Saharan dust as a microbial breakfast?

Researchers are feeding mysterious thunder plankton in the Atlantic Ocean with unusual food

Satellite image of a Sahara sandstorm over the Cape Verde Islands. © NASA
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Kiel scientists collected air and water samples on the Cape Verde Islands in the Atlantic to detect a connection between dust storms from the Sahara and the biological productivity of the ocean. The first results were astonishing: The waters around Cape Verde contain large amounts of the recently discovered cyanobacteria UCYN-A, a mysterious fertilizer algae whose properties are still unexplainable for researchers.

By feeding Saharan dust to the alga, the Kiel biogeochemists are now testing whether the presence of UCYN-A is promoted by the abundant Saharan dust there.

Nutrients are falling from the sky

The waters of the tropical Atlantic around Cape Verde - 800 kilometers off the West African coast - have very low levels of plant nutrients. Especially nitrogen is exceptionally scarce and limits the growth of phytoplankton, the tiny plants that form the basis of the food chain in the ocean.

The nutrients fall in this area from the sky: trade winds carry Saharan dust, which is rich in iron and phosphorus, and fertilize the sea surface. This was one of the reasons for the Leibniz Institute for Marine Sciences (IFM-GEOMAR), together with other German and British institutes to build an observatory on the Cape Verdean island of Sao Vicente. The Tenatso Observatory now supports long-term measurements of dust and greenhouse gases, an oceanographic measurement anchorage and regular sampling with the small Cape Verdean research boat Islandia.

Does Saharan dust stimulate growth?

"We're checking to see if Sahara dust can stimulate the growth of a particular microbial species, the cyanobacteria. These bacteria can fertilize the surface layer of the ocean by fixing the dissolved gaseous nitrogen present in seawater, "says Professor Julie LaRoche of IFM-GEOMAR, co-director of the recently completed six-week expedition. There is plenty of gaseous nitrogen in the atmosphere, but it needs to be taken up so that it can be converted into a fertilizer, making it useful for phytoplankton. display

Enigmatic cyanobacteria

The enigmatic cyanobacteria of the species UCYN-A seem to be very special nitrogen fixers. Until recently, scientists thought that cyanobacteria can only fix nitrogen at night because oxygen released during photosynthesis prevents nitrogen fixation by poisoning the responsible enzyme. The cyanobacteria UCYN-A do not seem to work that way. They lack photosystem II genes that are needed to release the oxygen, and they can not convert carbon dioxide into sugars.

Thus, UCYN-A seems to exploit light energy in a different way and bypass the photosynthesis normally associated with land plants and other algae. Although these organisms could not previously be isolated in the laboratory, a first description of their genome was published in 2008 by the group of Jonathan Zehr at the University of California, Santa Cruz.

Difficult working conditions

Trade winds and frequent dust storms make the Cape Verde region so important for marine research, but also make the work of scientists more difficult. Dust samples are taken with filters on the tower of the atmospheric observatory. In order to take water samples, however, one must drive with the Islandia several hours to the measuring point of the sea observatory. It is located 130 kilometers from the land and has a surrounding water depth of 3, 600 meters.

In laboratories set up in the Cape Verdean National Institute for Fisheries Development, the scientists carry out the dust experiments. Now the researchers are laden with numerous data from their experiments and confident that their experiments will provide new discoveries, having returned to Kiel.

(idw - Leibniz Institute for Marine Sciences, 23.06.2009 - DLO)