Bio-Accumulation

Bio-Accumulation

	Bio-accumulation Various heavy metals like mercury, cobalt, tin, nickel, cadmium and thallium are used in metal alloys or as catalysts, and during their mining and ultimate disposal, cause heavy metal pollution problems. All these metals are substantially toxic to plants, animals and many microorganisms. Microorganisms, owing to their large surface to volume ratio and high metabolic activity, are important vectors in introducing heavy metal and radionuclide pollutants into food webs. From neutral to alkaline pH, heavy metals in soils and sediments tend to be immobilised by precipitation and/or adsorption to cation exchange sites of clay minerals.
Microbial production of acid and chelating agents can reverse this adsorption and mobilise the toxic metals. Microbial metabolic products that can chelate metals include dicarboxylic and tricarboxylic acids, pyrocatechol, aromatic hydroxyl acids, polyols and some specific chelators such as enterochelins and ferrioxamines. Mobilisation is often followed by uptake and intracellular accumulation of the heavy metals, both by microorganisms and by plant roots. It is not entirely clear why some of these toxic metals are taken up and stored by microorganisms, but intracellular sequestering seems to confer heavy metal resistance on at least some bacteria. Filamentous fungi were shown to transport heavy metals and radionuclides along their hyphae.
This has some implication for the potential role of mycorrhizal fungi in transmitting such pollutants into higher plants. Direct root uptake of heavy metals mobilised by microbial acid production or chelation is an alternative possibility. The heavy metal cadmium is of special concern in this respect. Cadmium is highly toxic and tends to accumulate with even very low exposures because it is excreted extremely slowly. Its approximate half-life in humans is ten years. Cadmium in humans causes, at low chronic exposure, hypertension and kidney damage.
Higher exposures through rice grown on industrially contaminated fields have caused the painful and crippling bone and joint disease (itai-itai disease). Cadmium occurs in low concentrations in phosphate fertiliser spread on agricultural fields. Another potential source of cadmium and other heavy metals is sewage sludge. Such sludge is being considered for use as soil conditioner. As a substantial portion of sewage sludge is derived from microbial biomass, the relatively high concentration of heavy metals in this material also reflects the ability of microorganisms to concentrate these pollutants. Microbial accumulation of radionuclides, have clear human health implications. Lichens are extremely effective in concentrating the radionuclides such as 9OSr and 137CS from atmospheric fallout.
	During periods of snow cover, when the lichens are shielded from direct fallout, concentrations of radionuclides in the lichens decrease. During periods of rain, concentrations of radionuclides increase. Because lichens serve as the primary producers in a food chain of lichen-caribou-humans, there can be an efficient transfer of such concentrated elements to the highest member of the food chain. These radionulcides, being deposited in bone tissue, may affect blood cell synthesis in the bone marrow and cause leukemia.

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