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Biogeosciences Discuss., 6, 1677-1712, 2009
www.biogeosciences-discuss.net/6/1677/2009/ doi:10.5194/bgd-6-1677-2009 © Author(s) 2009. This work is distributed under the Creative Commons Attribution 3.0 License. |
Bioavailability of organically bound Fe to model phytoplankton of the Southern Ocean
1Centre for Australian Weather and Climate Research (CAWCR), a partnership between CSIRO and the Bureau of Meteorology, Castray Esplanade, Hobart, TAS, Australia
2Ecologie des Systèmes Aquatiques, Université Libre de Bruxelles, Campus de la Plaine, CP 221, Boulevard du Triomphe, 1050 Bruxelles, Belgium
Abstract. 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 keystone phytoplankton species of the Southern Ocean (Phaeocystis sp., Chaetoceros sp., Fragilariopsis kerguelensis and Thalassiosira antarctica Comber) to measure the influence of various organic ligands on Fe solubility and bioavailability. Based on the Fe:C ratios, the strength of Fe limitation was inversely related to the surface area to volume ratio, with the chain forming Fragilariopsis being the most Fe-limited strain. 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 of organic ligands on Fe bioavailability cannot be solely explained by their effect on Fe solubility. For all strains studied, the bioavailability of Fe can be enhanced in presence of porphyrin, cathecolate and saccharides whereas it was decreased in presence of hydroxamate and organic amine. 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.
Citation: Hassler, C. S. and Schoemann, V.: Bioavailability of organically bound Fe to model phytoplankton of the Southern Ocean, Biogeosciences Discuss., 6, 1677-1712, doi:10.5194/bgd-6-1677-2009, 2009.