What toxicants can teach us about metabolic organisation

Tjalling Jager, Vrije Universiteit Amsterdam, Dept. Theoretical Biology

Organisms that are exposed to chemicals respond with changes in their life-history behaviour.
These unlucky organisms may, for example, show a deviating growth curve compared to their 
unexposed relatives, and differ in the timing and rate of their reproduction. Clearly, such 
changes must reflect underlying changes in their energy budget. It is therefore essential to 
view toxicant effects as the response of a stressed system, where metabolic processes change 
under the influence of toxicants. The study of toxicant effects on (non-mammalian) organisms 
is the realm of ecotoxicology. Unfortunately, a process-based view is rare among 
ecotoxicologists, who generally appear to prefer a more descriptive view of toxic effects. In 
contrast, scientists working on metabolic organisation usually have little interest in 
toxicants. However, I will argue that the combination of energy budget theory and toxicants 
is a fruitful one for both fields. Firstly, existing theory on dynamic energy budgets can be 
applied to the practical problems of dealing with the enormous number of man-made chemicals, 
and aid in assessing their impacts on human and environmental health. Regulatory decisions 
require extrapolation from the results of laboratory tests to populations under field 
conditions, and any sensible extrapolation requires a thorough understanding of the system. 
Secondly, and more interesting scientifically, chemical stress offers a unique opportunity to 
test the allocation rules of energy-budget theory. Because of the huge diversity of chemicals, 
and their wide range of toxicity mechanisms, the organism can be stressed in far more (and far 
more specific) ways than with natural stressors such as food limitation. 

The application of DEB theory to toxicants has become known as DEBtox. DEBtox started as a 
pragmatic simplification of DEB theory to be able to work with the data from standardised 
toxicity tests. However, in the last five years, we have taken the development of DEBtox 
further to analyse full life-cycle tests and mixtures of toxicants. The comparison of 
theoretical predictions to (dedicated) experimental data sets confirmed the usefulness of DEB 
theory, but also revealed limitations and raised new research questions. In this lecture, I will 
reflect on the history of DEBtox, provide an overview of the recent developments on life-cycle 
analysis and mixtures, and focus on the avenues that need to be explored in the near future.