Biogeosciences www.biogeosciences.net 1726-4170 1726-4189 6 10 2009 10.5194/bg-6-2281-2009 http://www.biogeosciences.net/6/2281/2009/ http://www.biogeosciences.net/6/2281/2009/bg-6-2281-2009.html http://www.biogeosciences.net/6/2281/2009/bg-6-2281-2009.pdf 2281 2296 2009-10-30 Bioavailability of organically bound Fe to model phytoplankton of the Southern Ocean C. S. Hassler V. Schoemann Centre for Australian Weather and Climate Research (CAWCR), a partnership between CSIRO and the Bureau of Meteorology, Castray Esplanade, Hobart, 7000, TAS, Australia Ecologie des Systèmes Aquatiques, Université Libre de Bruxelles, Campus de la Plaine, CP 221, Boulevard du Triomphe, 1050 Bruxelles, Belgium now at: Plant Functional Biology and Climate Change Cluster, University of Technology, Sydney, P.O. Box 123 Broadway, 2007, NSW, Australia Iron (Fe) is known to be mostly bound to organic ligands and to limit primary productivity in the Southern Ocean. It is thus important to investigate the bioavailability of organically bound Fe. In this study, we used four phytoplankton species of the Southern Ocean (<i>Phaeocystis</i> sp., <i>Chaetoceros</i> sp., <i>Fragilariopsis kerguelensis</i> and <i>Thalassiosira antarctica Comber</i>) to measure the influence of various organic ligands on Fe solubility and bioavailability. Short-term uptake Fe:C ratios were inversely related to the surface area to volume ratios of the phytoplankton. The ratio of extracellular to intracellular Fe is used to discuss the relative importance of diffusive supply and uptake to control Fe bioavailability. The effect of excess organic ligands on Fe bioavailability cannot be solely explained by their effect on Fe solubility. For most strains studied, the bioavailability of Fe can be enhanced relative to inorganic Fe in the presence of porphyrin, catecholate siderophore and saccharides whereas it was decreased in presence of hydroxamate siderophore and organic amine. For <i>Thalassiosira</i>, iron bioavailability was not affected by the presence of porphyrin, catecholate siderophore and saccharides. The enhancement of Fe bioavailability in presence of saccharides is presented as the result from both the formation of bioavailable (or chemically labile) organic form of Fe and the stabilisation of Fe within the dissolved phase. Given the ubiquitous presence of saccharides in the ocean, these compounds might represent an important factor to control the basal level of soluble and bioavailable Fe. Results show that the use of model phytoplankton is promising to improve mechanistic understanding of Fe bioavailability and primary productivity in HNLC regions of the ocean. % vor jede Referenz Abraham, E. 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