Heavy metal toxicity testing in environmental samples

The toxicity of heavy metals in the environment depends on a number of physicochemical and biological factors. The complexity of these relationships has encouraged the use of bioassays for direct measurement of the [table: see text] impact of toxic metals on selected test species. Fish and daphnid bioassays are well accepted by the scientific and regulatory communities, but their length (48 h or more) and the considerable time and effort needed to culture the test organisms make their application to sample screening problematical. Microbial and biochemical assays based on the inhibition of bioluminescence, enzyme activity, enzyme biosynthesis, growth, respiration, and heat production are typically faster and less expensive than the traditional and fish bioassays. Some of these tests approach or equal the sensitivity of daphnids to heavy metals. Since the soil acts as a sink for airborne and waste-applied metals, the uptake of metals by plants and the associated toxic impacts are important. Growth inhibition, enzyme induction, and production of stress proteins have been considered as toxicity end points. Enzymatic tests have been developed that are specific for heavy metal toxicity. Such tests can facilitate toxicity reduction evaluations. Detection of individual metals in the environment may eventually be possible using biosensors consisting of genetically engineered microorganisms. Direct solid-phase tests for soil, sediment, or sludge toxicity, using bacterial bioluminescence or enzyme activity as end points, have been developed. Such tests may complement traditional solid-phase toxicity tests using nematodes or earthworms as indicator organisms. Based on the work reviewed, we draw the following conclusions: 1. The Microtox assay is sensitive to mercury but would fail to detect the toxicity of certain metals, such as cadmium. Among all the microbial assays reviewed, the bioassay based on growth inhibition of the alga Selenastrum capricornutum appears to give the lowest EC50s, similar to those seen for daphnid bioassays. 2. Biosensors, using genetically engineered microorganisms, offer an elegant means of detecting the presence of specific heavy metals in environmental samples. However, at the present time, they are not designed for assessing heavy metal toxicity. 3. The use of bioassays specific for heavy metal toxicity can be useful for directly assessing the bioavailability of these toxicants in environmental samples, thus avoiding the need for fractionation.+4