Abstract
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 (68)Ge(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.
Highlights
Models of Michaelis-Menten-type saturable kinetics of nutrient uptake and assimilation in phytoplankton have guided our understanding of how the cell translates nutrient availability into growth (Eppley et al, 1969; Sullivan, 1976, 1977; McCarthy, 1981; Del Amo and Brzezinski, 1999)
Electron spectroscopic imaging indicates intracellular Si is not sequestered in vesicles (Rogerson et al, 1987), and other data suggest that maintenance of supersaturated levels of silicic acid is likely through an association of intracellular Si with as-yet-uncharacterized organic components (Azam et al, 1974; Sullivan, 1979; Hildebrand, 2000; Hildebrand and Wetherbee, 2003)
Si(OH)4 Uptake in Thalassiosira pseudonana Transitioned from Nonsaturable to Saturable Kinetics over Time
Summary
Models of Michaelis-Menten-type saturable kinetics of nutrient uptake and assimilation in phytoplankton have guided our understanding of how the cell translates nutrient availability into growth (Eppley et al, 1969; Sullivan, 1976, 1977; McCarthy, 1981; Del Amo and Brzezinski, 1999). Conclusions regarding uptake kinetics of nutrients have been variable, due in part to differences in the methods applied, making comparison of data difficult Factors contributing to these variations include species-specific or cell size differences, the physiological state of cultures, and the incubation period and nutrient concentration range over which uptake is measured (Sullivan, 1976; Wheeler et al, 1982; Harrison et al, 1989; Collos et al, 1992; Lomas and Glibert, 1999; Leynaert et al, 2004). Electron spectroscopic imaging indicates intracellular Si is not sequestered in vesicles (Rogerson et al, 1987), and other data suggest that maintenance of supersaturated levels of silicic acid is likely through an association of intracellular Si with as-yet-uncharacterized organic components (Azam et al, 1974; Sullivan, 1979; Hildebrand, 2000; Hildebrand and Wetherbee, 2003) The balance between these binding components and free Si(OH) will influence the equilibrium state of Si(OH). Listed are species used in the study, whether cultures were prestarved for Si prior to measuring uptake, the incubation period, Si(OH) concentrations over which uptake was measured, the method, the results, and the reference
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