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Abstract
Human activities have been increasing the cadmium levels in soils and waters, disturbing
many organisms in the primary trophic levels such as microalgae. Toxic metal pollution
is a focus point of serious concern and the examination and monitoring water quality
are becoming essential procedures. Diatoms are important bioindicators to monitor
the metal concentrations in diverse habitats. The present study was planned to determine
the biochemical mechanisms used by freshwater diatoms to cope with cadmium stress
and to identify biomarkers of metal stress. For this, Nitzschia palea (Kützing) W.
Smith was grown under different concentrations of Cd (0.01-0.1 mg l(-1)) and the IC(50)
determined. Three concentrations (0.1, 0.2 and 0.3 mg Cd l(-1)) and a control (no
cadmium) were used to undergo the experimental assays which allowed the determination
of cadmium accumulation and several biochemical markers currently used to assess metal
stress. N. palea was sensitive to cadmium, as the IC(50) calculated was 0.0276 mg
Cd l(-1). Cadmium accumulation increased sharply and was mainly associated to the
frustule. Total protein content increased with cadmium exposure, inducing increases
and decreases in polypeptide expression, indicating an attempt of N. palea cells to
adjust to the new prevailing conditions induced by metal stress. In order to cope
with cadmium stress, cells induced the synthesis of chelating molecules such as phytochelatins
(PCs). The enzymatic (SOD and CAT) and non-enzymatic (glutathione and proline) ROS
scavenging mechanisms were also induced. Our results indicate the existence of diverse
metal stress-mediated mechanisms in order to lessen metal damages to the cell. PCs
showed to be a suitable biomarker of metal stress; besides being metal specific and
concentration respondent it also allows to infer about the level of stress imposed
to cells, constituting a useful tool to complement the evaluation of diatom communities
when accessing aquatic metal toxicity.
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