Cell-cycle effects on the kinetics of silicic acid uptake and resource competition among diatoms

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Abstract
Current models of nutrient utilization by planktonic algae generally assume that cells transport nutrients continuously. This assumption is violated in the case of Si utilization by diatoms. Silicic acid transport is confined to specific portions of the cell cycle such that only a fraction of the cells in a population are likely to be transporting silicic acid at any given time. A theoretical framework for describing the growth and nutrient-uptake kinetics of populations of cells exhibiting cell-cycle-dependent nutrient uptake is presented. Thalassiosira weissflogii was used as a model organism. Kinetic parameters determined at steady state, assuming cells take up Si continuously, underestimated both the Vm and Ks of individual cells by nearly an order of magnitude. Likewise, kinetic constants determined from short-term experiments assuming continuing uptake were shown to be dependent on the initial physiological state of the population and to dramatically underestimate the kinetic parameters of individual cells. The response of steady-state populations to large pulses of dissolved Si was also examined. The ratio of the average maximum specific uptake rate of the culture to the average maximum observable specific growth rate (V′ mavg/ m)increased from unity at a relative growth rate of 1 to over 24 at low relative growth rates. The increase in (V′ mavg/m) with decreasing relative growth rate under nitrogen limitation has been hypothesized to be an adaptation by cells to exploit micropatches of NH3. In contrast, the predicted increase in this ratio for Si-limited cells was due entirely to changes in the distribution of cells within the cell cycle caused by Si stress. These findings, if empirically verified, will require a revision of our views of how diatoms are adapted to low-nutrient environments.

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