Multiple stressors—a challenge for the future

Environmental pollutants such as trace metals, organics and radionuclides rarely occur alone. Sources contributing to contamination often contain a mixture of contaminants. Information on processes affecting ecosystem behaviour, mobility, biological uptake, metabolism and accumulation are therefore equally relevant for all contaminants. Radiation induced free radicals resulting in umbrella endpoints such as reproduction failure, mutation, morbidity, mortality as well as synergistic and antagonistic interactions are equally relevant for metals and organics. Thus, integrating concepts for all contaminants within environmental impact assessment and risk models should be highly beneficial.

In mixed contaminated areas, organisms are exposed to a cocktail of contaminants, i.e. multiple stressors, which could potentially lead to umbrella biological endpoints. To protect the environment, however, authorities apply Environmental Quality Standards (EQSs) that are based on a one-dimensional concept, assessing one component independently of others. Uncertainties associated with EQS are also questionable. Problems arise from extrapolating toxicity data; from acute to chronic effects, from laboratory to field conditions, from effect concentration to no-effect concentration and from isolated test-species to complex systems. According to OECD, uncertainties in extrapolating can be covered by Safety factors, being 100, 10, and 1 for acute, chronic and field data, respectively. Thus, there is a need to improve the scientific bases for setting EQSs, and as contaminants often are a mixture of stressors, a multiple stressor approach is essential.

No EQSs are derived for radionuclides and radiation protection has previously focused only on man. According to the International Commission on Radiation Protection (ICRP): if man is protected, the environment is adequately taken care of, considering the environment only as a conduit for the transfer to man. Since the 1990s, however, radiation protection of the environment has become a major issue internationally and is also implemented in regulations and legislations. The scientific basis for protecting the environment from radiation represents a challenge, as concepts associated with low dose chronic exposure and related effects are to a certain extent based on cancer in man. Thus, knowledge on dose–effect units, radiation effects, doserate effects, internal–external radiation effects and dose–biological endpoints relationships for biota is crucial.

Long-term chronic exposure to low concentrations of contaminants may result in a series of negative biological responses; free radical production and induced oxidative stress, causing important biomolecules such as chromosomes to change or degrade; effects on the immune system, altering susceptibilities for infectious diseases; effects on the neurological system, affecting developmental and differentiation processes. Traditional endpoints, like survival and growth will typically not be sufficiently sensitive to detect the series of potential chronic effects. One single stressor may induce multiple biological effects, if multiple interactions occur or if interactions with different biological targets take place. In mixtures with several different stressors, multiple types of interactions and interactions with multiple target sites may occur. For contaminants having the same mode of interactions with the same target sites, effects will be concentration-additive and synergistic and antagonistic effects appear. If contaminants have different modes of interactions, and act at different target sites, they should act independently. As information on interaction modes and target sites for most contaminants is scarce, well-controlled mechanistic experiments, utilising advanced molecular and genetic tools are needed to identify early biological responses.

Several stressors, such as radiation and trace metals induce free radicals due to excitation and ionisation of water molecules in cells and Haber–Weiss and Fenton reactions. Free radicals produced (˙H, ˙OH) are extremely reactive and will recombine and produce various reactive compounds in cells (e.g. HO₂, H₂O₂, H₂, O₂) which may result in damage to membranes, tissues, enzymes, and proteins, DNA/RNA. Following free radical induction, a series of biological endpoints is influenced, such as SOD, catalase, the glutathion cycle, and lipid peroxidation, enzyme inactivation and DNA strand breakage may also occur. Thus, mechanistic studies performed under well-controlled exposure conditions utilising modern advanced molecular and genetic tools to identify induced effects are a prerequisite for studying effects following low dose chronic exposures of multiple stressors.

It is internationally recognised that there are severe gaps in basic knowledge with respect to biological responses to multiple stressor exposures. Identification of biological responses to low dose chronic exposure calls for early warning biomarkers, utilising modern molecular and genetic tools. Furthermore, information on dose–response relationships (on/off mechanisms), sensitivity (detection limits, thresholds), and synergetic and antagonistic effects, as well as the role of protecting agents such as antioxidants is highly needed. Development of analytical strategies, methods and biomarkers that can be utilised to increase knowledge on the biological impacts from multiple stressors represents a challenge for the future.

To strengthen the understanding within ecotoxicology of the effects of multiple environmental stressors to living organisms is a topic of international concern, not only within the scientific communities but also for authorities, responsible for directives, regulations and legislations. A major challenge within ecological risk assessment is how to relate scientific information to judgements about the functioning of the ecosystem, and policy decisions based on potentially adverse affects. This requires a sophisticated risk characterisation evaluation, particularly focusing on extrapolation issues and a systematic assessment of the uncertainties and assumptions involved within that extrapolation. In this respect, improved knowledge on the effects from multiple stressors has relevance for both ecotoxicology and human toxicology.

 

Brit Salbu, Bjorn Olav Rosseland and Deborah H. Oughton

Norwegian University of Life Sciences,

Department of Plant and Environmental Sciences,

Isotope Laboratory, P.O. Box 5006,

1432 Aas, Norway

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