TOXICITY, UPTAKE AND METABOLISM OF THE N-METHYLCARBAMATE PESTICIDE CARBOFURAN BY THE FRESHWATER BIVALVE MOLLUSC GLEBULA ROTUNDATA (LAMARCK)
ZAKOUR, HELEN ROBINSON
Doctor of Philosophy thesis
The uptake, toxicity and metabolism of the N-methylcarbamate pesticide carbofuran (2,3-dihydro-2,2-dimethyl-7-benzofuranyl methylcarbamate) by the nontarget freshwater bivalve mollusc Glebula rotundata (Lamarck) were examined. Uptake and toxicity studies were also conducted with another bivalve species, Rangia cuneata (Lea). Both G. rotundata and R. cuneata absorbed water-borne carbofuran. During long-term (>48 hours) exposure experiments, R. cuneata did not appreciably concentrate this pesticide; generally, tissue and water concentrations of carbofuran were almost equivalent. In short-term (<48 hours) exposure studies, carbofuran uptake by G. rotundata was proportional to the wet weight of whole body tissue. Glebula rotundata tissues did not differentially absorb this pesticide. When exposed to carbofuran concentrations greater than 75 ppm (waterborne or injected), both species exhibited symptoms of carbofuran poisoning; these included gaping of the shell valves, extension and flaccidity of the foot, incoordination and loss of tactile response. The effects of carbofuran were reversible except in R. cuneata that had lost the ability to respond to tactile stimulation. During in vivo experiments, G. rotundata converted radiolabeled carbofuran primarily to polar compounds; a variety of less abundant free metabolites were also recovered. The polar metabolites produced by water immersion incubated clams appeared to be conjugated products that were refractory to chemical and enzymatic conditions known to hydrolyze glycosidic conjugates. The possibility of the involvement of an amino acid moiety in the formation of the polar metabolites was suggested. Glebula rotundata degraded carbofuran more slowly than most other animals but more rapidly than plants and microorganisms. When in vitro methods were used, G. rotundata hemolymph metabolized carbofuran more rapidly than homogenates of gill, mantle and hepatopancreas tissues. The extracellular portion of this body fluid was more active than the cellular portion in metabolizing this pesticide. Hemolymph preparations converted radiolabeled carbofuran to hydrolysis products and polar metabolites. The in vitro produced polar carbofuran metabolites were tentatively identified as glycosidic conjugates. Bacteria isolated from clam tissue and incubation water were tested for the ability to metabolize this carbamate pesticide. The bacterial contribution to the observed metabolism of carbofuran by G. rotundata appeared to be negligible in both in vivo and in vitro studies. Future experiments are also discussed.