooms

National Oceanography Centre, UK

Observations made by Southampton scientists help understand the massive blooms of microscopic marine algae - phytoplankton - in the seas around Madagascar and its effect on the biogeochemistry of the southwest Indian Ocean.

The observations were made by researchers based at the National Oceanography Centre, Southampton (NOCS) during a 2005 hydrographic survey south and east of Madagascar while aboard the royal research ship RRS Discovery. The fully analysed results are published in the journal Geophysical Research Letters.

Nitrogen-fixing bacteria convert atmospheric nitrogen into nitrogen compounds that organisms can then use as food. This process is thought to be important in areas of the ocean where nitrogen-based nutrients are otherwise in short supply, and the researchers confirm that this is indeed the case in the region south of Madagascar.

But there were some surprises. Previously, it has been thought that the large-scale autumn bloom that develops in this region is driven by nitrogen-fixing blue-green algae, or cyanobacteria, called Trichodesmium, colonies of which the researchers found to be abundant. However, the 2005 bloom was dominated by a diatom - a type of phytoplankton - the cells of which play host to another nitrogen-fixing cyanobacterium called Richella intracellularis, with Trichodesmium apparently playing second fiddle.

Diatoms have relatively large cells, and when they die they sink down the water column, carrying with them carbon that is ultimately derived from carbon dioxide drawn from the atmosphere though the process of photosynthesis. "Carbon dioxide is a greenhouse gas, and enhanced export of carbon to the deep ocean in the bodies of diatoms is an important natural mechanism by which the ocean regulates atmospheric carbon dioxide and our climate," says team member Dr Alex Poulton of NOCS.

The researchers believe that their findings will have an impact on modelling and satellite studies of the Madagascar bloom. "Future research will also need to account for the magnitude of carbon export associated with diatoms and their nitrogen-fixing guests in the southwest Indian Ocean, and indeed other subtropical oceanic settings," says Dr Poulton.

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Reference:

Poulton, A. J., Stinchcombe, M. C. & Quartly, G. D. High numbers of Trichodesmium and diazotrophic diatoms in the southwest Indian Ocean, Geophys.

Res. Lett., 36, L15610, (2009. doi:10.1029/2009GL039719).

The authors are Alex Poulton, Mark Stinchcombe, and Graham Quartly, who are all based at the National Oceanography Centre, Southampton.

http://www.agu.org/pubs/crossref/2009/2009GL039719.shtml

Contact information:

For more information contact the NOCS Press Officer Dr Rory Howlett on +44 (0)23 8059 8490 Email: r.howlett@noc.soton.ac.uk

An image is available from the NOCS Press Office (Tel. 023 8059 6100).

Scientist contact

Dr Alex Poulton: email Alex.Poulton@noc.soton.ac.uk; telephone +44 (0) 23 8059 7086

The National Oceanography Centre, Southampton is the UK's focus for ocean science. It is one of the world's leading institutions devoted to research, teaching and technology development in ocean and earth science. Over 500 research scientists, lecturing, support and seagoing staff are based at the centre's purpose-built waterside campus in Southampton along with over 700 undergraduate and postgraduate students.

The National Oceanography Centre, Southampton is a collaboration between the University of Southampton and the Natural Environment Research Council. The NERC royal research ships RRS James Cook and RRS Discovery are based at NOCS as is the National Marine Equipment Pool which includes Autosub and Isis, two of the world's deepest diving research vehicles.

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