EFFECT OF THE SINKING RATE OF TWO DIATOMS (THALASSIOSIRA SPP.) ON UPTAKE FROM LOW CONCENTRATIONS OF PHOSPHATE1

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Abstract
SUMMARY: The effect of the sinking rate, or rate of medium flow (φ) on the rate of phosphate incorporation (V) by the planktonic diatoms Thalassiosira fluviatilis Hust. and T. pseudonana Hasle & Heimdal in batch and chemostat cultures was determined by passing medium at defined flow rates (0.5–25.0 mm·min−1) over algae on membrane filters. At concentrations from 1 to 100 μg phosphorus·l−1 V, increases with increasing velocity of flow, approaching a maximum value (Vm) as described by the empirical relationship: image where Kφ is the sinking rate value when V = 1/2 Vm+ Vo and Vo is the uptake at 0 rate of flow. By comparing uptake at controlled flow with uptake in a vigorously stirred medium, the phosphate concentration in the cell boundary layer can be determined. The sinking rate that reduces the phosphate concentration in the boundary layer to half of nominal concentration in the medium is much lower for the larger T. fluviatilis than for T. pseudonana. For both diatoms, it is inversely related to the nominal concentration.