A new way to understand the effects of toxic compoundsby Flore Emonnot and Anne Michaud
Published by the September 7, 2020 on 8:07 AM
Water pollution is a major concern. It can be induced by many elements. For example, Cadmium (Cd) which belongs to the heavy metals family can be source of pollution in certain concentrations. This element is naturally present in the environment, but the use of agricultural chemicals has been indicated as the main anthropogenic source of Cd pollution in aquatic environments. The organisms living in these aquatic ecosystems are exposed to this pollution. Moreover, this compound is bioaccumulated in organs and tissues, so it can induce damages.
Daphnia magna
That is why it is important to evaluate the effects of this pollutant on organisms. Daphnia magna (a cladoceran crustacean) is one of the most widely used animals in aquatic toxicology. In terms of sensitivity to toxic substances, it is generally thought to be representative of other zooplankters (Anderson, 1944). It plays an important role in the balance of an ecosystem, because of its position on the first levels of the food chain. Also, D. magna enhances water purification by filtering water and retaining food particles, it is its way to eat. This animal spends its whole life in a variety of freshwater environments. As long as the conditions remain favourable, it reproduces predominantly by parthenogenesis.
Bioassays are used in aquatic toxicology to provide individual-level information, while ecotoxicology is assessing the impact of pollution on populations. The DEB theory (Dynamic Energy Budget) is a theory that describes the aspects of metabolism (energy and mass budgets) at the individual level. Food assimilation results in energy, which can then be used for reproduction, growth or maintenance (life-history traits). The DEBtox is a toxicological application of the DEB theory which attempts to assess the effects of pollutants on life-history traits over time.
The key challenge is how to infer the impact of toxic effects observed in individuals and apply it to an entire population. Elise Billoir and her team combined the following tools to extrapolate the individual effects to the entire population:
The DEBtox is a good way of modeling survival, reproduction and growth continuously as a function of time and exposure concentration and at the individual level.
The population growth rate (which incorporates lethal and sublethal effects), is the best parameter to evaluate the risk of a pollutant on a population, hence matrix population models are a useful tool. Billoir explains that in population matrix, population is divided into classes based on development stage, and individuals transfer from a class to the next one depending on their survival and their fecundity.
By combining DEBtox theory and matrix population models, it is possible to extrapolate every effect of the toxic compound on the individual to the population level (as explained in the synthetic diagram below).
This technique used by Billoir, has not yet been used in an ecotoxicological context. It consists of reorganizing all the age-specific information in a stage-specific way. This way it makes possible to compare the sensitivity in the face of cadmium and in relation to the age of the individual.
Diagram of the method developed in the study of Billoir et al.
In this case, the sensitivity analyses showed that the effects of cadmium at the individual level were not significant but the application of the model proved that the population growth rate is highly affected through the cadmium contamination. Moreover, we think that this model could be applied to similar aquatic organisms and other pollutants such as heavy metals and could be useful to enhance existing bio-indicators of water quality.
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