Chlamydomonas reinhardtii: duration of its cell cycle and phases at growth rates affected by light intensity

Abstract

In the cultures of the alga Chlamydomonas reinhardtii, division rhythms of any length from 12 to 75 h were found at a range of different growth rates that were set by the intensity of light as the sole source of energy. The responses to light intensity differed in terms of altered duration of the phase from the beginning of the cell cycle to the commitment to divide, and of the phase after commitment to cell division. The duration of the pre-commitment phase was determined by the time required to attain critical cell size and sufficient energy reserves (starch), and thus was inversely proportional to growth rate. If growth was stopped by interposing a period of darkness, the pre-commitment phase was prolonged corresponding to the duration of the dark interval. The duration of the post-commitment phase, during which the processes leading to cell division occurred, was constant and independent of growth rate (light intensity) in the cells of the same division number, or prolonged with increasing division number. It appeared that different regulatory mechanisms operated through these two phases, both of which were inconsistent with gating of cell division at any constant time interval. No evidence was found to support any hypothetical timer, suggested to be triggered at the time of daughter cell release.

Chlamydomonas reinhardtii: duration of its cell cycle and phases at growth rates affected by temperature

“…They can be entrained to external light-dark cycles with a periodicity close to 24 h, and the period is independent of temperature within the physiological range. The cell cycle of C. reinhardtii, as shown in the present Wndings and those described in Vítová et al (2011), has none of these properties. Progression through the cell cycle is driven through several checkpoints, and attaining each commitment point is the main controlling factor.…”

“…At 15°C, the cell cycle lasted about 36 h, the growth rate was about 0.057 doubling h ¡1 and the cells divided into four daughter cells as a Table 1 Duration of the cell cycle and its phases and number of daughter cells per mother cell in synchronized populations of Chlamydomonas reinhardtii grown at diVerent growth rates, using diVerent temperatures, mean light intensities, illumination regimes and methods of culturing T culture temperature in °C, I mean light intensity in mol m ¡2 s ¡1 , Light regime: LL continuous illumination, LD alternation of light and dark periods (numerals before L and D indicate number of hours spent in light and dark, respectively, growth rate in doubling h ¡1 , DC average number of daughter cells per mother cell, PreCP pre-commitment phase (interval from the beginning of the cell cycle to the midpoint of the Wrst commitment curve) in hours, PostCP post-commitment phase (interval from the midpoint of commitment curve to daughter cell release) in hours, CC duration of the cell cycle (from the beginning of the cell cycle to the midpoint of daughter cells release) in hours a Data taken from Vítová et al (2011) Table 2 Temperature coeYcients (Q 10 ) and the ratios of the lengths of the cell cycles for a given 10°C increase in temperature in synchronized populations of C. reinhardtii grown at diVerent mean light intensities °C limit of temperatures diVering by 10°C, Q 10 ratio of growth rates ( ) at temperatures (T) diVering by 10°C (T + 10, T respectively), CC 10 ratio of length of the cell cycles at temperatures diVering by 10°C, I mean light intensity in mol m ¡2 s ¡1…”

“…The cultures were aerated by an air mixture containing 2% of CO 2 . For a detailed description of the culture unit and the composition of the mineral nutrient solution, see Vítová et al (2011).…”

“…While the eVect of light intensity on the duration of the cell cycle and its phases has been examined extensively in autotrophic algae (Wanka 1959(Wanka , 1968Pirson et al 1963;Ketlík et al 1972;Zachleder et al 1975Zachleder et al , 2002Zachleder and Ketlík 1982;Zachleder and van den Ende 1992;Vítová and Zachleder 2005), studies on the eVects of temperature are less intensive (Morimura 1959;Ketlík et al 1975;Donnan and John 1984;Donnan et al 1985) and data interpretations are controversial (McAteer et al 1985;Zachleder and van den Ende 1992;Goto and Johnson 1995;Vítová et al 2011).…”

“…Within circadian systems, temperature-compensated clocks exist to ensure the same length of cell cycle over a range of temperatures such that the Q 10 values should always be about 1 (usually varying from 0.8 to 1.4) (Sweeney and Hastings 1960;Goto and Johnson 1995). Recently, circadian rhythms and endogenous timers were found not to play a role in regulation of the cell cycle of C. reinhardtii (Vítová et al 2011). These studies, however, were carried out at only one temperature (30°C) and diVerent eVects may occur with varying temperatures.…”