Author's personal copy Sediment toxicity tests involving immobilized microalgae (Phaeodactylum tricornutum Bohlin) I. Moreno-Garrido ⁎ , L.M. Lubián, J. Blasco Instituto de Ciencias Marinas de Andalucía (CSIC), Campus Río S. Pedro, s/n 11510, Puerto Real, Cádiz, Spain Available online 6 December 2006 Abstract Populations of calcium-alginate immobilized marine diatom Phaeodactylum tricornutum were exposed to two sediments containing different levels of surfactant (LAS). Toxic responses were compared for free and immobilized algae. Although there is a direct relation between LAS content in sediment and inhibition, immobilized algae suffered less inhibition than free cells, over all when fluorescence is chosen as a biomarker for biomass. When cells are counted from dissolved beads, inhibition of growth is closer to the values found for free cells. Immobilization can be useful for in situ experiments but protection of cells inside the alginate beads against toxic capacity of xenobiotics must be taken into account. © 2006 Published by Elsevier Ltd. Keywords: Phaeodactylum tricornutum; Bioassays; LAS; Sediment toxicity 1. Introduction Microalgae conform the basis of the trophic chain in different coastal and estuarine biocenosis (MacIntyre et al., 1996). Biomass of these organisms can match and even surpass biomass of bacteria in certain conditions (La Rosa et al., 2001). Organisms from microphytobenthos are known to be respon- sible for part of the stability of the surface sediments in estuaries and mudflats (Blanchard et al., 2000; de Brower et al., 2000). Coastal and estuarine sediments use to act as a sink and potential posterior source of xenobiotics, because solubility of most of those substances is modified (usually reduced) when chemicals dissolved in freshwater enter marine and estuarine conditions (pH values higher than 8, salinity values trending to 35). This is the case of some surfactants such as LAS (Lineal Alkylbenzene Sulfonate), a widely used tenside that can be found at high concentrations in coastal and estuarine sediments of developed countries (Blasco et al., 1999; González-Mazo et al., 1997; Tolls et al., 1997). The absence of normalized toxicity bioassays on coastal and estuarine sediments is a restriction in the integrated toxicity study of coastal management from an environmentalist point of view. It is well known that microalgae can be very sensitive to toxicants in toxicity bioassays (Radix et al., 2000; Stauber and Florence, 1990), and the key role of these organisms has been pointed out yet. In spite of all this, there is not a common agreement in the use of microalgae for toxicity testing involving coastal and marine sediments (Lamberson et al., 1992; SETAC, 1993). Some efforts have been made with extracts or elutriates, but modification of natural conditions is unavoidable when these fractions are used. Direct exposition of microalgae to sediment would improve in a great way the ecological relevance of the tests (Moreno-Garrido et al., 2003a,b; Adams and Stauber, 2004). This philosophy is extended to the extreme when in situ bioassays are designed: instead of carrying samples (surface sediments, in this case) to the laboratory, the aim of the in situ bioassays is to install experimental organisms in actual locations for a determinate period of time, then to recover them and carry the organisms to the laboratory for the analysis of any parameter susceptible of being compared between control (non polluted) locations and potentially polluted locations (Munawar and Munawar, 1987; Pereira et al., 1999; Moreira dos Santos et al., 2002). In the case of other organisms this is easier by the use of cages of different designs (Pereira et al., 1999), but in the case of microalgae their small size implies an additional problem. Two solutions have been proposed to the problem of the in situ bioassays involving microalgae: the design of cages incorporating filters or di- alysis membranes (Munawar and Munawar, 1987) or the Environment International 33 (2007) 481 – 495 www.elsevier.com/locate/envint ⁎ Corresponding author. E-mail address: ignacio.moreno@icman.csic.es (I. Moreno-Garrido). 0160-4120/$ - see front matter © 2006 Published by Elsevier Ltd. doi:10.1016/j.envint.2006.10.003