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

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 silicon (Si)-replete conditions, Si(OH)₄ uptake was initially nonsaturable but became saturable over time. Cells prestarved for Si for 24 h exhibited immediate saturable kinetics. Data suggest nonsaturability was due to surge uptake when intracellular Si 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 Si transporters. 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 Si transporters 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.