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

Abstract

A fast‐growing marine diatom Thalassiosira pseudonana (3H) was grown under NH4+‐limited steady state conditions in continuous culture. Growth rate could not be described as a function of external or residual NH4+ because of analytical limitations in measuring NH4+ concentrations <0.03 µg‐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 (kQ) to the maximum (Qm) 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 B12 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 kQ 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 kQ and µ̂ are variable.