Practical Toxicity Issues
The general factors which influence toxicity have already been set out earlier in this discussion, but before moving on to consider wider practical issues it is helpful to mention briefly the manner in which the toxic action of pollutants arises. There are two main mechanisms, often labelled ‘direct’ and ‘indirect’. In the former, the effect arises by the contaminant combining with cellular constituents or enzymes and thus preventing their proper function. In the latter, the damage is done by secondary action resulting from their presence, typified by histamine reactions in allergic responses.
The significance of natural cycles to the practical applications of environmental biotechnology is a point that has already been made. In many respects the functional toxicity of a pollution event is often no more than the obverse aspect of this same coin, in that it is frequently an overburdening of existing innate systems which constitutes the problem. Thus the difficulty lies in an inability to deal with the contaminant by normal routes, rather than the simple presence of the substance itself. The case of metals is a good example. Under normal circumstances, processes of weathering, erosion and volcanic activity lead to their continuous release into the environment and corresponding natural mechanisms exist to remove them from circulation, at a broadly equivalent rate. However, human activities, particularly after the advent of industrialisation, have seriously disrupted these cycles in respect of certain metals, perhaps most notably cadmium, lead, mercury and silver. While the human contribution is, clearly, considerable, it is also important to be aware that there are additional potential avenues of pollution and that other metals, even though natural fluxes remain their dominant global source, may also give rise to severe localised contamination at times.
The toxicity of metals is related to their place in the periodic table, as shown in Table 4.1 and reflects their affinity for amino and sulphydryl groups (associated with active sites on enzymes).
In broad terms, type-A metals are less toxic than type-B, but this is only a generalisation and a number of other factors exert an influence in real-life situations. Passive uptake by plants is a two-stage process, beginning with an initial binding onto the cell wall followed by diffusion into the cell itself, along a concentration gradient. As a result, those cations which readily associate with particulates are accumulated more easily than those which do not. In addition, the presence of chelating ligands may affect the bio-availability and thus, the resultant toxicity of metals. Whereas some metal-organic complexes (Cu-EDTA for example) can detoxify certain metals, lipophilic organometallic complexes can increase uptake and thereby the functional toxic effect observed.
Although we have been considering the issue of metal toxicity in relation to the contamination of land or water, it also has relevance elsewhere and may be of particular importance in other applications of biotechnologies to environmental problems. For example, anaerobic digestion is a engineered microbial process commonly employed in the water industry for sewage treatment and gaining acceptance as a method of biowaste management. The effects of metal cations within anaerobic bioreactors are summarised in Table 4.2, and from which it is apparent that concentration is the key factor.
However, the situation is not entirely clear cut as the interactions between cations under anaerobic conditions may lead to increased or decreased effective toxicity in line with the series of synergistic/antagonistic relationships shown in Table 4.3.
Toxicity is often dependent on the form in which the substance occurs and substances forming analogues which closely mimic the properties of essential chemicals are typically readily taken up and/or accumulated. Such chemicals are often particularly toxic as the example of selenium illustrates.
Often wrongly referred to as a toxic metal, and though it has some metallic properties, selenium is a nonmetal of the sulphur group. It is an essential trace element and naturally occurs in soils, though in excess it can be a systemic poison with the LD50 for certain selenium compounds being as low as 4 micrograms per kg body weight.
In plants, sulphur is actively taken up in the form of sulphate SO42−. The similarity of selenium to sulphur leads to the existence of similar forms in nature, namely selenite, SeO32− and selenate SeO42−.
As a result, selenium can be taken up in place of sulphur and become incor-porated in normally sulphur-containing metabolites.
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