ENVS 149 Effects of toxic chemicals on developing seeds
Toxic Assays
In toxicity assays, we attempt to determine the dose of a chemical substance required to cause acute effects on living organisms. In performing toxicity assays, there are many considerations in experimental design.
When selecting a species for toxicity testing, usually “indicator species” are chosen. An indicator species is a species of organism that is used in the laboratory to act as an indicator for what may be happening in a more complex ecosystem. The choice of indicator organisms varies depending on the problem. For example, if we were concerned with water pollution, we would use aquatic organisms. If we were concerned about a chemical leaking into the soil, we might use a plant. For human toxicity, ideally we’d use humans, but since this is obviously not ethical, we use “model species.” A model species is used as a surrogate for the species of interest. Often rats and mice are used as model species for humans.
Once a species is chosen, the dosing method must be determined. Chemicals can be applied dermally (through the skin), given in food or water, injected, or supplied in the air or water in which the organisms are living. When using living organisms, there are also different endpoints that can be considered. Usually, the easiest is death. We can also use other endpoints such as growth rate, birth rate, birth defects, a biochemical change (e.g. hormone levels) or cell function.
When examining the results of a toxicity assay, we can use simple statistical methods to evaluate data. For example, the average (mean) is calculated. This average can be the number of surviving organisms, average shoot length, average life span, average egg laying rate, etc. In addition, we can calculate a standard deviation and the coefficient of variation (CV) for each average. The CV is the standard deviation divided by the mean and is a relative measure of data dispersion compared to the mean. The smaller the CV, the more reliable the data. In toxicology, we generally like the CV to be less than 5%. It is then common to use graphical methods to display the data. The most common way to display data for toxicity assays is a dose-response curve, where dose is plotted on the x-axis and response is plotted on the y-axis. From this type of graph we can estimate a number of variables. The LD50 is the dose of chemical that causes death in 50% of the population. Similarly, the ED50 is the dose of chemical that causes an effect in 50% of the population. In addition, the NOEL is the concentration of chemical that causes no effect (no observed effect level).
Please take a look at the lab handout before coming to lab. There will be a couple of pre-lab questions directly from it and it will be helpful to have read the procedure at least once before executing the steps. In addition read the following handout from a different textbook – Environmental Science: Principals, Connections and Solutions by G. Tyler Miller Jr. The supplement focuses on the LD50 dose response curve.
Handout: Toxic Seeds Part 1_2015
Supplement: Milller_sup17_Toxicology
Pre-Lab questions –
Question 1 (4 points):
What is the difference between a LD50 and ED50 experiment?
Question 2 (2 points):
According to the Miller supplement (pdf above) , what a does of 1 mg/kg of nicotine mean for an experimental population?
Question 3 (2 points):
What are two factors that can affect the toxicity of chemicals in humans?
Question 4 ( 2 points):
How many seed types and chemical types will you use in your experiment?