Sublethal effects biomarkers

The following brief account identifies only major groups of herbicides not mentioned elsewhere in the text, and is far from comprehensive. Their mode of action is only dealt with in a superficial way. From an ecotoxicological point of view, there has not been as much concern about their sublethal effects upon plants as there has been in the case of mammals, and there has not been a strong interest in the development of biomarker assays to establish their effects. The major concern has been whether weeds, or nontarget plants, have been removed following herbicide application—a rather easy matter to establish as plants are fairly sedentary. For a more detailed account of herbicide chemistry and biochemistry, see Hassall (1990). 

For avian wildlife, data are incomplete on PAH background concentrations and on acute and chronic toxicity. Studies with mallard embryos and PAHs applied to the egg surface showed toxic and adverse sublethal effects at concentrations between 0.036 and 0.18 pg PAH/kg whole egg (Hoffman and Gay 1981). Additional research is needed on petroleum-derived PAHs and then-effects on developing embryos of seabirds and other waterfowl. There is an urgent need for specific avian biomarkers of PAH exposure (Murk et al. 1996). 

Pesticides can cause lethal and sublethal effects on nontarget organisms such as wildlife. However, it is sublethal effects that have been most important in terms of their long-term survival. In this section, we will study two cases that show population declines in wildlife due to pesticide contamination in the environment. We will also study biochemical changes in wildlife induced by sublethal doses of pesticides as biomarkers for the detection of pesticide contamination in the environment. 

Table 3.6.1 Biomarkers of exposure and of sublethal effect. The examples chosen are relatively simple to perform and have been applied to a wide range of species in environmental monitoring programmes...

Concomitant with an appreciation of the importance of sublethal effects of xenobiotics on natural populations, there has been increased interest in alternative procedures for assessing these effects. Considerable interest has centered on the application to fish of procedures developed in clinical medicine these have used biochemical assays for specific enzyme activities and physiological parameters in blood and serum samples. The term biomarker has been used, but it should be pointed out that this has also been applied in a completely different context to compounds isolated from samples of sediment, coal, and oil and which plausibly have a biological origin (Simoneit 1998). There seems, however, little reason for confusion in the application of the same term in these widely different contexts. 

The inhibition of brain cholinesterase is a biomarker assay for organophosphorous (OP) and carbamate insecticides (Chapter 10, Section 10.2.4). OPs inhibit the enzyme by forming covalent bonds with a serine residue at the active center. Inhibition is, at best, slowly reversible. The degree of toxic effect depends upon the extent of cholinesterase inhibition caused by one or more OP and/or carbamate insecticides. In the case of OPs administered to vertebrates, a typical scenario is as follows sublethal symptoms begin to appear at 40-50% inhibition of cholinesterase, lethal toxicity above 70% inhibition. 

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