Organisms belonging to several taxonomic groups have been used in order to detect and measure toxicity of different substances in benthos (SETAC 1993): Recently, efforts have been made in standardization of suitable organisms able to be used in sediment toxicity tests. Related to this subject, microalgae have not received much attention, even taking into account the relevant role of this group in benthic environments. This is especially noticeable in mudflats and coastal shelf systems (Light and Beardall 2001); Blanchard et al. 2000), where biomass of microalgae can match or even exceed bacterial biomass in intertidal sediments (La Rosa et al. 2001). References of sediment toxicity test involving benthic microalgae have been found in the literature (Moreno-Garrido et al. 2001). The diatom Cylindrotheca closterium is a very widespread benthic species, that demonstrated fast growth in laboratory cultures, even in poorly enriched media. Taking growth inhibition as the endpoint, a novel approach to sediment toxicity testing has been developed, by exposing benthic microalgal populations to laboratory sediment spiked with linear alkylbenzene sulphonate (LAS), a synthetic organic surfactant produced in large quantities (Tolls et al. 1997). Surfactants can alter membrane permeability, enzyme and lysosomal activity and tissue structure in organisms, inducing alterations in electron-transfer chains (Argese et al. 1994; Blasco et al. 1999; Bragadin et al. 1996; Lewis 1992). Surfactants tend to accumulate in high quantities in urban-polluted coastal sediments (González-Mazo et al. 1997).

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