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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