Carbon dioxide: riding the sugar express into the abyss

In the world's oceans, considerably more of the greenhouse gas carbon dioxide than previously suggested can be transported to the deep sea. This was pointed out in an article authored by scientists from the Alfred Wegener Institute for Polar and Marine Research (AWI) in Bremerhaven, and the Laboratoire d’Océanographie de Villefranche in France in the recent issue of the scientific journal Nature (see below). In their work, the scientists show that water-soluble sugar molecules produced by algae form particles in the process of aggregation. These particles play a considerable role in the vertical transport of organic carbon compounds to ocean depths. Up to now, it was assumed that dissolved organic substances do not contribute to the vertical transport of carbon to the deep sea.

There is a fundamental difference between dissolved and particulate substances in the ocean. While dissolved substances do not sink to the bottom, particles such as algal material descend through the water column and, in this manner, may carry elements such as carbon from the surface into great depths. However, most of the algal-produced organic carbon compounds exist in dissolved form in the ocean. Some such dissolved special sugar compounds, the acid exopolymer polysaccharides, can stick to one another and develop into particles. “We have now shown that the formation of these particles during the course of algal blooms is faster than the bacterial decomposition of the dissolved sugars. Hence, these particles, which are rich in carbon, can sink into the deep ocean,” explains the lead scientist of this study, Dr Anja Engel.

The scientists conducted their experiments in model ecosystems (mesocosms), each with a volume of 11 cubic meters, deployed in a Norwegian fjord. Over 16 days, dissolved and particulate material was sampled in a bloom of the calcium carbonate-producing alga Emiliania huxleyi. Subsequently, scientists succeeded for the first time in simulating dynamics of particle build up from dissolved sugar molecules in a computer model.

The production of exopolymer polysaccharides depends on the physiology and species composition of the algae. In addition, global environmental changes, such as the increase in carbon dioxide concentration, or the input of nutrients in the upper, productive water layers, likely will influence the formation of sugars. This could have consequences for the absorption and storage of carbon dioxide in the ocean.

Polysaccharide aggregation as a potential sink of marine dissolved organic carbon.
Anja Engel (Alfred Wegener Institute; currently guest scientist at the State University New York at Stony Brook), Silke Thoms (Alfred Wegener Institute), Ulf Riebesell (Alfred Wegener Institute, currently at the Institute for Marine Research, Kiel), Emma Rochelle-Newall (Laboratoire Océanographie de Villefranche, France; currently at the Centre IRD de Noumea, New Caledonia) & Ingrid Zondervan (Alfred Wegener Institute). Nature, April 29 2004.

Bremerhaven, April 29 2004