Cell cycle events in the green alga Chlamydomonas eugametos and their control by environmental factors

Abstract

ABSTRACT A procedure for routine synchronization of large amounts of the unicellular green alga Chlamydomonas eugametos in liquid culture by alternating light and dark periods is described. The synchronized populations were grown at various light intensities and temperatures. The effect of these variables on the lengths of parts of the cell cycle and the number of daughter cells per cell division was followed. The cell cycle of C. eugametos started with a period in which the cells increased in size only (precommitment period). The length of this period was dependent on both the light intensity and the temperature. At the end of this period, a key point of the cell cycle (called commitment point) was attained. From this point, the cell were committed to divide and cell reproduction was triggered. The following period (postcommitment period), during which daughter cells were formed, could be traversed without supply of external energy, and without further growth of the cells. However, if sufficient energy was supplied during this period, the cells were able to attain more commitment points, leading to a higher number of daughter cells. The postcommitment period was fairly constant within a certain range of light intensity. At light intensities leading to more commitment points, however, this period was prolonged. No evidence was found for circadian rhythms or endogenous factors of “Zeitgeber” type playing a role in the control of growth and reproductive sequences in the cell cycle of C. eugametos.