On the Nature of the Three-Phase Response of Scenedesmus quadricauda Populations to the Action of Imazalil Sulfate

Abstract

Three-phase dose responses of biological systems of different levels of organization are often called “paradoxical” because the biological effects are clearly manifested under low- and high-intensity treatments, but are absent during moderate-strength treatments. In this work, we found anomalous changes in the cell number of a green alga Scenedesmus quadricauda (Turp.) Breb. grown in the presence of the fungicide imazalil sulfate. At low imazalil concentrations (2.5 × 10^–9–2.5 × 10^–6 M), the slow increase in the cell number as compared to an untreated culture was not related to cell death. As seen by the dynamics of the population structure and cell functional characteristics (photosynthesis, thermal stability of photosynthetic membranes, etc.), the decrease in the growth rate at low concentrations of imazalil (2–10 × 10^–9 M) was due to a long-term arrest of cell division in a fraction of the cell population rather than to a decrease in the rate of division. The absence of a toxic effect or even a slight stimulation of culture growth at moderate concentrations (0.05–1.25 × 10^–6 M) was due to the resumption of cell division after a temporal cessation. At these concentrations, imazalil induced cell stress and adaptive elevation of cell tolerance to the fungicide (acclimation). Cell death was observed only at a high fungicide content in the medium (6.25 × 10^–6 and higher). Thus, the three-phase (bimodal) dose response corresponds to two regimes (steady-states) of cell functioning which differ in cell sensitivity to external stimuli. The low-sensitivity state, which is characteristic of cells that have experienced stress, is likely to be the state known as “hormesis.”