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-replenis hment did not differ for the three types of nitrogen (nitrate, ammoniu m and nitrite) added, but death rate responses did. Resupply of 200 mu M nitrate to cultures preconditioned at a steady-state growth rate (m u) of 0.53 d(-1) resulted in enhanced algal cell lysis, with average m aximum specific cell lysis rates (d(b)) of 0.24 +/- 0.07 d(-1) compare d to d(b) < 0.06 d(-1) for the control populations. Enhanced cell lysi s was also found upon addition with nitrite, but not with ammonium. Al gal 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 cel l lysis strongly, whereas subsaturating irradiance levels reduced alga l mortality. Accumulation of nitrite intracellularly indicated that th e reduction of nitrite to ammonium was the rate-limiting step in nitra te assimilation. It is argued that the hypothetical mechanism behind t he enhanced death responses concerns excretion of intracellular nitrit e by active transport and deregulation of the protonmotive across the algal cell membranes, but this remains to be studied. (C) 1998 Elsevie r Science B.V. All rights reserved.