Abstract
The marine chlorophyte Dunaliella tertiolecta was grown in continuous cultures under NH4+-N, NO2−-N, NO3−-N, and urea-N limitations. The effect of the nitrogen cell quota (Qn) on the steady-state growth rate (μ) was the same regardless of the N source. The relationship between μ and Qn was well described by the Droop equation, but only up to the true maximum growth rate ^μ (= cell washout rate). The ratio between the minimum cell quota (kQ) and the maximum cell quota (Qm) was 0.19. Hence, there is no substitute for determining ^μ experimentally. That there was no difference in growth response to different N sources suggests that no internal pooling of inorganic nitrogen occurred. Both the carbon (Qc) and phosphorus (Qp) cell quotas under N limitation increased with increasing μ in a threshold fashion: virtually no change in either cell quota up to ∼0.8 ^μ, followed by a rapid and large increase up to ^μ. In addition, in the region of low μ, there was an increase in Qp with a decreasing medium N/P ratio of between 15 and 5 (by atoms). The results generally indicate the physiological limits in cellular constituency under N limitation. The usefulness of this information, however, in describing the response of natural populations of marine phytoplankton to transient nutrient exposures on the temporal and spatial microscales that most likely exist is of limited value.