Bioacumulación de distintas especies de selenio y sus efectos en organismos marinos

Résumé

Selenium (Se) is present in proteins and has an antagonistic effect against toxic elements, what makes it an essential trace element. Human beings need Se in quantities smaller than 100 pg day-1. However, differences between its deficiency, appropriate intake and toxicity are rather slight, being 300 pg Se day- the maximum tolerable dose. Living organisms take Se by direct absorption, mainly through the trophic chain. On the other hand, bivalve molluscs are of great relevance for the human diet, due to having a high nutritional value as well as a high quantity of essential elements. It is thus important to do research concerning Se supplementation in species of aquatic interest. In this work, the bioaccumulation, biotransformation and elimination of several species of selenium were investigated in a filtering organism of aquatic interest (Ruditapes decussatus clams). R. decussatus were exposed to a daily renewal protocol with dissolved selenite (Se(IV)), selenomethionine (SeMet) and seleno-methylselenocysteine (SeMetSeCys), and to a microalgae (Se-Tiso) diet for a period of 120 days. The concentration range was 0-1000 pg Se L-1, 0-50 pg Se L-1, 0-25 pg Se L-1, and 0,042-4,37 pg Se Kg-1 Se(IV), SeMet, SeMetSeCys and Se-Tiso, respectively. Elimination was studied for clams previously exposed to 500 pg Se L-1, 10 pg Se L-1, 5 pg Se L-1 y 4,37 pg Se Kg-1 of Se(IV), SeMet, SeMetSeCys and Se- Tiso, respectively. Time of exposure was 60 days. The total amount of Se was determined, as well as its speciation in clams’ soft body and tissues (mantle, foot, gut, gill and hepatopancreas). The amount of Se was also determined at the sub-cellular level, distinguishing between the biologically active and non-active content. Additionally, other parameters were determined, as those of mortality and sublethal effects. These latter depended on the level of concentration and on the type of Se used. Results showed a mortality rate of 100% for those clams exposed to 1000 pg Se L-1 Se(IV), 50 pg Se L-1 SeMet, 25 pg Se L- 1 and SeMetSeCys of Se(IV) at 28, 7 and 21 days, respectively. For clams fed with Se-Tiso, the mortality rate was of 35% at 60 days. Regarding physiological effects, clarification rate decreased with respect to the control in those groups of clams which had been exposed to the highest concentrations from 7-14 days on. No significant differences were found for the group of clams with Se-Tiso. Overall, GST enzyme activity decreased and that of GPx increased in those groups of clams treated with Se. Total accumulation of Se in whole-body and in the different clam tissues conformed to first-rate kinetic models, displaying increasing bioconcentration factors (BCFs) in the following order: Se(IV) < SeMetSeCys < SeMet. Trophic transfer factors for clams fed with Se-Tiso were in the range of 3,75-16,5. Likewise, Se distribution in tissues increased as follows: foot = mantle < gut < gill < hepatopancreas. The different sub-cellular sections of gill and hepatopancreas showed that between 35% and 81% of Se is found in biologically-detoxified areas (maximum heat stable proteins). On the other hand, less than 28% of Se is found in biologically-active areas (mainly in mitochondria). Elimination of Se conformed to one- and two- compartment kinetic models. The biological half-life values of elimination in whole-body were between 1 and 10 days and between 23 and 46 days in the short and long term for all treatments, respectively. Finally, three known species of Se (Se (IV), SeMet, SeMetSeCys) and four unknown species (U1, U2, U3, U4) were identified for the whole body, tissues and sub-cellular sections. Known species account for between 4-16%, 7-27%, 31-66%, and 6-29% in clams exposed to selenite, SeMet, SeMetSeCys and fed with Se-Tiso, respectively. SeMetSeCys was detected in all groups of clams. Se (IV) was only detected in those groups of clams exposed to Se (IV), SeMet and SeMetSeCys. For clams exposed to SeMet, only this latter was detected.