BIOMETHYLATION AND BIOTRANSFORMATION OF ARSENIC IN A FRESH-WATER FOOD-CHAIN - GREEN-ALGA (CHLORELLA-VULGARIS)-]SHRIMP (NEOCARIDINA-DENTICULATA)-]KILLIFISH (ORYZIAS-LATIPES)

Tolerance, bioaccumulation, biotransformation and excretion of arsenic compounds by the freshwater shrimp (Neocaridina denticulata) and the killifish (Oryzias latipes) (collected from the natural environment) were investigated. Tolerances (LC50) of the shrimp against disodium arsenate [abbreviated as As(V)], methylarsonic acid (MAA), dimethylarsinic acid (DMAA), and arsenobetaine (AB) were 1.5, 10, 40, and 150 mug As ml-1, respectively. N. denticulata accumulated arsenic from an aqueous phase containing 1 mug As ml-1 of As(V), 10 mug As ml-1 of MAA, 30 mug As ml-1 of DMAA or 150 mug As ml-1 of AB, and biotransformed and excreted part of these species. Both methylation and demethylation of the arsenicals were observed in vivo. When living N. denticulata accumulating arsenic was transferred into an arsenic-free medium, a part of the accumulated arsenic was excreted. The concentration of methylated arsenicals relative to total arsenic was higher in the excrement than in the organism. Total arsenic accumulation in each species via food in the food chain Green algae (Chlorella vulgaris) --> shrimp (N.denticulata) --> killifish (0. latipes) decreased by one order of magnitude or more, and the concentration of methylated arsenic relative to total arsenic accumulated increased successively with elevation in the trophic level. Only trace amounts of monomethylarsenic species were detected in the shrimp and fish tested. Dimethylarsenic species in alga and shrimp, and trimethylarsenic species in killifish, were the pre-dominant methylated arsenic species, respectively. freshwater food chain, green alga, shrimp, killifish.