Ecotoxicological evaluation of metals and marine sediments using phytoplanktonic algae immobilized in Na-alginate beads.

Marine sediments are a reservoir of organic and inorganic pollutants, that can be adsorbed in suspended particulate matter and can occur in higher concentration in whole sediments than in the water column; so, the evaluation of sediment ecotoxicity provides for general information on pollutant bioavailability. Microalgae are widely employed in ecotoxicity testing because they are known to be sensitive to toxicants; they are at the basis of the food web in coastal marine waters, as well as in the other aquatic environments, and their key-role in the ecosystems has been pointed out. In this connection, the direct exposition of microalgae immobilized in Na-alginate to sediments could greatly improve the ecological relevance of the tests. Nevertheless, to date algae are recommended only for the ecotoxicological evaluation of the elutriate matrix of sediments; so, taking into account such assumption, this work aims to evaluate the employment of Na-alginate immobilized algae Tetraselmis suecica to study the toxicity of the whole sediment matrix. An extension of this study concerned the evaluation, on both free and immobilized algae, of the ecotoxicity of some metals (Cu, Cd, Zn) that are known to occur in sediments. The immobilization of algae was carried out by using a 3% p/v sodium alginate solution in deionized water; exponential growing algal cells were added to this solution. Algal growth was determined, after 24 and 48 hours, by counting with haemocytometer Thoma; the growth of immobilized cells was determined after treatment of beads with 5% sodium hexametaphosphate; the end-point for algae growth was the percent variation in comparison to the control. Zn and Cd showed a similar toxicity for both immobilized and free algae after 48 hours of exposition. As concerns the ecotoxicological evaluation of sediments, the tests carried out with elutriates showed a low variability between free and immobilized algae; the tests carried out on immobilized algae with the whole sediments showed greater growth decrease than those with the elutriate. On the whole, the obtained results show that in the elutriate some pollutants could not be detected; this points out the importance of the application of new methods able to detect the underestimated ecotoxicological effects of sediments.