Ecological Systems and Devices Annotation << Back Variability of the Effect of Hydrogen Peroxide on the Physiological Parameters of Algae and Bacteria in an Enrichment Culture Dallakyan G.A., Akulova A.Yu., Khazanova K.P., Rostanets D.V., Ilyinsky V.V. The photosynthesis effi ciency for the release of oxygen (PEO) of algae in the fi rst weeks of culture growth at low concentrations of hydrogen peroxide is signifi cantly higher than in the control. From the 9th day of growth to 21st days, the PEO decreases. At a concentration of 10–3 M, PEO starts to increase from the 6th day of culture growth and by day 21, PEO is the same for all samples. Exposure to hydrogen peroxide immediately after addition on day 1 resulted in an increase in the total number of bacteria and the number of saprophytic bacteria. Quantitative microbiological indicators then decreased compared to the control. Hydrogen peroxide has an inhibitory effect on the development of Scenedesmus quadricauda at concentrations of 5×10–4 M and 10–3 M. The proportion of living cells responds most rapidly. Algae recovery begins on day 2 and is complete on day 21. At the same time, the proportion of living cells in the control and at a concentration of 10–4 M was 99–100 % throughout the experiment. By day 21, all physiological parameters of the bacteria and algae returned to control levels, indicating the end of exposure to the test substance. It has been shown that PEO is infl uenced by two factors: hydrogen peroxide at low concentrations acts as an additional electron donor for chlorophyll a, thereby increasing PEO, and at high concentrations of hydrogen peroxide there is a sharp increase in the adverse effects of reactive oxygen species on algal cells; the growth of saprotrophic bacteria in the environment is also negatively affected, leading to an increase in their oxygen consumption. Thus, hydrogen peroxide, depending on its concentration in the medium, can be used to both stimulate and suppress the intensity of algae photosynthesis, also for solving practical problems. Keywords: Bacteria, algae, chlorophyll, hydrogen peroxide, oxygen, abundance. Pp. 42-52.