AUTOLYSIS KINETICS OF THE MARINE DIATOM DITYLUM-BRIGHTWELLII (BACILLARIOPHYCEAE) UNDER NITROGEN AND PHOSPHORUS LIMITATION AND STARVATION

Autolysis kinetics in axenic cultures of the diatom Ditylum brightwell ii (West) Grunow were studied under nutrient limitation in continuous cultures and under nutrient starvation in batch-mode cultures obtained by switching off nutrient supply in the continuous cultures. Under N limitation, the specific algal autolysis rates (delta, day(-1)) were f ound constant at 0.014 +/- 0.002 day(-1) over a broad range of specifi c dilution rates (D, day(-1)) (0.09-0.56 day(-1)), implying an intrins ic death factor independent of the physiologic state of the algal cell s. Under P limitation, delta was inversely related to D and ranged bet ween 0.067 and 0.005 day(-1) at D = 0.17-0.44 day(-1). Under condition s of nutrient starvation, the degree of algal nutrient deficiency prio r to starvation affected autolysis rates (delta(b), day(-1)) and subse quently survival of the algal cultures. Nitrogen-starved D. brightwell ii showed highest delta(b) (maximum, 0.10 day(-1)) when precultured at the higher growth rates. Phosphorus starvation led to highest delta(b ) (maximum, 0.21 day(-1)) in the cultures preconditioned at the lower steady state growth rates. The lower death rates for D. brightwellii u nder limitation and starvation of N compared to P suggest that D. brig htwellii was better equipped to handle N than P deficiency. The presen t results showed that cell lysis induced by nutrient stress was a sign ificant cause of mortality in. D. brightwellii and provided more insig ht into the field distribution of this neritic diatom.