Steady‐state Luxury Consumption and the Concept of Optimum Nutrient Ratios: A Study With Phosphate and Nitrate Limited Selenastrum Minutum (Chlorophyta)1

ABSTRACT Selenastrum minutum (Naeg.) Collins was grown over a wide range of growth rates under phosphate or nitrate limitation with non‐limiting nutrients added to great excess. This resulted in saturated luxury consumption. The relationships between growth rate and cell quota for the limiting nutrients were well described by the Droop relationship. The observed variability in N cell quota under N limitation as reflected in k Q ·Q max −1* , was similar in magnitude to previously reported values but k Q ·Q max −1* for P under P limitation was greater than previously reported for other species. These results were evaluated in light of the optimum ratio hypothesis. Our findings support previous work suggesting that the use of a single optimum ratio (k Qi ·K Qj −1 ) is inappropriate for dealing with a species growing under steady‐state nutrient limitation. Under these conditions the optimum ratio should be viewed as a growth rate dependent variable. Two approaches for testing the growth rate dependency of optimum ratios are proposed. The capacity for luxury consumption differed between nutrients and was growth rate dependent. At low growth rates, the coefficient of luxury consumption (R sat ) for P was ca. four times that for N. The set of all possible relationships between N and P cell quota under these conditions was reported and these values were then used to establish the cellular N:P niche boundaries for S. minutum. Cell quotas of non‐limiting nutrients were not described by the Droop equation. Analysis showed that as the cellular N:P ratio deviates from the optimum ratio, the ability of the Droop equation to describe the relationship between growth rate and non‐limiting cell quotas decreases. When non‐limiting nutrient cell quotas are saturated, the Droop equation appears to be invalid. Previously reported patterns of non‐limiting nutrient utilization are summarized in support of this conclusion. The physiological and ecological consequences of luxury consumption and growth rate dependent optimum ratios are considered.