PHOSPHATE UPTAKE KINETICS IN EUGLENA GRACILIS (Z) (EUGLENOPHYCEAE) GROWN IN LIGHT/DARK CYCLES. II. PHASED PO4-LIMITED CULTURES1

The characteristics of phosphate uptake and photosynthetic capacity were studied in P-limited populations of Euglena gracilis Klebs (Z), using both P-limited batch cultures in stationary phase and cyclostat cultures grown on 14:10 LD. P uptake obeyed Michaelis-Menten kinetics between 0 and 150 μM PO₄ under both growth conditions. The value of V_max was 35% lower in the dark than in the light in the stationary phase cells. The value of K₈ was not affected by light conditions, and uptake was completely inhibited in the presence of 1 mm KCN. P uptake (at 2.0 μM PO₄) and photosynthetic capacity showed diel periodicity with peak rates occurring just before the beginning of the dark period for P uptake, and 8 h into the light period for photosynthetic capacity. V_max for P uptake increased by a factor of 1.5 over the light period, whereas K₈ remained constant at 1.4 μM PO₄. These patterns were displayed by both nondividing stationary phase cells and populations in which less than a third of the cells divided each day, indicating that the rhythmicity is not coupled to cell division.