Steady state growth and ammonium uptake of a fast‐growing marine diatom 1

A fast‐growing marine diatom Thalassiosira pseudonana (3H) was grown under NH 4 + ‐limited steady state conditions in continuous culture. Growth rate could not be described as a function of external or residual NH 4 + because of analytical limitations in measuring NH 4 + concentrations <0.03 µ g‐atom N · liter −1 . Neither did the data fit the internal nutrient model of Droop because the washout growth rate (µ̂) under N limitation is substantially less than the maximum growth rate term used in the Droop expression (µ̄). The ratio of the minimum ( k Q ) to the maximum ( Q m ) cell quota was the key term in determining the ratio µ̂:µ̄, and hence the applicability of the Droop expression. For limiting nutrients such as vitamin B 12 and P, µ̂ ~ µ̄, and the expression is applicable; but, when µ̂ < µ̄, as for N and Si, the usefulness of the expression is diminished, and when µ̄ → ∞, as for inorganic carbon, the Droop equation is completely invalid. In a general sense the usefulness of the Droop expression diminishes as the limiting nutrient:cell weight ratio increases. The generality of the concept of cell “shift up,” or increasing k Q and µ̄ with increasing growth rate, is questioned as no evidence for this phenomenon was found in T. pseudonana. Moreover, the demonstration of discontinuities in kinetic curves of Q vs. µ does not a priori imply that the coefficients k Q and µ̂ are variable.