Iron ‘Vacuuming’ checks algal growth in oceans

14-11-2011   ‘The availability of iron varies in different oceans’. This is one of the results of research carried out by Charles-Edouard Thuróczy, a scientist working at NIOZ, in the Arctic and Antarctic Oceans. Thuróczy found trends in the availability of iron, both horizontally and vertically in the sea water. Iron importantly combines with dissolved organic matter, which means that the iron remains available for phytoplankton. Thuróczy will defend his thesis on this research project at the University of Groningen on 21 November 2011.

Phytoplankton (plant plankton), the foundation of the food chain, combines with carbon dioxide dissolved in the upper layers of the oceans. Phytoplankton requires iron for growth. Iron concentrations in sea water, however, are very low, which often makes iron a limiting factor on growth. The availability of iron is mainly determined by two factors, on the one hand iron supply (from land or from glaciers) and on the other hand the solubility of iron.

Iron can be found in sea water in various different forms. When it is dissolved, it is usually combined with organic substances called ligands. However, unfiltered water also contains particles that combine with iron, which can then no longer be taken in by phytoplankton. These particles lump together and then sink to the bottom of the sea, leaving the upper layers of the ocean, where phytoplankton is found. This is why these particles are sometimes called vacuum cleaners: they clean away the iron from the water and remove it.

Thuróczy found a balance between dissolved iron that has combined with ligands, and the iron combined with the ‘vacuuming’ particles. When iron concentrations in sea water are low, the ligands can draw some iron out of the ‘vacuum cleaners’ so that some of it becomes available to phytoplankton.

During research expeditions to both the Arctic and the Antarctic Oceans, Thuróczy studied the location of iron in the sea water by analyzing both dissolved iron that combined with ligands and the total amount of iron, including the iron combined with the particles. On the basis of this, Thuróczy has developed a method for expressing the saturation of the ligands with iron. This method uniquely demonstrated a clear geographical distribution of different forms of iron. It turns out that there is a correlation between the distance to the source of iron and the iron saturation of the ligands. The closer the sea water is to the source of iron, the more saturated the ligands are. In areas with lots of phytoplankton, however, the ligands are far from saturated because iron from the ligands has been taken in by phytoplankton.

The research cruises with the vessels Polarstern and Nathaniel B. Palmer in 2007 and 2009 were part of the International Polar Year project, which was organized by the Netherlands Organisation for Scientific Research.

PhD Thesis
Thuróczy, C.-E. Physical and Chemical Speciation of Iron in the Polar Oceans, 206 pp.

Photo captions:
Charles-Edouard Thuróczy on the ice at the Arctic in front of the Polarstern.
Sample points at the Antarctic.