Effects of N:P atomic ratios and nitrate limitation on algal growth, cell composition, and nitrate uptake 1

Scenedesmus sp. was grown in chemostats at a fixed growth rate ( µ ) in an inorganic medium with nitrogen to phosphorus atomic ratios (N:P) ranging from 5 to 80, to investigate the effect of double nutrient limitation. There was no additive or multiplicative effect of the two nutrient limitations: below the optimal cell N:P of 30, growth was determined solely by N limitation and above 30, by P limitation. Cell N remained constant up to the optimal ratio and increased linearly with N:P above it. The level of cell P was high at low N:P (N‐limited state) but decreased rapidly until N:P approached the optimal and remained constant at a low level at high N:P (P‐limited state). Protein was the major fraction in which excess N accumulated under P limitation. Cell free amino acids were a constant proportion of cell N at all N:P ratios. RNA concentration was the same regardless of N:P, its level being determined by µ independent of the type of limiting nutrients. Cell carbon (C) concentration was higher in the P‐limited than in the N‐limited state. The C fixation rate per unit chlorophyll a , however, was constant under both P‐ and N‐limited states because the variation in chlorophyll a content was similar to that of C. The apparent maximum uptake rate for nitrate ( V ) in N‐ and P‐limited cultures decreased with increasing cell N or N:P. In N‐limited cultures the half‐saturation constant ( K m ) also decreased at higher cell N or N:P. The variation of V appeared to be affected by the level of free amino acids.