Bioaccumulation of cadmium in marine organisms

It has been established that, although Cd occurs in the marine environment in only trace concentrations, most marine organisms, especially molluscs and crustaceans, can accumulate it rapidly. Cadmium is not uniformly distributed in the body and selectively accumulates in specific organs like liver, kidney, gills, and exoskeleton. The concentrations in muscle tissues are several orders of magnitude lower. The disposition of Cd in the organisms in the laboratory studies generally parallels those in nature. A number of biotic factors like body size, maturity, sex, etc. influence bioaccumulation but extensive studies are still lacking. The chemical form of Cd in the environment is of prime importance in bioaccumulation by marine organisms. Salinity can affect the speciation of Cd, and bioaccumulation is affected by both temperature and salinity. The ultimate level of Cd in the organisms will depend not only on the biotic and abiotic factors but also on metabolism of the metal by the organisms. A few studies indicate depuration of Cd by some bivalves but other organisms show very effective retention of Cd. Metallothionein formation for detoxification and storage has been observed in a large variety of marine organisms. Recent reports indicate an alternate storage and excretion mechanism in the formation of membrane-limited vesicles or granules. There seems to be a common link between intracellular localisation of Cd in metal-binding proteins and Cd containing vesicles as detoxifying mechanisms in the marine organisms. Much of what is known about Cd bioaccumulation by marine organisms has come from laboratory studies and there are inherent dangers in trying to extrapolate the results to field situations. In spite of tremendous progress made over the years, the basic understanding of the bioaccumulation process is still very nebulous and will remain so until the uptake, storage, and elimination processes are fully understood.