Abstract:
Microcystis is well known bloom-forming toxic algae indicating eutrophication in freshwater ecosystems. As consequence, a reduction of light penetration through the water column, depletion of oxygen in water, and disruptions of the ecological integrity of lentic freshwater ecosystems have been observed. However, research on the allelopathic effects of Microcystis on other species in phytoplankton communities is still scarce. The objective of the present research was to study the allelopathic effects of Microcystis sp. (blue-green algae) on Scenedesmus sp. (green algae) under laboratory conditions with reference to variations in cell density and morphology. Monoculture series of Microcystis and Scenedesmus were prepared and maintained under controlled light (12 hours day and 12 hours dark) and temperature (26°C) in the laboratory. Two algae cultures (initial cell density: Scenedesmus - 9.34×106 cells mL-1, Microcystis - 12.66×106 cells mL-1) were mixed under different ratios of Scenedesmus: Microcystis as 1:0, 3:1, 1:1 and 1:3. Microcystis was added to Scenedesmus cultures in two different forms: live cell cultures and extract of Microcystis. Cell density and morphological variations were observed to determine the allelopathic interactions. A reduction of Scenedesmus cell density was observed when increasing the cell density of Microcystis in the live mixed cultures for 10 days. Similarly, a reduction of cell density of Scenedesmus was observed when using the cell extract of Microcystis in different volume ratios. The Scenedesmus colonies with regular four cells were changed to irregular shape colonies with 32-50 cells when increasing the live Microcystis cell density in cultures. The ability to perform allelopathic effects on other prominent phytoplankton species in lentic freshwater ecosystems, such as green alga, may lead to preserving the superiority of Microcystis blooms. Understanding such species interactions and underlying mechanisms related to algal blooms are necessary to manage the ecological conditions of freshwater ecosystems under continuous anthropogenic pressure.
