Nutritional Studies of Two Red Algae. Ii. Kinetics of Ammonium and Nitrate Uptake¹,²

Similar NH 4+ and NO 3 − .uptake kinetic patterns were observed in Neoagardhiella baileyi (Harvey ex Kiitzing) Wyinne & Taylor and Gracilaria foliifera (Forssk?l) Borgesen. NO 3 − was taken up in a rate‐sturating fashion described by the Michaelis‐Menten equation. NH 4+ uptake was multicomponent: a saturable component was accompanied by a diffusive or a high K component showing no evidence of saturation (at ≤50 μM [NH 4+ ]). Nitrogen starved plantsi(C/N atom ratios > ca. 10) showed higher transient rates of NH 4+ uptake at a given concentration than plants not N‐Iimited. Only plants with high N content exhibited diel changes inNH 4+ uptake rates, and showed transient rates of NH 4+ accumulation which did not greatly exceed the capacity to incorporate N in steady‐state growth. NH 4+ was preferred over NO 3− even in plants preconditioned on NO 3− as the sole N. source, NO 3− uptake was suppressed at 5μM [NH 4+ ], but simultaneous uptake occurred at unsurpressed rates at lower concentrations. Potential for N accumulation was greater via NH 4+ uptake than via NO 3− uptake. Changing capacity for NH 4+ uptake with N content appears to be a mechanism whereby excessive accumulation of N was avoided by N‐.satiated plants but a large accumulation was possible for N‐depleted plants.