EFFECTS OF NITROGEN LIMITATION ON GROWTH AND NITRITE EXCRETION RATES OF THE DINOFLAGELLATE PROROCENTRUM-MINIMUM

Continuous nitrate-limited cultures of the dinoflagellate Prorocentrum minimum were grown under saturating photon flux densities to study th e 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 stoppi ng of the renewal supplies, received successively 1 pulse of nitrate a t 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 cu ltures subjected to longer starvation times. In all cases, the amount of nitrite excreted before reabsorption represented an important propo rtion of the initially supplied nitrate (up to 45 %). This suggested t hat for nitrogen-deprived cells of P. minimum reduction of nitrite by the nitrite reductase is the more limiting step in the nitrate assimil atory pathway. The proportion of pulsed nitrate which is excreted as n itrite increased for decreasing growth rates. For 1 and 2 d of nitrate deprivation, the ratio nitrite excretion rate/nitrate uptake rate int egrated during each perturbation experiment increased, but decreased a fter longer times of starvation. This suggests that the processes of n itrate uptake and nitrite reduction are affected at different rates du ring nitrogen deprivation. The implications of nitrite excretion in N- limited cells on the determination of new production are discussed.