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Published on December 27, 2007; 10.1104/pp.107.107094

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Received August 8, 2007
Accepted December 20, 2007

Silicon uptake in diatoms revisited: a model for saturable and nonsaturable uptake kinetics and the role of silicon transporters

Kimberlee Thamatrakoln * and Mark Hildebrand

Institute of Marine and Coastal Sciences, Rutgers University, 71 Dudley Road, New Brunswick, NJ 08901 USA; Marine Biology Research Division, Scripps Institution of Oceanography, 9500 Gilman Drive, La Jolla, California 92093-0202 USA

The silicic acid uptake kinetics of diatoms were studied to provide a mechanistic explanation for previous work demonstrating both nonsaturable and Michaelis-Menten type saturable uptake. Using 68Ge(OH)4 as a radiotracer for Si(OH)4, we showed a time-dependent transition from nonsaturable to saturable uptake kinetics in multiple diatom species. In cells grown under Si-replete conditions, Si(OH)4 uptake was initially nonsaturable, but became saturable over time. Cells prestarved for silicon for 24 h exhibited immediate saturable kinetics. Data suggest nonsaturability was due to surge uptake when intracellular silicon pool capacity was high, and saturability occurred when equilibrium was achieved between pool capacity and cell wall silica incorporation. In Thalassiosira pseudonana at low Si(OH)4 concentrations, uptake followed sigmoidal kinetics, indicating regulation by an allosteric mechanism. Competition of Si(OH)4 uptake with Ge(OH)4 suggested uptake at low Si(OH)4 concentrations was mediated by silicon transporters (SITs). At high Si(OH)4, competition experiments and nonsaturability indicated uptake was not carrier-mediated, and occurred by diffusion. Zinc did not appear to be directly involved in Si(OH)4 uptake, in contrast to a previous suggestion. A model for Si(OH)4 uptake in diatoms is presented that proposes two control mechanisms; active transport by SITs at low Si(OH)4, and diffusional transport controlled by the capacity of intracellular pools in relation to cell wall silica incorporation at high Si(OH)4. The model integrates kinetic and equilibrium components of diatom Si(OH)4 uptake, and consistently explains results in this and previous investigations.






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