Physiology of an Obligate Photoautotroph (Chlamydomonas moewusii) I. Characteristics of Synchronously and Randomly Reproducing Cells and an Hypothesis to Explain Their Population Curves*

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Abstract
SYNOPSIS. Cultures of Chlamydomonas moewusii begun with log‐phase cells of any age had the same lag phases and showed slight but demonstrable synchronous division. Those begun with stationary‐phase cells had longer lag phases and showed pronounced synchronously reproductive tendencies. Cultures grown under favorable or unfavorable conditions, and their log‐phase cells compared, behaved like log‐ and stationary‐phase cells respectively. Both stationary phase and unfavorably grown cultures shared some of the characteristics of cultures synchronized by light‐dark cycling. A light‐dark cycled population divides synchronously in direct response to the cycle, and not as a result of being transferred from one medium to another. Synchronous division will occur as 8‐fold increases with initial cell densities as low as 10 cells/ml and as high as 100,000 cells/ml. Initial concentrations above 100,000/ml will result in synchronous bursts of less than 8 times the cell number. Cycled cells will continue to divide synchronously when exposed to continuous illumination for an additional 24 hr (i.e., 1 entire “cycle” period). After 12 more hr, they divide exponentially. Continuously illuminated young log cells when subjected to a light‐dark regimen begin growing and dividing synchronously by the end of the 1st 12‐hr dark period (i.e., during the 2nd 24‐hr cycle). The synchronized cell cycle is described cytologkally and quantitatively, and the degree of synchrony for each of the phases of the cell cycle analyzed. After comparing the data from the synchronously and randomly growing cultures, an hypothesis is suggested to explain exponential and synchronous growth and division.