Distribution et effets du chlorure de tributylétain et de ses produits de dégradation sur la croissance de l'algue marine Pavlova lutheri en culture continue

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Organotin compounds have been used as biocides, wood preservatives, plastic stabilizers and antifouling agents for many years (Maguire, 1991). The acute toxicity as well as the sublethal effects of tributyltin (TBT) derivatives have been tested for numerous freshwater and marine species (Alzieu, 1989; Kelly et al., 1990). The biodegradation of TBT in dibutyltin (DBT) and monobutyltin (MBT) is well established for some phytoplankton species (Lee et al., 1989) but little is known about the physiological adaptation of microalgae to a long term sublethal contamination by TBT. The objective of this study was to evaluate the ability of marine alga Pavlova lutheri, cultured in “chemostat mode” under axenic conditions, to accomodate increasing cocentrations of TBTCl in the culture medium and to determine its capacity to degrade TBT in DBT and MBT. Chemostat culture of P. lutheri were exposed to contaminated nutrient supplies containing 18.5, 74, and 185 nmol l−1 of TBT chloride (expressed as TBT+) respectively. For comparison purposes, a batch culture of P. lutheri was exposed to a concentration of 13 nmol l−1 of TBTCl for a 48 h-period. Organotin species (BuSn) were monitored by GC/ITD in three different fractions: dissolved in the culture medium, adsorbed on the external cellular walls, and dissolved in the cellular fluid. Within a 24 h-period, TBT was observed in the cellular fluid indicating a rapid uptake of the contaminant. The ability of P. lutheri to degrade TBT was confirmed and found more effective in a continuous culture than in a batch culture under similar contamination conditions. MBT was observed in both cellular fractions of all cultures (Tables 2 and 3) but P. lutheri seems particularly efficient to form MBT at low TBT concentration. The amount of BuSn adsorbed onto the cell was directly related to TBT concentration in the nutrient supply but intracellular BuSn decreased from 150 nmol g−1 (dry weight) to less than 60 nmol g−1 (dry weight) when the TBT concentration increased from 74 to 185 nmol l−1 (Fig. 2). The cell density and the growth rate of a continous culture of P. lutheri (Figs 3 and 4) were not affected by a contamination level of 18.5 nmol l−1 of TBT. In a continuous culture receiving 74 nmol l−1 of TBT P. lutheri suffered a toxic impact in the first few days of the experiment, loosing 40% of its cell density, However, the culture recovered its initial growth rate in the next 10 days in spite of an uninterrupted contamination. Finally, a severe toxic shock was observed for a culture receiving 185 nmol l−1 of TBT leading to a major dysfunction of the culture in only three days. The toxic effects observed in culture productivity (Fig. 5) for treatments at 74 and 185 nmol l−1 are also illustrated by a significant decrease of chlorophyll α concentration in culture [Fig. 6(A)] but not in the individual cells [Fig. 6(B)] which was an indication of the preservation of life functions of cells in all experiments. These results evidence the ability of P. lutheri, cultured in chemostat mode, to resist and accommodate to a continuous input of a high level of TBT to the culture medium degrading TBT into much less toxic species MBT and DBT. As a chemostat culture is comparable in many aspects to an open natural environment (Rhee, 1980; Wangersky and Maass. 1991). The behavior of P. lutheri observed in this study might be extended to the natural coastal environment where this alga is present. If such an assumption is correct at least regarding the general processes of bioaccumulation and degradation, this study clearly indicates that P. lutheri might play an important role in the food uptake of BuSn by filter feeders and other herbivorous species due to its ability to bioaccumulate large quantities of BuSn [Fig 2(A) and 2(B)] without being irreversibly affected. Fortunately, the natural environment as simulated by our continuous cultures seems to strongly accelerate the degradation rate of TBT into DBT and MBT, two much less toxic tin species. Results of previous studies on the toxicity effects of TBT on micro-algae, all conducted in batch cultures, should be interpreted with some caution since the ability of P. lutheri and possibly other marine algae to survive and grow in TBT contaminated media seems to have been grossly underestimated.