Christel S. Hassler et al., PNAS, Saccharides enhance iron bioavailability to Southern Ocean phytoplankton

Proceedings of the National Academy of Sciences, published online before print December 15, 2010; doi: 10.1073/pnas.1010963108

Saccharides enhance iron bioavailability to Southern Ocean phytoplankton

1. Christel S. Hassler^a,^b,^c,*, 
2. Véronique Schoemann^d,^e,
3. Carol Mancuso Nichols^b,^f, 
4. Edward C. V. Butler^a,^b, and 
5. Philip W. Boyd^g

+Author Affiliations

1. ^aCentre for Australian Weather and Climate Research, a partnership between Commonwealth Scientific and Industrial Research Organisation (CSIRO) and the Bureau of Meteorology, Hobart, Tasmania 7000, Australia;
2. ^bWealth from Ocean Research Flagship, CSIRO, Clayton, South Victoria 3169, Australia;
3. ^cPlant Functional Biology and Climate Change Cluster, University of Technology Sydney, Broadway, New South Wales 2007, Australia;
4. ^dEcologie des Systèmes Aquatiques, Université Libre de Bruxelles 1050, Bruxelles, Belgium;
5. ^eRoyal Netherlands Institute for Sea Research, 1797 SZ 't Horntje (Texel), The Netherlands;
6. ^fMaterial Science and Engineering, CSIRO, Hobart, Tasmania 7000, Australia; and
7. ^gNational Institute of Water and Atmospheric Research Center for Chemical and Physical Oceanography, Department of Chemistry, University of Otago, Dunedin 9012, New Zealand

1. Edited by David M. Karl, University of Hawaii, Honolulu, HI, and approved November 19, 2010 (received for review July 26, 2010)

Abstract

Iron limits primary productivity in vast regions of the ocean. Given that marine phytoplankton contribute up to 40% of global biological carbon fixation, it is important to understand what parameters control the availability of iron (iron bioavailability) to these organisms. Most studies on iron bioavailability have focused on the role of siderophores; however, eukaryotic phytoplankton do not produce or release siderophores. Here, we report on the pivotal role of saccharides—which may act like an organic ligand—in enhancing iron bioavailability to a Southern Ocean cultured diatom, a prymnesiophyte, as well as to natural populations of eukaryotic phytoplankton. Addition of a monosaccharide (>2 nM of glucuronic acid, GLU) to natural planktonic assemblages from both the polar front and subantarctic zones resulted in an increase in iron bioavailability for eukaryotic phytoplankton, relative to bacterioplankton. The enhanced iron bioavailability observed for several groups of eukaryotic phytoplankton (i.e., cultured and natural populations) using three saccharides, suggests it is a common phenomenon. Increased iron bioavailability resulted from the combination of saccharides forming highly bioavailable organic associations with iron and increasing iron solubility, mainly as colloidal iron. As saccharides are ubiquitous, present at nanomolar to micromolar concentrations, and produced by biota in surface waters, they also satisfy the prerequisites to be important constituents of the poorly defined “ligand soup,” known to weakly bind iron. Our findings point to an additional type of organic ligand, controlling iron bioavailability to eukaryotic phytoplankton—a key unknown in iron biogeochemistry.

• *Correspondence e-mail: Christel.hassler@uts.edu.au.
• Link:  http://www.pnas.org/content/early/2010/12/14/1010963108.abstract