Biologically-mediated increase of
bioavailable Fe in the ocean

Maria T. Maldonado-Pareja, Ph.D.
University of California, Santa Cruz

Friday, June 11, 1999
12:00 Noon—Pacific Forum

Thalassiosira oceanica, a marine centric diatom, possesses an extracellular reductase that reduces iron (Fe(III)) bound to organic complexes as part of a high-affinity Fe transport mechanism. A number of Fe(III) organic complexes are reduced, including siderophores —effective Fe chelates produced by microorganims in response to Fe stress-. Under Fe-limiting conditions, reduction rates increase and the ability of T. oceanica to transport Fe from siderophores is enhanced. Iron bound to the siderophore desferrioxamine B (DFB) is reduced two times faster than it is taken up, suggesting that the reductase is well coupled to the Fe transporter, and can provide all the inorganic Fe to account for the measured Fe uptake rates in the presence of excess DFB. The efficacy of the reductase in providing inorganic Fe for uptake and growth is ultimately dependent on the relative concentrations of excess ligands in solution and cell surface Fe transporters competing for inorganic Fe. The rates of Fe reduction and uptake are twice as fast in cells grown in NO3- compared to those grown in NH4+, suggesting a link with cellular N metabolism and with NO3- utilization in particular. Experiments in the subarctic Pacific, an Fe-limited oceanic region, demonstrated that even indigenous plankton have the ability to acquire Fe bound to strong organic chelates. Because the predominant form of dissolved Fe in the sea is bound to strong organic complexes, a reductive mechanism as described here may be a critical step in Fe acquisition by phytoplankton.

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