Distribution of arsenic species in Mytilus galloprovincialis of the Venice lagoon

The toxicological effects and the environmental behaviour of arsenic are tightly linked to the chemical form in which this element appears. The inorganic species, arsenite and arsenate are more toxic than the simple methylated forms monomethylarsonic acid and dimethylarsinic acid; the more complex compounds like arsenobetaine, arsenocholine, tetramethylarsonium ion and arsenosugars are, however, considered non-toxic for living organisms. The determination of the total arsenic content in a sample therefore turn out to be not very adequate for evaluating its impact upon the organisms. The quantification of the different species can, however, provide information about the toxicological risk and about the transformations and the metabolism of arsenic in the environment. The cycle of arsenic in the marine environment is complex and many aspects regarding the processes of accumulation and biotransformation are not yet fully known and require further investigation. To this end, it is important to be able to make use of methodologies of analysis which permit the identification and quantification of the different species of arsenic in the various environmental matrixes. This research forms part of a study concerning the characterisation of As pollution in the Venice lagoon, in relation to the metabolic processes which occur in the lagoon itself. The preliminary results are reported regarding the development of an arsenic speciation procedure in biological matrices and its subsequent application to samples of bivalve molluscs (Mytilus galloprovincialis). Mytilus galloprovincialis collected from the Venice lagoon (Italy) were investigated for total arsenic concentrations by ICP-MS, ICP-AES, AAS and for arsenic compounds by HPLC-ICP-MS. For this purpose, analytical procedures for the sensitive and efficient speciation of the arsenic species As(III), As(V), monomethylarsonic acid (MMA), dimethylarsinic acid (DMA), arsenobetaine and arsenocholine were optimised. The total arsenic concentration and the arsenic compounds were determined in both the hepathopancreas (digestive gland) and all the soft tissue to verify the different As accumulation in the mussels’ body parts. Arsenic compounds were extracted from the animals sample with a methanol/water mixture; the extracts were chromatographed in an anion- and cation-exchange column. Only small quantities or traces of inorganic arsenic were detected in tested organisms. For example, the majority of arsenic compounds detected in the extracts were organic species, with a predominance of arsenobetaine and an unidentified compound that we supposed to be a product of biotransformation of the arsenosugars. Moreover, we observed a greater As accumulation in the mussels’ hepatopancreatic tissue.