Fossil fuel combustion causes annual emissions of 6 billion tons of carbon in the form of carbon dioxide (CO2) which significantly contributes to the greenhouse effect. Different strategies have been proposed in order to reduce anthropogenic input of CO2 into the atmosphere. Microalgae in the oceans (phytoplankton) take up large amounts of CO2 via photosynthesis and influence the carbon budget of our planet by transferring carbon from the surface to the deep ocean through sinking. This process called the biological pump leads to a long-term withdrawal of carbon from the atmosphere. An enhancement of the biological pump of the ocean through iron fertilization of the phytoplankton has been proposed as away to reduce atmospheric CO2. To test the feasibility of this approach, the Alfred Wegener Institute conducted an European iron fertilization experiment (EIFEX) on board R.V. Polastern in collaboration with several national and international partners.Phytoplankton constitutes the first link of the food chain in the global ocean the largest ecosystem in our planet. Although phytoplankton constitutes only a fraction (0.5 %) of the total plant biomass of the planet, it contributes about 30 % to the annual carbon-uptake via photosynthesis equivalent to approximately 150 billion tons. Consequently high turnover rates occur within the plankton, where growth, mortality and sinking rates are nearly balanced. In three major oceanic regions, the equatorial and subarctic Pacific as well as the Southern Ocean the growth of the phytoplankton is limited low iron concentrations. Previous iron fertilization experiments in these region led to a rapid increase of phytoplankton biomass and a clear decrease in dissolved inorganic carbon concentrations (including CO2) in the water column. However, the fate of these phytoplankton blooms had not been studied yet. The aim of EIFEX was to follow the fate of the iron-induced phytoplankton bloom (growth and decay) and determine the influence of the bloom on the carbon budget of the fertlized patch. During EIFEX we fertilized an area of over 150 km2 with 13 tons of iron sulphate dissolved in seawater. The temporal development of the phytoplankton bloom was followed over a period of more than five weeks. In the final week of the experiment a significant increase in turbidity of the deeper water layers down to the ocean floor could be observed as a result of the dying and sinking phytoplankton bloom.Microscopic analysis of water samples revealed the presence of aggregates composed of phytoplankton from the bloom together with zooplankton faecal material that sank out to deeper water layers. Geochemical measurements at the sediment/water boundary layer as well as the analysis of sediment cores indicated that a five millimeter thick layer of fresh plankton material had sedimented in the deep ocean. A detailed budget of the amount of carbon exported into the deep ocean is in preparation.The EIFEX results indicate that iron fertilization can lead to transport of organic material to the deep ocean. However, the amount of CO2 which could be biologically removed by fertilization of the entire Southern Ocean amounts to only a minor fraction of the annual CO2emissions. The overriding aim for a sustainable energy policy should be efficient energy use in combination with reduced CO2 emissions.