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The freshwater green alga Scenedesmus opoliensis proves to be a suitable bioindicator …


Biology Articles » Hydrobiology » Stress-physiological reactions of the green alga Scenedesmus opoliensis to water pollution with herbicides » Introduction

Introduction
- Stress-physiological reactions of the green alga Scenedesmus opoliensis to water pollution with herbicides

Molecular indicators may be very useful parameters for an early and precise detection of changes in physiological processes of living organisms caused by stressful environmental conditions related to anthropogenic pollution. In this context, one major direction of the present-day biological investigations points toward identification of molecular markers directly related to stress conditions sensed by living organisms, suitable for bioindication of the quality of both terrestrial and aquatic ecosystems [8, 12, 23]. Microalgae have a key role in biomonitoring the changes that occur in aquatic habitats not only because they are the main primary producers of new organic substances in these ecosystems, but also because of their pronounced physiological plasticity that supports their acclimation to changes in the physical and chemical properties of the environment [3, 6, 11, 16]. Xenobiotic organic compounds, like herbicides that accumulate over the time in inland waters surrounded by agricultural fields, represent a real challenge for the survival of algae because these substances are not present in the natural, unpolluted aquatic ecosystems, and in consequence no adaptive strategies could be developed during the evolution of the algal species in order to cope with the harmful effects of these chemicals on the vital processes of photoautotrophic cells [4, 14, 27, 29]. Planktonic green algae, such as different species of the Scenedesmus genus, are useful test organisms in ecotoxicological studies, because they are globally distributed and are applicable to all aquatic habitats and a wide range of environmental stressors, they respond rapidly to changes in ecosystem condition, they allow for nondestructive sampling and they often provide one of the first signals of ecosystem impacts [2, 13, 21, 31].

While a large number of papers deal with influence of herbicides on terrestrial vascular plants, there are only a few publications referring to their impact on aquatic organisms, even though these organic compounds usually exhibit a high solubility in water and they may easily accumulate in aquatic habitats, especially in lakes [5, 7, 19, 26]. Unwanted side effects of agricultural application of herbicides are to be expected on non-target organisms, for example on algae in the aquatic ecosystems adjacent to areas subject to agricultural activities. These side effects consist of toxicity, disturbance of developmental processes and the appearance of resistant ecotypes. On the basis of their site and mode of action, the different herbicides may act as specific inhibitors of the light reactions of photosynthesis, inhibitors of biosynthesis of carotenoid pigment, fatty acids and aromatic amino acids, growth regulators administrated in excessive amounts, and compounds that affect tetrapyrrole biosynthesis, causing damage by peroxidation in light. The non-selective herbicides usually impair the energy flux in plants, disturb vital metabolic processes and generate highly reactive oxygen species [18, 25, 28]. Microalgae have a certain ability to bioaccumulate, to immobilize, to sequester and to biotransform different herbicides and related organic compound, thus contributing to a substantial remediation of the aquatic habitats polluted with xenobiotics. This is why the knowledge of how they react to stress conditions imposed by water pollution is important in order to use them in bioindication and bioremediation of disturbed aquatic ecosystems [17, 24].

The aim of the present study is to investigate some of the physiological reactions of the highly adaptable green alga Scenedesmus opoliensis to long-term exposure to micromolar amounts of three largely used non-selective herbicides, and to reveal molecular indicators suitable for a good establishment of environmental stress condition imposed by water pollution with the applied herbicides.


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