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Natural bioaccumulation of manganese and iron in the soft tissues of anodonta cygnea mollusca lamellibranchia metabranchia


, : Natural bioaccumulation of manganese and iron in the soft tissues of anodonta cygnea mollusca lamellibranchia metabranchia. Malacologia 29(2): 405-417

The accumulation of manganese in the soft tissues of the bivalve mollusc Anodonta cygnea has been studied by photon and electron microscopy, by classical cytochemistry and by the modern methods of analytical microscopy (secondary ion emission, X-ray emission and Raman diffusion). Contents of Mn and Fe in the soft tissues and in water and sediments were quantified by electrothermal atomic absorption spectrophotometry. The main tissues accumulating manganese are the connective tissues of digestive gland, mantle and gills. Mn is concentrated in extracellular concretions, in great amounts, together with P, Ca, Fe, and some other metals in lesser quantities. Manganese is in the Mn4+ state, iron in the two oxidation forms, Fe2+ and Fe3+; the Fe2+ is less common in animal tissues. The concretions in the gills are made predominantly of calcium phosphate but, in the concretions of the other organs, no mineral form is detected. This led us to consider the binding of the metals with the organic part of the spherocrystals. Our hypothesis is that, as well as classical mineral precipitation within an organic matrix, the spherocrystals might be in another form of organo-metallic complexation. The two might exist separately or together. The epithelial cells of gills, mantle and digestive gland also contain mineral inclusions, but these have little manganese; thus, these cells are not the place of manganese accumulation. Leydig cells present in connective tissue contain other intracellular concretions, rich in mangenese. This intracellular Mn accumulation is not very important compared to what is extracellular in the same tissue. We observed and described bacteria in the soft tissues of this mollusc, for the first time to our knowledge. These procaryotes contain Mn concretions. We suggest that Mn may originate, at least in part, from the activity of these bacteria. In this connection, the reduction of Fe3+ to Fe2+ leading to the oxidation of Mn2+ to Mn4+ might be a source of their energy and explains the great quantity of the unusual divalent form of the iron. Thus, the exceptionally high content of manganese in soft tissues would not be correlated with a particular metabolic pathway of the mollusc, but to its procaryotic biota. Our results emphasize the total independence between the accumulation of manganese in soft tissues and in the periostracum, and its deposition at the surface of molluscs shells.

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