Timing of events in overlapping cell reproductive sequences and their mutual interactions in the alga Scenedesmus quadricauda

Synchronous populations of the chlorococcal alga Scenedesmus quadricauda were subjected to various irradiances, photoperiods, heterotrophic growth and chloramphenicol treatment in order to induce different cell cycle patterns. The courses of cellular and nuclear division, and of commitments to them, were followed to measure duration of growth and reproductive steps and to analyze their mutual interactions in overlapped cell reproductive sequences. Duration of the growth steps (precommitment periods) was inversely proportional to the irradiance applied. It was prolonged if the growth was retarded by chloramphenicol. It is argued that light has an exclusively trophic function in regulating growth steps and can be replaced by glucose. The number of growth steps, and consequently the number of reproductive sequences, at a given irradiance can be experimentally changed by the photoperiod. The reproductive steps (postcommitment periods) are triggered by the termination of corresponding growth steps. They do not require any supply of external energy, and can therefore be terminated in darkness or in the presence of chloramphenicol. By varying the number of growth steps and their duration, a different extent of overlapping consecutive reproductive steps can be obtained. This overlapping is the major factor affecting the duration of reproductive steps which are proportional to the extent of overlapping. Complex interactions between growth conditions and overlapped reproductive steps resulted in substantial variations in the duration of the whole cell cycle. The present findings do not support the operation of a timing mechanism such as a pacemaker (Zeitgeber) or circadian rhythms, either in regulating the duration of the cell cycle or in the duration of reproductive steps.