Diel Periodicity of Photosynthesis and Cell Division Compared in Thalassiosira Weissflogii (Bacillariophyceae)1

Diel patterns of photosynthesis and cell division were examined in Thalassiosira weissflogii Grun. (clone Actin) grown in nitrogen‐limited cyclostat culture. Ammonia (NH4+) was either supplied continuously or as a daily pulse to cultures grown in constant light or in a light: dark cycle. When either nitrogen or light was supplied periodically, both cell division and photosynthetic capacity were periodic. When both nitrogen and light were supplied periodically, cell division was coupled to the N‐pulse whereas periodicities of photosynthetic capacity were modified but remained coupled to the light‐dark cycle. Diel oscillations in photosynthesis were i) largely independent of cellular pigmentation and ii) similar for light‐limiting and saturating irradiances. Periodicity in photosynthetic capacity also persisted following transfer of non‐dividing batch cultures to constant light. Results suggest that photosynthesis but not cell division was coupled to a circadian clock in T. weissflogii. A circadian rhythm of photosynthesis may optimize carbon assimilation in phytoplankton exposed to intermittent nutrient supply by ensuring that maximum photosynthetic capacity occurs during the day.