Mechanisms of heavy metal cation homeostasis in marine invertebrates

The cellular processes involved in metal metabolism in molluscs are reviewed, with emphasis on the contribution of microscopy (AMG, ARG, EPMA, and SIMS) to both basic research of metal cell biology and applied environmental research. In molluscs, metal uptake may occur by facilitated diffusion, active transport, or endocytosis, and can be enhanced by MT synthesis or formation of mineralized granules. In aquatic molluscs, gills constitute a key interface for dissolved metal uptake, where metals are bound to MT, incorporated into lysosomes, and released basally towards the blood plasma and circulating hemocytes. However, particulate metal uptake is mainly achieved via the digestive tract by endocytosis; further metals are transferred first to lysosomes and then to residual bodies, especially in the digestive cells of the digestive gland. Additionally, metals can be accumulated selectively in specific cell types. As ligands pools differ from cell to cell, different metals may be retained in different cell types. Class "a" metals are localized in cells with granules composed of carbonate, oxalate, phosphate, and sulfate (oxygen donors), whereas "b" metals are associated with those cell types rich in sulfur and nitrogen ligands (sulfur donors). In molluscs, oxygen donors occur in connective tissue calcium cells and basophilic cells, whereas sulfur donors are present in digestive cells, podocytes, nephrocytes, and rhogocytes. Hemocytes, which constitute the most relevant system for metal transport between tissues, move around the body and may penetrate tissues and remove metals from the inner medium to be accumulated in lysosomes as nondigested products. Rhogocytes also participate in metal mobilization, accumulation, and release. The assessment of metal levels in target cells of sentinel molluscs by microscopic techniques provides an early-warning measure, with promising applications as an exposure biomarker for environmental monitoring programs.

Cellular and subcellular distribution of metals in molluscs.

1. The uptake of heavy metals via the alimentary tract can be an important factor for the metal budget of fish. 2. Concepts such as biomagnification, bioaccumulation, biotransference, or concentration factors, convey little information about the real threat originating from heavy metals in an aquatic food chain. 3. In polluted aquatic ecosystems the transfer of metals through food chains can be high enough to bring about harmful concentrations in the tissues of fish. This relationship is called the food chain effect. 4. Two kinds of ecological factors influence the food chain effect: firstly, high levels of contamination of the food, and, secondly, the reduction of species diversity. When susceptible species are eliminated, metal-tolerant food organisms may become dominant. Their tolerance may be based either on their ability to accumulate excessive amounts of metals or to exclude heavy metals from the tissues. These two strategies represent feedback mechanisms which may enhance or weaken the food chain effect. 5. It is concluded that future investigations on transference of heavy metals to fish must take into more careful consideration the specific ecological situation of a given environment.

Contaminated food and uptake of heavy metals by fish: a review and a proposal for further research.

Models of regulation and accumulation of trace metals in marine invertebrates

Patterns of accumulation, distribution and liberation of zn, cu, cd and pb in different organs of the land snail Helix pomatia L

Publisher Summary This chapter provides a general understanding of metal ion involvement in the various physiological compartments of the mollusk. Five elements—potassium, sodium, magnesium, calcium, and chlorine—are involved in establishing the ionic composition of the intracellular and extracellular fluids. These elements are also manipulated by cells so as to produce the osmotic equilibria that are necessary for living systems. There is a large group of elements that are required in only trace amounts for a whole variety of metabolic activities. Many of these elements are metals and they perform a wide variety of functions of which the activity of metalloenzymes, the transport of oxygen, and redox activities are the best understood. A large number of metalloenzymes and metal ion-activated enzymes are now known. Some metals, such as zinc and magnesium, are frequently associated with enzymes, although they may not be structurally a part of the molecule. A second major group of metal-dependent functions involves the respiratory pigments.

A.Z. Mason

Papers

Metal Ions: Metabolic and Toxic Effects