Studies on synchronously dividing cultures of Euglena gracilis Klebs (strain Z). 3. Circadian components of cell division.

Abstract

AbstractEuglena gracilis Klebs (Z) was grown axenically and autotrophically in four‐liter serum bottles at 25°C on an aerated, continuously stirred, inorganic salt medium. Four fluorescent illumination regimes were employed: (1) continuous bright light of 3500 lux (LLb); (2) continuous dim light of 800 lux (LLd); (3) a LbD: 14, 10 (3500 lux) light‐dark cycle; and (4) a LdD: 14, 10 (800 lux) light‐dark cycle. Cell number was automatically monitored throughout all experiments.In LLb the generation time (G.T.) of the population was about 12 hours, whereas in LLd following LLb it was approximately five days; exponential growth occurred in either case. In LbD: 14, 10 synchronous growth occurred with a doubling of cell number every cycle of 24 hours. In LdD: 14, 10, however, although rhythmic cell division took place every 24 hours, the average increase in cell number during the division burst which occurred in each dark period was only 13.4%, so that the G.T. of the culture was about five days, as was the case for LLd. In the constant conditions of temperature and continuous dim light (LLd), following synchronous growth in LbD: 14, 10, small (17.0%) rhythmic division bursts lasting 14.5 hours continued to occur for at least ten days, with a period of 24.2 hours. The overall G.T. of the culture was about five days. These data demonstrating the circadian, endogenous nature of rhythmic cell division under certain conditions of continuous dim illumination were discussed in relation to the synchronous division observed in temperature and light‐dark cycles.