Metallothioneins (MTs) and δ-aminolevulinic acid dehydratase (ALAd) as biomarkers of metal pollution in great tits (Parus major) along a pollution gradient

Biomonitoring allows an integrated evaluation of different aspects of exposure, accumulation and effects to environmental pollution, simultaneously accounting for the natural variety between individuals in an ecosystem. In this study, the effects of increased metal accumulation were evaluated at the biochemical level in terms of two biomarker responses in the great tit (Parus major), a small insectivorous songbird, along an established metal pollution gradient. Metal concentrations in internal tissues (liver and kidney) and blood indicated that lead and cadmium were the most important metals in the pollution gradient under study. At the biochemical level, induction of metal binding protein metallothionein (MT) in liver and kidney reflected cadmium concentrations in these tissues (R2 = 0.42 and R2 = 0.94 respectively, n = 19), although in kidney, MT induction was not sufficient to complex all cadmium present. Secondly, the activity of the enzyme δ-aminolevulinic acid dehydratase (ALAd) in blood decreased exponentially in response to increased lead accumulation (R2 = 0.70, n = 18) and represents therefore a specific effect marker for lead exposure. In the highest polluted area, an ALAd inhibition of 85% was reported. Since a higher metal exposure resulted in an increased metal accumulation and subsequent biomarker responses in a dose-dependent way, this study indicates the applicability of ALAd and MT levels in great tits for biomonitoring responses to heavy metal pollution.