Algae are gaining attention for their application in aquaculture as a highly sustainable source of useful products. As microalgae have a significant role in primary production in aquatic ecosystems and are the basis of many food chains, it is important to understand the processes that provide them with better survival in a toxicant-polluted environment. In this study the Chlorella bioassay was evaluated: (1) as a potential method for algae farming, (2) as a method for testing advantages or disadvantages of symbiotic association, including two species of aposymbiotic algae, i.e. endosymbiotic algae isolated from green hydra Mychonastes homosphaera (Skuja) Kalina and Punčochářová and Desmodesmus subspicatus (Chodat) Hegewald and Schmidt) and two related free-living algal species (Chlorella kessleri Fott and Novak. [K&H, 1992] and C. vulgaris Beij. [K&H, 1992]), (3) through algal bioindicator responses related to comparative toxicity and ecotoxicological pollution of iron, and (4) by using algal bioindicators for microscopical and morphometrical application in environmental stress. Increasing concentrations of iron led to cell changes (dry dotted clusters of dying cells, intensive green wet bubbles representing a mucous structure, area, diameter and length), deformations (empty cells, aberrant divisions, irregular coenobia, tetrads and transitional forms) and ultrastructural changes (chloroplasts and nuclei). All modifications were more pronounced in aposymbiotic algae, suggesting a lower degree of adaptation to iron toxicity than their free-living relatives. A free-living species C. kessleri showed the best ability to survive in given unfavorable environmental conditions. High statistical significance was noticed in the cell division parameter, underlining the hormetic effect of increasing the biomass in free-living algal species. This increasing of the cell divisions at the specific concentration of iron demonstrated that the Chlorella bioassay may represent a useful tool for evaluating the growth of different microalgal species, and has a prospective application in a comparative study of algae farming.
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