Cell cycle dynamics and their application to calculating in situ growth rates in two heterotrophic protozoa : a flow cytometric approach

Whiteley, Andrew Steven (1994) Cell cycle dynamics and their application to calculating in situ growth rates in two heterotrophic protozoa : a flow cytometric approach. University of Southampton, Doctoral Thesis.

Abstract

Cell cycle analysis using flow cytometry has been performed upon two heterotrophic protozoa, Oxyrrhis marina (Dujardin) and Chilomonas paramecium (Ehrenberg) in order to the microzoopankton.

Both species displayed cell cycle kinetics which were independent of photoperiodic regime, but which were dependent upon environmental temperature. The division rate response to lowered temperature was primarily mediated by the influence of low temperatures upon pathways in the G₂ phase of the cell cycle. In contrast, nutrient-dependent control of the cell cycle occurred via a restriction point control of both the G₁ and G₂ transit in exponentially dividing populations. The S phase, once initiated, tended to be completed independently of nutrient supply. The specific dependences of both restriction points in each species have been discussed.

The influence of cell cycle controls upon the rate of cell cycle resumption in arrested populations was studied to provide a framework of cell cycle progression in environments subjected to periodic nutrient supply. Both factors studied; arrest period and nutrient supply, modulated cell cycle kinetics in both species when cell cycling resumed. Further, the results indicated that in both these protozoa, the 'restriction point' control of G₁ and G₂ progression probably comprised of two elements; a deterministic pathway for the initiation of cell cycling and probabilistic pathway concerned with the maintenance of cycling toward DNA synthesis or mitosis.

The basic cell cycle data obtained during exponential growth experiments was then examined in an attempt to calibrate the cell cycle method for calculating the in situ growth rates of the two species of protoza. The method, based upon asynchronously dividing populations, was found not to be feasible for either of the species studied. Principally, this was due to the presence of a G₂ restriction point in both species, which violated the key assumption behind the method.

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