Agriculture Reference
In-Depth Information
The bioavailability problem may be addressed by measuring biomarker responses accurately.
This would facilitate the setting of environmental quality criteria, for generic use by regulatory
authorities, in risk assessment. The heterogeneity and complexity of soil and other systems may
force us to think in terms of site-specific criteria for each unique situation (differentiated environ-
mental quality criteria). This would require a completely different approach (Slijkerman et al. 2000)
to the standardized one currently implied or accepted within the framework of existing laws and
regulations in many developed countries.
At the subcellular level, progress has been made in showing that certain biomarker responses,
such as the neutral red retention time assay in earthworms (Weeks 1995; Svendsen and Weeks
1997; Spurgeon et al. 2000; Reinecke et al. 2002), can be used reliably to link changes in the
permeability of the lysosomal membranes to ecologically relevant life cycle effects caused by
certain substances. Although this does not constitute a definite link to structural and functional
properties of the ecosystem, these authors demonstrated dose-related responses in earthworm weight
change and reproduction that correlated with neutral red retention times. This neutral red retention
technique is therefore well established for measuring the cellular toxicity of certain chemicals to
earthworms and other invertebrates. It is sensitive and easy to use and has now been used for a
few metals and organic chemicals on a range of earthworm species, such as
Lumbricus terrestris,
Lumbricus rubellus, Lumbricus custaneas, Aporrectodea rosea, Allolobophora caliginosa, Eisenia
fetida, Eisenia andrei,
sp. In all cases, reliable dose-response relationships
were established by various researchers, emphasizing the reliability of the technique in spite of the
fact that it relies heavily on subjective observations (S.A. Reinecke and Reinecke 1999).
The use in ecotoxicology of stress proteins, metallothioneins, ultrastructural changes, and
isozymes has also gained ground as potential tools for toxicant hazard identification and exposure
assessment (Kammenga et al. 2000). The usefulness of these biomarkers should become clearer
once a large enough database for different species of earthworms and soil and contaminant types
becomes more readily available. Some biomarkers may not be practical for regular use in field
monitoring, but others, such as the neutral red retention assay, may be used at low cost by a trained
technician on organisms sampled from the field to provide a quick indication of toxic stress. The
exposure of uncontaminated organisms with a known biomarker response under uncontaminated
conditions could provide a more practical and realistic tool for measuring bioavailability in con-
taminated soil, or at least circumventing some of the problems in assessing toxicant bioavailability
(Weeks 1997), than is currently appreciated. This can be done either by placing earthworms directly
in the field in outdoor enclosures or microcosms or by exposing them to field soil in the laboratory
and subsequently monitoring the changes in biomarker response. This may in the future provide
an excellent biomonitoring tool for evaluating or at least estimating bioavailability and eventually
for predicting earthworm population changes caused by continued toxic stresses.
Biomarker responses provide an early indication that toxicant uptake and internal exposure has
occurred, and that a toxic response may have been initiated. In comparison with chemical toxicity
measurements, biomarkers have a striking advantage in that they are indicative of a reaction that
has commenced and not just of a chemical concentration. This already gives the biomarker a higher
relevancy in the assessment of toxicity than the measurement of a chemical concentration could.
This is especially true not only for biochemical responses to toxicants (Strzenbaum et al. 1998),
but also for cellular responses such as changes in the lysosomal membrane stability that reflect
various degrees of cytological damage in a dose-related manner. This response may vary among
different earthworm species but is expected to reflect differences in the relative sensitivity of
earthworm life cycle traits (Spurgeon et al. 2000). It is, however, a dose-related response and
probably presents a more reliable reflection of the bioavailable fraction of the toxic substance taken
up over time from the substrate, thereby causing the effect.
Although nonspecificity is generally seen as a drawback in relating cause and effect, this
response may nevertheless be useful precisely because it integrates the background toxicant con-
centrations, in both the organism and the substrate as well as other stress factors, with the effect
and a
Microchaetus
