Dynamics of Growth Control in a Marine Yeast Subjected to Perturbation

Abstract

Normalized specific growth rates of cultures of the yeast Rhodotorula rubra apparently indicate the activity of a growth control mechanism, when growth is perturbed with low concentrations of a toxic inhibitor (cadmium). Data from a number of experiments, in which different cadmium concentrations were used, indicate non-linear growth control dynamics and some features of a model structure, but do not permit it to be defined. Interpretation of the oscillatory behaviour as the response of a control mechanism to perturbation suggests that inhibition of mean growth rates, indicated by a threshold on dose-response curves, is due to overloading of the control mechanism or 'saturation'. Hormesis, the tendency of subinhibitory levels of typically toxic agents to stimulate growth, is a consequence of transient or sustained overcorrections by the control mechanism to low levels of an inhibitory stimulus.

Elucidating the Mechanism(s) of Hormesis at the Cellular Level: The Universal Cell Response

“…This is broadly consistent with the aforementioned general scheme of cell life [42] (Fig. 2) and is fundamental to a well-established and widely-discussed explanation of hormesis [10,11,19,43,48,49] . In these and other publications, Stebbing proposed that hormesis is a consequence of non-specific adaptive (homeostatic) effects.…”

“…Intuitively, conditioning hormesis may be pictured as a shift of the biphasic dose-effect curve to the right (cf. [19] ), so that the effect becomes maximal at a higher stressor dose. Because of the difficulty of data interpretation at low doses, this simple representation of the phenomenon does not provide a reliable experimental method for quantifying the effect of conditioning (pre-exposure).…”

“…The techniques described in the references cited above, notably [8,9,18] , are likely to be particularly useful, but they are not alone; in Stebbing's method for extracting growth-control system output, for example, the errors grow with each step so the raw data must be of high quality (errors <5%) [19] . However, statistical issues will not be considered further in this article; it will be assumed that unequivocally biphasic effects to wide ranges of stimuli are commonplace among cells, organisms and populations.…”

“…These difficulties have been discussed in the literature (e.g. [8,9,[17][18][19] ) but they are not always considered when possible instances (or mechanisms) of hormesis are explored. This article therefore begins with a brief survey of the data interpretation problem.…”

Interpreting ‘Dose-Response’ Curves using Homeodynamic Data: With an Improved Explanation for Hormesis

“…This interpretation is confirmed by experimental data showing that at subthreshold concentrations, growth control mechanisms are active in counteracting inhibition (freshwater Hydra littoralis, Stebbing and Pomroy, 1978; marine hydroid Laomedea flexuosa, Stebbing 1981a; marine yeast Rhodotorula rubra, Stebbing et al 1984). That such an equilibrium exists can be seen by the sudden removal of the inhibitor, which briefly reveals the counter-response as a relaxation stimulation (Stebbing, 1981a).…”