Effet sublétal du cadmium sur Donax trunculus : Evaluation de la valeur nutritionnelle.

Cadmium (Cd) is a transition metal, very toxic and ranked seventh on the list of hazardous substances. Previous studies have reported markedly increased levels of metal pollution in the Gulf of Annaba, especially by Cd. The present study was designed to investigate under laboratory conditions the impact of the acute toxicity (96h) of cadmium on the nutritional aspect of an edible species and bioindicator of pollution, Donax trunculus (Mollusca, Bivalvia); and also aims to assess the depuration capacity of this species. The experiments were carried out in two phases: a first phase of acute exposure (96h), followed by a second restoration phase of the same duration (96 hours), and taking into consideration the periods of the sexual cycle: the period of sexual activity and the period of sexual rest. The samples were collected at El-Battah, a site far from the sources of pollution and then reared in the laboratory. The physicochemical parameters of the seawater samples, including temperature, pH, salinity and dissolved oxygen, were measured. Cadmium was added to culture water at two sublethal concentrations (LC10 and LC25) previously determined by Merad and Soltani (2015). The evaluation of the sublethal effects of cadmium on the nutritional aspect of Donax trunculus was considered according to two parts; the first part concerns the proximate composition, our results showed that cadmium leads to an increase in moisture content and total ash content. However, it causes a decrease in dry matter content, fat content, carbohydrate content, lipid content and protein content in the flesh of D. trunculus. These responses vary with time, concentration of sex, and period of the sexual cycle. The second part relates to the composition of metallic trace elements (MTE), our results show highly significant concentration, time and gender effects. Indeed, the contents of MTE were significantly reduced with the exclusion of Cd which presented a significant increase. All the results indicated that the predominant MTE is Ca in the individuals of D. trunculus collected in El-Battah. Although, the least predominant MTE is Cd, however these contents are lower than the admissible limit recommended by the CEFAS (1997) and the NHMRC (1987). In addition, Zn was found to be the second most abundant element in D. trunculus males, in comparison to females which are richer in Fe more than Zn, this was explained by the cellular uptake of Zn, which was higher in sperm than in eggs. On the other hand, our data indicated higher levels of immediate composition during the period of sexual rest compared to the period of sexual activity; this could be due to the high energy consumption during sexual activity. In addition, the responses of the immediate composition and the composition of metallic trace elements are more marked in females compared to males. In addition, D. trunulus revealed a relatively high restoration potential after 96 hours, which testifies to the effectiveness of the depuration phase in order to reduce the stress suffered after metal contamination. Keywords: Nutritional value, proximate composition, bivalve mollusc, Donax trunculus, metallic trace elements (MTE), cadmium.

Effet sublétal du cadmium chez Donax trunculus : aspect biochimique et cytologique. Soutenus

Cd is a very toxic heavy metal whose presence has been detected in the Gulf of Annaba. The research conducted during the realization of this thesis, aims to study under laboratory conditions, the sublethal effects of cadmium (Cd) on the reproductive aspect of an edible species and bioindicator of pollution, Donax trunculus (Mollusca, Bivalvia) ; and also aims to assess the ability of this species to overcome the chemical stress induced by Cd, using biochemical and cytological responses in Donax trunculus. The studies were carried out in two phases : a first phase of acute exposure to Cd lasting 96 h, followed by a second phase of depuration of the same duration. Considering the periods of the sexual cycle : the period of sexual activity and the period of sexual rest. The sampling was carried out at the beach of El Battah, a site far from the sources of pollution. The samples were reared in the laboratory and physico-chemical parameters such as temperature, pH, salinity and dissolved oxygen were measured. Cd chloride was added to the rearing water at two sublethal concentrations (LC10 and LC25) determined previously by Merad & Soltani (2015). Concerning the biochemical responses, our results showed that cadmium causes an increase in the levels of vitellogenins and vitellines in D. trunculus with a concentration-effect relationship. Moreover, it causes an induction in the number of protein fractions of vitellogenins and vitellins ; these inductions vary according to time, concentration and period of the sexual cycle. Regarding the cytological responses, our results showed that cadmium could cause ovarian follicle hypertrophy due to the increase in oocyte width and volume ; these responses varied according to time and concentration ; on the other hand, no time or concentration effect was observed for the number of oocytes and no histological lesions were revealed. Regarding the depuration phase, the results of our study revealed a progressive reduction in the levels of Vtg and Vn followed by a decrease in the number of protein fractions; this reduction varies according to time, concentration and period of the sexual cycle. All the results confirm that cadmium has the character of a metallo-estrogen for D. trunculus, it could act on its endocrine system by inducing the synthesis of Vtg and Vn, and could also modify the expression of the electrophoretic profiles of the latter. Moreover, it could induce changes in the morphometry of ovarian follicles and oocytes of D.trunculus, which are probably related to the increase of Vn contents in the oocyte. On the other hand, the influence of the sexual cycle on the biomarker of reprotoxicity Vtg is due to vitellogenesis and the resumption of gonadal activity. On the other hand, this species revealed a rather important restoration potential with a possible recovery of anti-estrogenic effects which confirms the effectiveness of the depuration phase to reduce the stress undergone after a metallic contamination at low concentration. Key words : Donax trunculus, Cadmium, Biomarkers of reprotoxicity, Ovaries, Hemolymph, Depuration.