Recovery of nitrogen-starved cultures of the diatom Ditylum brightwellii (Bacillariophyceae) upon nitrogen resupply

N-limited continuous cultures of the diatom Ditylum brightwellii were starved for nitrogen to study the effects of nitrogen pulses on algal growth and cell lysis kinetics. Growth rate responses after N-replenishment did not differ for the three types of nitrogen (nitrate, ammonium and nitrite) added, but death rate responses did. Resupply of 200 mu M nitrate to cultures preconditioned at a steady-state growth rate (mu) of 0.53 d(-1) resulted in enhanced algal cell lysis, with average maximum specific cell lysis rates (d(b)) of 0.24 +/- 0.07 d(-1) compared to d(b) < 0.06 d(-1) for the control populations. Enhanced cell lysis was also found upon addition with nitrite, but not with ammonium. Algal mortality rates reduced with decreasing concentrations of nitrate additions, and with decreasing starvation time that the cultures were subjected to. Populations of D. brightwellii precultured at 0.30 d(-1) did not show accelerated death responses to nitrate addition. The pH in the cultures was generally > 8.5. Cultures preconditioned at 0.53 d(-1) and pulsed with nitrate gave no enhanced d(b) when buffered at pH 8 +/- 0.1 with 20 mM HEPES. A light/dark cycle appeared essential to obtain accelerated a]gal cell death. Increased irradiance enhanced cell lysis strongly, whereas subsaturating irradiance levels reduced algal mortality. Accumulation of nitrite intracellularly indicated that the reduction of nitrite to ammonium was the rate-limiting step in nitrate assimilation. It is argued that the hypothetical mechanism behind the enhanced death responses concerns excretion of intracellular nitrite by active transport and deregulation of the protonmotive across the algal cell membranes, but this remains to be studied. (C) 1998 Elsevier Science B.V. All rights reserved.