The cycling of various naturally occurring elements, including toxic pollutants, is a well-regulated and balanced process but man’s industrial activity often tends to perturb this equilibrium and upset the balance towards greater accumulation and lesser destruction.
Human activities also provide newer sources of toxic elements to the environment and such elements, like naturally oncoming ones, may pass through the biogeochemical cycle and, in turn become accessible to living organisms. A very significant role in the mobility or dispersion of various elements in the environment is played by microorganisms.
Mercury is one of the most important toxic elements which are now being increasingly discharged into soils and water as an unwanted by-product of certain industrial and agricultural activities. In aquatic habitats mercury gets biologically converted into the poisonous compound methyl mercury.
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Elemental mercury has fairly low vapour pressure and this facilitates its global dispersal. The bioaccumulation of mercury is greatly facilitated by the natural synthesis of stable alkyl mercury compounds (Wood, 1974). Elemental mercury can exist in three alternative states, viz., Hg22+, Hg2+ and Hg° and certain micro-organisms are capable of interconnecting the three forms.
Naturally occurring methyl vitamin B12 compounds can aid the synthesis of methyl mercury in natural habitats, and certain microorganisms can in turn degrade the mercury compounds, albeit slowly, into elemental mercury.
The biological cycle for mercury in, for instance, a pond, is summarized in this. Besides the processes indicated in dimethyl-mercury, which is highly volatile, passes into the air and decomposes into CH4, C2H6, and Hg°, thus causing air pollution.
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Mercury pollution can be best assessed by measuring the concentration of total mercury in sediments and also the rate of uptake of methyl mercury by fish.
Sayler et al. (1975) have studied the bio-amplification of mercury levels in the oyster (Crassostrea virginica. This bio-amplification is mediated by bacteria, indicating the fundamental role of bacteria in mobilization and accumulation of mercury at higher levels in food-webs.
Where there is a significant mercury-resistant bacteria population actively metabolizing as well as accumulating mercury compounds, their involvement in the bioaccumulation of mercury may well be very significant.
According to Wood, certain other elements, e.g., tin, palladium and thallium probably have cycling mechanisms similar to those of mercury whereas others, e.g., cadmium and zinc fail to become methylated.
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Apart from mercury, arsenic also has a biological cycle in nature and alkylarsenic compounds have been shown to accumulate in Norwegian shell- fish.
Arsenic is more abundant in nature as compared to mercury. In water supplies, arsenic can occur at levels of up to 50 ppm whereas mercury levels commonly do not exceed 1 ppm. Arsenic occurs in rocks, soils and waters at much higher levels than doe’s mercury.
Pentavalent arsenic occurs naturally in properly treated wood; it is after used as a constituent of preservatives for timber. The biological cycling of arsenic is shown in. Arsenate is reduced to a lower valency state and then microbially methylated to form dimethylarsine ad trimethylarsine.
Some species involved in the conversion of dimethylarsenic acid to trimethylarsine include Scrophulariopsis spp., Aspergillus sydowi, A. fisheri, A. virens, A. glaucus, Mucor mucedo, Lenzites trabea, Monilia sitophila, Phoma spp. and Graphium spp.
Dimethylarsine is highly toxic to fish and other organisms. The volatile arsines become oxidized in air to the less toxic dimethylarsinine acid. The conversion of arsenate through arsenite and methylarsenic acid to dimethylarsinic acid occurs in lake sediments; di- and tri-methylarsines are released in water, and dimethylarsinic acid is cycled between air and sediment (see Wood, 1974).
In soil, arsenic is thought to be held fairly tightly and is not readily leached. Some recent studies (see Dobbs and Grant, 1976) on the burning of wood previously treated with the preservative chromated copper arsenate have revealed that such burning does not add significantly to the quantity of volatilized arsenic in the air, in fact, the quantity added to the air in this way is much less than the amount added when coal is burnt.
Unlike mercury, arsenic compounds are not known to bioaccumulate or amplify through food chains (Summers and Silver, 1978).