BIOTIC INTERACTIONS BETWEEN TWO SPECIES OF MICROALGAE IN THE MIXED CULTURE

Biotic interactions in a mixed culture of two microalgae species — Scenedesmus quadricauda (Turp.) Breb. and Monoraphidium arcuatum (Korsch.) Hind., used in bioassay in monocultures as test objects, were studied. The toxic effect of cell-free filtrates from different “ages” monoculture (2, 7, 10, 15, 21 and 28 days) S. quadricauda on the growth of the “young” test culture M. arcuatum and, conversely, the toxic effect of cell-free filtrates from the different “ages” (2, 7, 10, 15, 21 and 28 days) monoculture M. arcuatum on the growth of the “young” test culture S. quadricauda was evaluated. Simultaneously the toxicity of their own filtrates of different “ages” was monitored by a test culture of each species. The interactions of the species in the mixed culture can be regarded as negative, such as antagonistic, when both populations inhibit the growth of each other through metabolites and food resource competition, while the effect of S. quadricauda on M. arcuatum is much stronger. The main factor constraining the growth of monoculture S. quadricauda is the rapid depletion of the food resource from the medium, and not the inhibition of growth by its own metabolites. The depletion of the food resources from the medium in monoculture M. arcuatum occurs much later than in monoculture S. quadricauda. Metabolites of S. quadricauda cause a strong inhibitory effect on the growth of M. arcuatum, and the metabolites of M. arcuatum—a weak inhibitory effect on the growth of S. quadricauda. The filtrates of the “old” culture of S. quadricauda (21–28 days) cause the greatest inhibitory effect on the process of cell division of M. arcuatum. The filtrates of the “old” culture S. quadricauda (21–28 days) cause the greatest inhibitory effect on the process of cell division of M. arcuatum. A comparative analysis of the cell number dynamics of two species S. quadricauda and M. arcuatum in mono- and two-species algal cultures, as well as experiments with filtrates of these monocultures, showed that the interaction of species can be explained by the food resource competition and allelopathic interaction (exometabolite effect).

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