Can acidic polysaccharides exuded by diatoms function as iron ligands, increasing the iron availability for diatoms?

• Applicant: Professorin Dr. Uta Desy Elisabeth Passow
• Subject Area: Plant Physiology
• Term: from 2003 to 2008
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 5415949

Of all trace elements, iron has the most profound impact on the biological pump. In many areas of the ocean (HNLC-areas) primary productivity can be limited by iron. In oxygenated waters the dominant redox state for iron is the poorly soluble Fe(III). Organic complexation of Fe(III) by natural ligands increases the overall solubility for iron. The majority of these ligands are not well characterized and their origin and production remain elusive. Diatoms exude acidic polysaccharides abundantly and these acidic polysaccharides, both in their colloidal and paricultate (called TEP) form, may represent an important fraction of the uncharacterized Fe-ligands in the ocean. We propose to investigate the following 4 hypotheses experimentally and in the field: 1. Biologically significant amounts of iron bind to acidic polysaccharides. 2. Diatoms exude acidic polysaccharides in response to iron limitation. The release of these substances is ongoing but may be enhanced by iron limitation (senescence) or by the sudden input of iron relieving cells of iron limitation (leaky cells). 3. The carbohydrate production of diatoms underlies a diel cycle, which influences iron speciation. 4. Iron bound to acidic polysaccharides profits diatoms, either because this iron is directly bioavailable for diatoms, or because binding of iron to these substances, prevents it from precipitation, keeping it in the surface layer, where it is released slowly in a bioavailable form (e.g. after photo-reduction). Understanding the cycling of iron in the ocean is a prerequisite for assessing the long-term effects of iron fertilization for the ocean and for the atmospheric CO2 concentration.

• DFG Programme: Research Grants