Types of responses of a model population of microalga <Emphasis Type="Italic">Scenedesmus quadricauda</Emphasis> (Turp.) Breb. under different intoxication conditions

The main ways of reaction of a green chlorococcal alga Scenedesmus quadricauda (Turp.) Breb. model population to the toxic action of different chemicals (the fungicide imazalil sulfate and heavy metal Cr⁶⁺ as a potassium bichromate) are analyzed. Regularities of the change in the size-age and functional population characteristics are considered, and four types of universal microalgal response to the toxic action are demonstrated. In the presence of low concentrations (0.001 mg/l), the slowdown of the culture growth was caused by the long stop of cell division in part of the algal population rather than by cell death or a decrease in their growth. The lack of effect (or even culture growth stimulation under medium concentrations (0.1 mg/l)) is related to the cell division proliferation after a temporary stop. After increasing the toxicant concentration up to 1.0 mg/l, we observed a long cell division inhibition; however, the cell death was under high toxicant concentrations of 5.0–10.0 mg/l). A significant inhibition of functional cell parameters (photosynthetic intensity) took place only under high toxicant concentrations (5.0 mg/l and higher). It is shown that the microalgal population state can be estimated on the basis of an analysis of its structural and functional parameters.     

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

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.