Single and combined effects of Zn, Mn and Fe on the Neotropical freshwater bivalve Anodontites trapesialis: Bioaccumulation and biochemical biomarkers

Abstract

Important concentrations of Zn, Mn and Fe were detected in a stream near a coal mining area and promoted, in field, biomarkers alterations in the bivalve Anodontites trapesialis. In order to understand the isolated and mixed effects of these metals on these Neotropical bivalves, we run short-term experiments under laboratory controlled conditions. After 96 h-exposure, tissues (gills, mantle, digestive gland, muscle, hemolymph) were removed for metal bioaccumulation analysis, oxidative stress biomarkers (reactive oxygen species (ROS), total antioxidant capacity, lipoperoxidation (LPO), proteins carbonylation (PC), metallothionein (MT), activity of superoxide dismutase and glutathione S-transferase and hemocytes DNA damage) and cholinesterase (ChE versus ASCh activity) activity evaluation. We run three independent tests. In Zn test, clams were exposed to three concentrations of Zn (0.18 mg L^-1, 1.0 mg L^-1, 5.0 mg L^-1); in Mn test, clams were exposed to three concentrations of Mn (0.1 mg L^-1, 0.5 mg L^-1, 5.0 mg L^-1) and in Mix test, clams were exposed to the mixture Zn (1 mg L^-1) + Mn (0.5 mg L^-1), with and without Fe (5.0 mg L^-1). After single exposure to 5.0 mg L^-1, Zn bioaccumulated in all tissues, but only in mantle and hemolymph after exposure to 1.0 mg L^-1. The increased MT in gills of A. trapesialis exposed to Zn appears to be sufficient to avoid damage, since LPO occurred only in digestive glands from animals exposed to 5.0 mg L^-1. We suggested that A. trapesialis had a metabolic suppression in consequence of Mn presence, based on the following results: the decrease of ROS in gills, the decrease of the Zn and Mn concentrations in tissues and the decrease of ChE versus ASCh activity in muscle. Despite this, animals exposed to Mn suffer oxidative damages (LPO and PC) in the mantle and digestive gland and MT increased in the mantle. These results showed A. trapesialis responded differently to each metal and Mn caused more damage. When exposed to Fe, gills level of ROS was increased, despite no changes in metal accumulation occurred. On the other hand, after exposure to the mixtures, tissues bioaccumulated Zn and previously observed damages caused by Mn and Fe disappeared. Consequently, biomarkers were less affected under mixture treatments, demonstrating mixtures effects or responses were not simply a combination of single exposures to Zn, Mn and Fe, but depend on metals toxicokinetics.

Keywords: Essential metals; Metabolic suppression; Metal mixture; Metallothionein; Oxidative damage.