|InterJournal Complex Systems, 755
|Manuscript Number: |
Submission Date: 2004
|The structure of algal population in the presence of toxicants|
Freshwater microalgae are used more frequently in toxicity tests than any other type of freshwater or marine plants, but less often than animal species. However, microalgae have been found to be often more sensitive than animal species to a variety of potential toxicants. Therefore, the use of microalgae is necessary to estimate the environmental hazard of chemicals. Laboratory populations of green chlorococcal algae are widely used as test object for the evaluation of the phytotoxicity of chemicals. This estimation is an essential component of the ecological risk assessment. The laboratory culture of microalga Scenedesmus quadricauda (Turp.) Breb. was studied under the influence of the fungicide imazalil sulfate and potassium dichromate. Population structure was used for quantifying signs on measurements of toxic action of these substances. The simultaneous presence of two groups of cells (“large”, 4.0-4.5 mm in width, mainly in the composition of two-cellular coenobia, and “small”, 3.0 mm, in the composition of four-cellular coenobia) proved to be a specific feature of the dimensional-age structure of the control population at different stages of its growth. The fungicide imazalil sulfate at concentrations of 0.001 and 1.0 mg/l was found to inhibit the division of cells and imparted to them anomalous increase in size and the formation of gigantic cells, but such state of algae was reversible: giant cells rapidly resumed their division after being transferred to a toxicant-free medium. At concentration of 0.1 mg/l the division of cells resumed within 1-2 days of intoxication. At high imazalyl concentrations (5, 10 and 20 mg/l) division stimulation preceded the fast death of algal population. The effect of potassium dichromate in concentrations of 0.001 to 10.0 mg/l on S. quadricauda was studied too. It was found that at concentrations of 0.001 and 1.0 mg/l the total cell numbers decreasing was not related with cell death. The decrease of the growth rate in this case was due to the cell division inhibition in a fraction of cell population. At concentrations over 3.0 mg/l a total cell number and proportion of living cells decreased. A positive correlation was found between the relative size and the cell numbers at 3.0 and 10.0 mg/l. The cells that survived during a 30-day exposure retained the ability to reproduce and with time attained the numbers and the proportion of the living cells in the control culture. A stable increase in cell size was observed. A culture exposed to 1 mg/l potassium dichromate had an increased resistance to higher concentrations of the toxin in terms of total cell number, alive cells share, photosynthesis efficiency. In the long-term presence of potassium dichromate 10.0 mg/l the selection of resistant cells took place. Then, these cells could restore the algal population. It is concluded that there are initial resistant cells within the heterogenous algal population, the share of this cells is 4-7 %. The resistance to the toxicant was low in autumn-winter and high in spring-summer. Data on the population structure will be appropriate for use to indicate the toxic action of different substances.
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