Immunotoxicity in ascidians : antifouling compounds alternative to organotins. The case of Diuron and TCMS pyridine.

Since ascidians are benthic filter-feeding organisms living in the water-sediment interface, they rapidly bioaccumulate great amounts of xenobiotics and can be employed as useful biosensors in marine pollution monitoring of a wide range of environmental contaminants. In particular, compound ascidians has been reported to be very sensitive to organotin compounds (OTCs), a class of biocides which present in the coastal ecosystems mainly due to the release of antifouling paints from boats and harbour structures. As previously shown, one of the most important toxic effects of these compounds is represented by an immunosuppressant activity both in vertebrates (teleosts and mammals) and filter-feeding marine invertebrates (bivalves and ascidians). After OTC ban by many world's countries due to their severe impact to coastal ecosystems, alternative biocides have been massively introduced in formulations of antifouling paints. Diuron (3-(3,4-dichlorophenyl)-1,1-dimetylurea) and TCMS pyridine (DENSIL100; 2,3,5,6-tetrachloro-4-(metylsulphonyl)pyridine) are two of these new generation biocides, previously used as herbicide and leather tanning additive, respectively. However, up to now, no data are available concerning their potential target organisms, long-term toxic effects on biocenoses, mechanisms of action, bioaccumulation and environmental fate. In the present study, short-term haemocyte cultures of the colonial ascidian Botryllus schlosseri were exposed to various concentrations of xenobiotics for evaluating the toxic effects by means of a series of bioindexes. Similarly to OTCs, at sublethal concentrations (from 25 to 500 µM for Diuron and from 5 to 20 µM for TCMS pyridine) both the pollutants negatively affected the phagocyte morphology and induced apoptosis, but did not inhibit the oxidative phosphorylation and Ca2+ homeostasis. Differently from OTCs, Diuron was able to increase the lysosomal hydrolytic activities of phagocytes, and TCMS pyridine caused glutathione oxidation and inhibited the phenoloxidase activity of the cytotoxic cells. Although these compounds showed a higher LC50 than their environmental concentrations, our assays do open many questions on the long-term effects related to bioaccumulation. On the other hand, as observed for OTCs, the settlement of new toxicity indexes will furnish crucial information about the pesticide impact on the coastal ecosystems and contribute to the choice of antifouling compounds with less dangerousness for the environment. This work was supported by CoRiLa