Ecological Systems and Devices Annotation << Back Growth and Physiological State of the Laboratory Population of Microalgae Scenedesmus Quadricauda in the Presence of Antibiotic Oxytetracycline Maksimova N.Yu., Ipatova V.I., Lazareva A.M. Pharmaceuticals are components of environmental pollution. The effect of the antibiotic oxytetracycline (OTC) in concentrations of 0.01–10 mg/l on the test culture of the microalga Scenedesmus quadricauda was studied in a series of chronic experiments. The toxic effect of OTC depended on its concentration, time of action, and the assessed test parameter. The minimum effective concentration of OTC on culture growth in terms of cell number in the case of growth stimulation was 0.01 mg/l, in the case of growth inhibition – 5 mg/l, and in terms of inhibition of photosynthesis effi ciency – 1 mg/l. Long-term inhibition of culture development at high levels of OTC exposure (5 and 10 mg/l) was accompanied by inhibition of cell division and signifi cant inhibition of photosynthesis effi ciency. The sensitivity of S. quadricauda culture to OTC varies depending on the season of the year. The seasonality factor should be taken into account when carrying out the bioassay procedure on algae. The observed effect of better culture growth upon repeated intoxication may indicate adaptation and increased resistance of the laboratory population of S. quadricauda cells to the antibiotic OTC. Keywords: Scenedesmus quadricauda, bioassay, oxytetracycline, season, adaptation. DOI: 10.25791/esip.5.2025.1522 Pp. 37-48.