Effects of nitrogen limitation on growth and nitrite excretion rates of the dinoflagellate Prorocentrum minimum

Abstract

Continuous nitrate-limited cultures of the dinoflagellate Prorocentrum minimum were grown under saturating photon flux densities to study the effects of nitrate pulses on the time variations of nitrate uptake, nitrite excretion, and cell division rate. In the first experiment, 5 chemostats were stabilized at the same dilution rate and, after stopping of the renewal supplies, received successively 1 pulse of nitrate at 24 h intervals. In the second, nitrate pulses were added about every 12 h in 1 chemostat. In the third experiment, 4 chemostats stabilized at different growth rates received 1 pulse of nitrate. Nitrate uptake process showed decreasing initial rates and lower maximum rates in cultures subjected to longer starvation times. In all cases, the amount of nitrite excreted before reabsorption represented an important proportion of the initially supplied nitrate (up to 45 %). This suggested that for nitrogen-deprived cells of L? minimum reduction of nitrite by the nitrite reductase is the more limiting step in the nitrateassimilatory pathway. The proportion of pulsed nitrate which is excreted as nitrite increased for decreasing growth rates. For 1 and 2 d of nitrate deprivation, the ratio nitrite excretion ratehitrate uptake rate integrated during each perturbation experiment increased, but decreased after longer times of starvation. This suggests that the processes of nitrate uptake and nitrite reduction are affected at different rates during nitrogen deprivation. The implications of nitrite excretion in N-limited cells on the determination of new production are discussed.