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Knowledge of the fractionation of nitrogen isotopes by phytoplankton is a key requirement for the calibration of the new 6"N paleotracer. An essential part of information required in this calibration concerns the magnitude of isotopic fractionation during the incorporation of N substrates by phytoplankton. To this end, the 61TN of batch cultures of Thalassiosira pseudonana grown on nitrate, nitrite, ammonium, and urea was determined. This paper reports the first 61SN study of phytoplankton growth on urea (e.g. organic N substrate). The 6"N of the particulate nitrogen (PN) collected during the logarithmic growth phase, thus for N-sufficient cells, was lower than the 6"N of the source due to kinetic isotope fractionation. With increasing drawdown of the N substrate, the 6"N of the accumulating PN increased in accordance with the Rayleigh distillation model. Enrichment factors (E) derived from a least-squares analysis of the accumulated 615N,, data were 5.2 + 0.2X0, 0.9 ? 0.6%0, 20 2 1%0, and 0.8 t 0.6%~ for NO,-, NOzm, NH,', and urea incorporation, respectively. Overall, E values for nitrate incorporation were consistent with field estimates of -6%0, and could be used to estimate past relative nitrate utilization as estimated from the 615N of bulk sedimentary organic matter.

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 kQ·Qmax−1*, was similar in magnitude to previously reported values but kQ·Qmax−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 (kQi·KQj−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 (Rsat) 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.