The latest developments in gene function determinations have opened up the possibility to use gene expression levels to measure the effects of toxicants on organisms. To our knowledge, most of the studies in metal toxicity on organisms have used high concentrations of metals in order to reveal genes that are, for the most part, involved in a general stress response, i.e. responses that could be solicited by other compounds. In order to obtain rigorous responses to metal concentrations that are environmentally realistic, the first part of this work focused on the determination of genes that respond to micromolar or lower concentrations of free Cd. Ten genes that could potentially be used as algal biomarkers were proposed.
As a starting point, the 10 genes were tested in the laboratory under various conditions that included variations in the concentrations of calcium and magnesium. The effect of protons was also assessed. The expression level of the biomarkers in complex media was affected by changes in the calcium concentrations while magnesium had little effect. More experiments will need to be performed to precisely assess the effect of pH.
More work will be required to assess the precise effect of interactions between Cd2+ and H+, including the determination of the affinity constant of the proton and Cd under a larger range of pH values. Overall, the role of the physicochemistry of the environment will be important to evaluate because the response of biomarkers may be affected by these variations.
Numerous reports have indicated that laboratory results cannot easily be extrapolated to the field, thus the biomarkers were tested for their response to environmental conditions.
They were exposed in two sites that included one more polluted (Riou Mort site) and one less polluted (Bouillac site). The results indicated that the response of the biomarkers was affected by the composition of the media (i.e. lack of nutrients in the Bouillac site). The response to cadmium concentrations is not straightforward i.e.
different biomarkers were activated at different levels during the two exposure seasons.
The field exposures revealed that the media composition can have an important influence on the biological response. The general conclusion of this part of the work is
used with complementarily methods already available for the measurements of the natural media.
The development of a battery of tests and methods will be required to detect pollution and predict effects on aquatic organisms. Nonetheless, to date, measurements of the dissolved Cd fraction (or other metals) does not predict well the effects on organism in natural media. It is well known that presence of organic matter and colloids have a strong effect on the speciation of the metal and its interaction with biological surfaces.
Nonetheless, there are no methods to measure and predict to what extent the complexity of the media can affect the interactions between metals and microorganisms.
Microorganisms that are used as biosensors can provide information about the effect of metals. One of the main challenges to overcome in order to use algal biosensors as a tool for monitoring toxicity will be the specificity of the gene that is used as a reporter gene. It is often observed that a grouped pathway of actions is used for a grouped class of compounds; i.e. similar compounds induce similar pathways of detoxification or toxicity. It is naïve to assume that every compound activates a specific gene that ultimately could be used as a biomarker. Nonetheless, this situation could be alleviated by the fact that it is possible to find some specificity in the gene expression. For example, Ni, Cu, Zn and other metals can activate production of phytochelatins (peptides involved in cadmium homeostasis). However, at low concentration ranges, cadmium is the primarily activator of their production (Cobbett and Goldsbrough, 2002). This demonstrates that the use of algal biosensors will be very promising and important if used in parallel to conventional analytical methods.
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