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The same concentration of the three herbicides used in the experiments triggers different changes in distinct physiological processes that underlie growth, development and reproduction of Scenedesmus opoliensis in batch cultures. It is known from studies with vascular plants that many environmental stress factors may cause damage of membrane structure which results in peroxidation of lipids with unsaturated fatty acids. The products of peroxidation, especially the malondialdehyde, leave the membranes and spread all over the cell compartments, causing structural damage of nucleic acids, proteins and photosynthetic pigments, thus triggering a cascade of disfunctions. These have to be quickly compensated by repair mechanisms that ensure a hardening process during the development of a certain tolerance against the stress factor. In this context the level of membrane lipid peroxidation products, called thiobarbituric acid reactive substances (TBARS), was measured in the algal cell cultures exposed for 10 days to 10 ìM of three different herbicides (diuron, methylviologen and glufosinate). Diuron did not cause any significant change in the level of TBARS, but in the presence of methylviologen and glufosinate the degree of lipid peroxidation increased more that two times as compared with the control, indicating that these two herbicides, even if they have different action sites in the plant cells, both induce membrane damages and impair transmembrane transport processes by causing structural changes in the lipid bilayer (Fig. 1). <p> The ascorbic acid (vitamin C) content of the algal cells, expressed on a dry weight basis, showed a statistically significant increase only when the cultures were exposed to 10 ìM methylviologen. No important change in the quantity of ascorbate was detectable in the algae treated with diuron and with glufosinate. As the most frequent non-enzymatic organic reducing agent in plant cells, vitamin C plays an important role in the antioxidative defense of algae and a higher ascorbate content is a prerequisite for a more efficient protection. It is worth mentioning that vitamin C occurs in plant cells in three interchangeable forms: as reduced ascorbic acid, as partly oxidized monodehydroascorbate and as fully oxidized dehydroascorbate. The method used in the present experiments allows only the determination of total vitamin C amount, without distinction between its different forms (Fig. 2). </p> <p> From among the enzymatic components of the antioxidative defense system, catalase exhibited the most prominent changes under the influences of the three herbicides. Its catalytic activity in decomposing the highly toxic hydrogen-peroxide was not significantly modified by diuron, but it was obviously decreased by methylviologen, while glufosinate induced a pronounced rise in its enzymatic activity. The changes in the intensity of the detoxification reaction of hydrogen peroxide may be due to changes in both the amount of the catalase protein molecules and the catalytic activity of the enzyme modulated by different regulatory factors (Fig. 3). </p> <p> The herbicides used in the experiments exerted a negative effect on the division rate of algal cells, resulting in changes of the dynamics of cell density in the populations during the first five days of exposure. Glufosinate caused only a delay in reaching the maximal cell density in the cultures, but did not lower significantly the final cell number in a unit of culture media volume. Diuron inhibited the growth of algal populations and forced the establishment of the steadystate growth phase at a much lower cell density than in the case of control. Methylviologen exerted a very pronounced inhibition of cell divisions, maintaining the cell density of the cultures close to the initial low values (Fig. 4). </p> <p> The final dry biomass of the algal populations, reflecting the efficiency of net photosynthetic primary production, was impaired by all the three herbicides used in the experiments. The most pronounced decline of the algal biomass was registered in the presence of methylviologen, while the mildest, but still significant effect was exerted by glufosinate. The adverse influence of diuron on net biomass production was more moderate than that of methylviologen but more intense than the decrease caused by glufosinate (Fig. 5). </p> <p> The net photosynthetic oxygen production of the constantly illuminated algal cells also changed under the influence of micromolar concentrations of herbicides. Oxygen production was mostly inhibited by diuron, and less (but still significantly) impaired by methylviologen. In contrast with the other two herbicides, glufosinate enhanced the net oxygen evolution of the algal cells, increasing it with almost 50% of the values measured in the control cultures (Fig. 6). </p>
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