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 brightwellii (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 found constant at 0.014 +/- 0.002 day(-1) over a broad range of specific dilution rates (D, day(-1)) (0.09-0.56 day(-1)), implying an intrinsic death factor independent of the physiologic state of the algal cells. Under P limitation, delta was inversely related to D and ranged between 0.067 and 0.005 day(-1) at D = 0.17-0.44 day(-1). Under conditions of nutrient starvation, the degree of algal nutrient deficiency prior to starvation affected autolysis rates (delta(b), day(-1)) and subsequently survival of the algal cultures. Nitrogen-starved D. brightwellii 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 under limitation and starvation of N compared to P suggest that D. brightwellii was better equipped to handle N than P deficiency. The present results showed that cell lysis induced by nutrient stress was a significant cause of mortality in. D. brightwellii and provided more insight into the field distribution of this neritic diatom.