Toxistoichiometry: Integrating Ecological Stoichiometry and Ecotoxicology

In my current job I haven’t been doing a lot of original research, but that hardly means I am abandoning my wide range of research interests. I wanted to explain today a little bit about the “big idea” that I’ve been chipping away at for years. What follows are pieces from a talk I’m going to give to the Society of Environmental Toxicology and Chemistry in Milwaukee on November 12^th.

Evaluation of toxicity is most often done by the use of standard bioassays on commonly used test organisms. For example, you can pull up an EPA-approved method for determining the acute or chronic toxicity of any chemical to Daphnia spp. Theoretically, anyone in the world can follow the same protocol and get the same result. In reality, considerable differences occur between labs and even between Daphnia strains (not to mention different species), but this overall approach has been accepted for years as a means of estimating toxicity. The problem is that this method only estimates relative toxicity within a specific context. Relating the results of these short-term, laboratory tests to real-world scenarios is not only difficult, it may be impossible. For example, frequently used protocols often provide organisms with an abundance of high-quality resources, while in nature organisms are often faced with either low quantity or low-quality resources. A number of studies have tried to understand how the effect of a toxin varies with food quantity, but few have adequately addressed how food quality alters the effect of a toxin. Imbalances between the nutritional quality of a food source and an organism’s dietary needs are common in nature and appear to play a role in individual physiology, population dynamics, community interactions, and ecosystem processes. The study of how nutrient imbalances alter ecological relationships is often referred to as ecological stoichiometry. Organisms respond to changes in the stoichiometric ratio of elements in their food with corresponding variations in growth, reproduction, assimilation, and excretion. Essentially, nutritional imbalances have a large effect on ecosystem function.

My interest is in linking ecological stoichiometry with ecotoxicology. Initially, I have focused on how food quality affects acute toxicity in aquatic organisms. My collaborators at the University of Notre Dame and Trent University and I have shown that some toxins (iodine, cobalt, fluoxetine in particular) are stoichiometrically explicit, meaning the effects on organisms vary based on the quality of food the organism is getting. Other toxins do not appear to be stoichiometrically explicit (bendiocarb, triclosan, methanol) although it is obviously harder to prove.

The implications of stoichiometrically explicit toxins are presently unknown, but one can imagine a situation where the concentration of a toxin allowed by law is harmless when organisms are fed high-quality food, but detrimental when they are given poor quality food. Obviously, in nature species are often faced with food shortages or poor food quality, thus the actual effect of toxins may be much greater than the estimated effects. Evaluating the risk a chemical poses to an ecosystem therefore requires a more context specific approach. Are the receiving ecosystems frequently nutrient stressed? Are times of pollutant release going to coincide with occasions when ecosystems are nutrient stressed? I’m not sure whether regulatory agencies are equipped to permit in this way.

This kind of work is obviously just a first crack at the idea, and in many ways a very basic approach. Without this kind of base data, however, addressing more interesting questions becomes difficult. I believe that toxistoichiometry has the potential to provide a new axis of understanding for ecotoxicology.