Reference: Gainer, A., Hogan, N. & Siciliano, S.D. (2019). Soil invertebrate avoidance behavior identifies petroleum hydrocarbon contaminated soils toxic to sensitive plant species. J Hazard Mater, 361, 338-347. https://doi.org/10.1016/j.jhazmat.2018.08.086
Plants are vital to our existence, yet we can go days without really taking notice of them. Plants provide shade on a hot day, delicious foods to fill our bellies, oxygen to breath and beauty to our world. If you are anything like me, you struggle to understand what makes some plants thrive and others not. We know plants require water, soil and sunlight to live, but the qualities of each of these can determine their health.
In particular, what if the soil a plant relies on is contaminated with chemicals? A plant is unable to take a stroll to a better location or tell us something is wrong. Traditionally, scientists have performed lab experiments to test the toxicity of specific chemicals on plant test species. However, these tests are time-consuming, and with thousands of new chemicals introduced to market each year, the need for efficient toxicity screening methods is critical for the protection of plant species.
Is there a better way?
Invertebrates (a.k.a. spineless critters; think worms and insects) alert scientists to soil toxicity. An invertebrate avoidance toxicity test provides valuable information on soil toxicity much faster than plant toxicity tests. This test relies on an organism’s ability to actively avoid contaminants in their habitat. The invertebrate detects a chemical using their chemoreceptor organs (like a nose) and moves away from the source of contamination. It would be similar to the response you have when smelling rotten eggs; you detect the smell, cover your nose and mouth, and run in the opposite direction. Could an invertebrate’s behavioral response to contaminated soil predict the quality of a plant’s habitat?
Creating the Link
Amy Gainer and team set out to answer this question with a Canadian twist. Seventy-five percent of Canada’s forests lie within the boreal zone and beneath a portion of these forests lies the oil sands. The petroleum that is produced in the oil sands lends itself as the experimental soil contaminant to determine whether spineless critters provide a warning system for plant toxicity.
The research team focused on a variety of plant species important to the local ecology and agriculture. These included Northern wheat grass, lettuce, alfalfa, radish, jack pine and white spruce. For plants, the team exposed each plant to varying levels of petroleum contaminated soil and measured how much the plant grew above ground (shoot system) and below ground (root system). For the invertebrates, the team selected five species: 2 types of worms, 2 types of mites and a spring tail.
The invertebrate avoidance response test was performed in a container divided into two sections using a plastic divider. One side of the container was filled with clean soil and the opposing side with petroleum treated soil. The plastic divider was then removed, and the invertebrate species were added and allowed to roam freely. After 24-48 hours, the divider was inserted again to separate the clean and treated soils. Each soil type was removed from the container to extract and count invertebrates in each half of the container.
Which plant species were more sensitive to petroleum contaminated soil?
Surprisingly, lettuce and alfalfa were the most sensitive to petroleum in soil overall. Both the root and shoot systems displayed stunted growth at much lower petroleum concentrations then the other plant species. The tolerance of boreal species like the jack pine and white spruce was attributed to their long-life span. Boreal trees have slower growth rates which allows energy to be used for other activities such as chemical defense, nutrient uptake and detoxification.
Which soil invertebrate was the most sensitive to petroleum contaminated soil?
All the soil invertebrates, except a worm (E. crypticus), avoided the contaminated soil. Interestingly, E. crypticus demonstrated no avoidance response to the contaminated soil. This was most likely due to its ability to remain in tiny pockets of water within the soil protecting it from the oil-based product (oil and water don’t mix).
A mite, O. nitriens, had the most sensitive avoidance response among all invertebrates. Along with its sensitivity, this particular species of mite inhabits surface soils of boreal forests thus making it an excellent candidate for toxicity testing in Canada.
Can plant toxicity be predicted by using the soil invertebrate avoidance test?
There is indeed a link between invertebrate avoidance response and plant growth and habitat quality when assessing petroleum toxicity. The concentrations of petroleum in the soil that impeded growth in plants were in the same range to those that elicited avoidance in soil invertebrates.
Hooray! Now what?
Scientists can use the invertebrate avoidance test as a first step in screening potentially hazardous chemicals. This could decrease the cost and time associated with testing the toxicity of new chemicals or mixtures on plants. It is very promising!
Before scientists rely too heavily on this approach, it is necessary to determine if a similar link between invertebrate avoidance response and plant toxicity is seen using different types of contaminants such as metals. A significant amount of metal mining occurs within the boreal forests of Canada. Metals such as copper or uranium interact with an organism’s cells differently than petroleum.