| Abstract: | A comparative study of nitrate-limited growth and nitrate uptake was carried out in chemostat cultures of Ankistrodesmus falcatus (Corda) Ralfs., Asterionella formosa Hass., and Fragilaria crotonensis Kit. In each species growth rate (μ) was related to total cell nitrogen or cell quota (q) by the empirical Droop growth function. Nitrate uptake was a function of both external N concentration and q. The apparent maximum uptake rate (Vm') at a given μ was inversely related to q – q0, where q0 is the minimum quota. The apparent half-saturation constant for uptake, (Km') appears to show a slight inverse trend with μ, although statistical analysis shows that this trend is inconclusive. When q approaches q0, Vm' is several orders of magnitude greater than μq, the calculated steady-state uptake rate. As q increases, however, the difference between these two variables decreases sharply until q approaches qm, the cell quota for nitrogen-rich cells. At this point the difference between μq and Vm' disappears. This behavior is explained by the feedback regulation of N uptake. The inverse relationship between Vm' and q – q0 can be described by an empirical three-parameter equation. |
