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Background  Levels of toxic metal exposure in indigenous inhabitants are key bioindicators of the severity of environmental contamination. This study measured the seasonal variation of heavy metals and metallothionein (MT) contents in Asian swamp eels ( Monopterus albus ) from a paddy field situated in Tumpat, Kelantan, Malaysia, to identify prevalence, patterns and associations and togain insight on the suitability of MT as a biomarker for metal exposure.    Methods  Gill, muscle and liver tissues of  M. albus  ( n  = 50) sampled during the ploughing, seedling, growing and harvesting phases of rice growing were collected. The concentrations of copper (Cu), zinc (Zn), lead (Pb), nickel (Ni), and cadmium (Cd) in these tissues were determined by flame atomic absorption spectrometry. MT from each sample was isolated and purified, and subsequently quantitated using UV spectrophotometry. Associations between metal and MT concentrations, season and tissue type were evaluated using Pearson correlation and ANOVA with post-hoc Tukey HSD analysis.    Results  Zn was present in higher quantities in gill and liver tissues, while Cu levels were elevated solely in liver. Patterns of non-essential metal accumulation were varied: Cd was detected in low concentrations in all tissues, while Pb and Ni were abundant in gill tissues across all seasons. MT concentration in liver tissue was consistently higher than that found in muscle or gill tissue, except during the growing phase. Moreover, significant correlations ( P  < 0.05) were observed for Cd, Ni, and Zn when MT was employed as metal exposure biomarker. However, no significant association was found between high Pb and Ni levels and MT concentration in gill tissue.Variation of bioaccumulation rates of heavy metals among the different tissues was observed. Some of these metal concentration differences were found to be associated with MT concentration and, by extension, to its high metal-binding capacity.    Conclusions  Significant liver MT-Zn, MT-Cd, and MT-Ni correlations found in this study emphasised the role of metallothionein as a biomarker for exposure of zinc, cadmium and nickel metals in  M. albus .

Seasonal variation of heavy metals and metallothionein contents in Asian swamp eels, Monopterus albus (Zuiew, 1793) from Tumpat, Kelantan, Malaysia