There is no justification for using biological monitoring where direct physical and chemical methods can achieve comparable results both as quickly and as cheaply. The decision of whether biological monitoring is appropriate or not must depend on the specific aims of, and the resources available for, the particular investigation.
The strengths of biological monitoring lie primarily in the close stimulation of biomonitors with the biological systems under study. Often the biomonitor will be part of that biological system.
Some of the criteria for selecting good biological monitoring species include:
-the organism must be capable of accumulating metals in measurable amounts.
-the organism, or relevant parts of it, must be readily available both in terms of quantity and distribution so that unbiased sampling is possible.
-it should be available throughout the year, or for the whole period of study, with relative ease of collection.
-The organism should show a differential uptake/accumulation which is related to exposure thus allowing either: (a) relative pollution levels to be determined, or (b) the establishment of a more quantitative relationship to deposition rate or air concentrations.
-for assessing airborne contamination, the organism should not be subject to substantial uptake or ingestion of metals from other sources.
-repeatability is essential
-cost of collection and analysis should be acceptable
Bacteria
All cell-based organisms have intricate mechanisms for detoxifying arsenic compounds that involve a wide variety of proteins that chemically modify, transport and extrude the arsenic from the cell. The biological synthesis and activation of these proteins is regulated by the presence of arsenic, often through specific genetic mechanisms. In one commonly employed mechanism, activation of the genes that encode the proteins for arsenic resistance depends on the reversible binding of a regulatory protein to a Deoxyribonucleic acid (DNA) control sequence associated with that gene. When the regulator is bound to the analyte it can switch the gene on to synthesize the required proteins to activate the arsenic detoxification system. Understanding the identity, specificity, and sensitivity of the genetic elements and their corresponding regulatory proteins is key to technologies employing biosensors. When creating an arsenic biosensor, the arsenic-responsive DNA control sequences are linked to an additional gene. This gene, called a reporter gene, produces a protein whose properties can be readily observed: as an enzyme that generates a highly colored material or a fluorescent protein. Using techniques developed from molecular biology, it is possible to develop a microbe that generates a visible signal, usually fluorescing bright yellow, when it comes in contact with arsenic compounds.
Genetically modified microbes were used in another recent study to develop a set of semi-quantitative assays for potable water. The investigators also developed an assay that produced a visible blue color with arsenite concentrations above 8 ppb.
Advantages:
- can detect arsenic down to ppb levels
-good potential for assaying arsenic
Disadvantages:
-apply only to water assays
- limited success rate
- it is not clear whether the microbes are measuring all of the arsenic in a sample or just the bioavailable arsenic.
Plants
However, far less research involving the use of plants to detect arsenic has been conducted than for the use of microbes. A strong research effort involving the study of plants that accumulate and store arsenic, primarily for the remediation of arsenic-contaminated sites, is underway. A recent study demonstrated changes in color pigmentation of two water plants upon exposure to arsenic. This effect requires an incubation period of three days and can be quantified with a series of standards. Although this is a very good “low tech” assay, it requires more study to rule out, for instance, the effect of other stresses, such as nutrient levels or microbial infection, which can generate the same pigment change as arsenic absorption.
In considering plant material for monitoring purposes it is important to reiterate that the size, shape, canopy structure and surface characteristics of the plants or plants organs used together with their degree of exposure will all contribute to the efficiency of particulate capture and retention. A further consideration is the relationship between the surface area and weight of the plant organs concerned because these will affect the expression of the results and may complicate the interpretation and comparison of data.
Advantage:
-its general ubiquity. Only in situations of extreme aerial contamination is vegetation likely to be sufficiently scarce to cause sampling problems.
Disadvantage:
- coloration changes in plant systems may be due to factors other than arsenic detection.
-samples may vary between general herbage of several species to leaves, whole leafy shoots and bark of single species.
A student team from the University of Edinburgh has used genetically engineered bacteria to detect arsenic in water. In combination with a drop of pH indicator (far right), samples turn red (middle) in the presence of arsenic and yellow in its absence.
For more information ,feel free to read up more on : http://www.technologyreview.com/Biotech/18103/
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