Assessing the impact and fate of microplastics on terrestrial isopods and springtails.

Porcellionides pruinosus isopod consuming polyethylene plastic that has been weathered in soil and water.

With growing awareness that microplastics (fragments smaller than 5 mm) are an increasing contaminant of concern in terrestrial ecosystems, it’s important that we understand the impact this may have on terrestrial organisms. My research involves terrestrial detritivores such as isopods and springtails. Detritivores play a vital role in leaf-litter systems, breaking down decaying organic matter and unlocking nutrients for other organisms and plants. This makes it vital that we assess the potential impact microplastics may have on their health and wellbeing.

In my experiments thus far, I have exposed three isopod species (Porcellio scaber, Porcellio laevis, and Porcellionides pruinosus) commonly found in North American leaf litter ecosystems to polyethylene plastic fragments derived from an everyday pool noodle. Initial experiments were conducted to assess whether these isopods would readily consume the plastic fragments and generate microplastics after ingesting them. Resulting data shows the isopods readily consumed the plastic fragments so long as they were weathered in soil and water, likely due to the organic matter growing on the substrate. Pristine polyethylene fragments were ignored entirely.

Plastic fragments used in isopod polyethylene consumption experiments were derived from an everyday common pool noodle (left). Cut fragments of the pool noodle (middle) are then weathered using soil and water for treatment vessels (right).

Example of an experimental vessel for isopod plastic fragment consumption experiments. Images show the contents of the vessel at day 0 (left) and day 7 (right). Much of the weathered soil on the outer layer of the plastic fragment has been consumed. The waste and other vessel contents are filtered post exposure and characterized.

Example of the microplastics generated through isopod consumption; filtered for observation under a microscope.

Experimental results have shown that not only do all three species consume weathered plastic with no observed negative health effects (significant changes in growth, mobility, or mortality), but also generate microplastics at a rapid rate. Exposure durations have covered a range from 1 to 28 days, all with no observed negative impact. Each isopod generated hundreds to thousands of microplastic fragments within the 28-day study. This led me to wonder whether microplastics generated through these methods could be small enough to be ingested by smaller organisms such as springtails. Further study using juveniles had shown that though some microplastics did indeed fall into this range, though the number was not significant for these durations. In the future, I will also be repeating this experiment with a dwarf species of isopod as the size of microplastic fragment generated seems to directly correlate to mandible (and therefore bite) size. The hope is to better understand if there is a risk of trophic transfer through the food chain to smaller organisms.

Isopod digestive tracts are also dissected to assess and characterize microplastics within. The digestive tract is removed from the isopod specimen (left) and the contents are tweezed apart so the contents can be examined under magnification (middle). On the right is a microplastic filtered from a treatment vessel and observed under 5x magnification using a microscope.

Moving on from the consumption studies I aim to assess the impact of exposure to other common microplastics such as polyester fibres and tire particles on the isopods and springtails (Folsomia candida). My experiments will assess the potential toxicity of these microplastics through exposure and direct consumption. Along with the control vessels, experimental exposures will include spiking food in one set of experiments vs. spiking soil in another set by mixing in increasing concentrations of the contaminants.

Throughout my thesis I aim to better understand the fate and impact these microplastics have on leaf-litter systems. Experiments consider two scenarios; 1) exposure as a result of environmental contamination, and 2) the potential fate of microplastics directly generated by terrestrial isopods. In the first scenario, we consider how microplastics commonly enter terrestrial systems, such as the application of wastewater or sludge (often contaminated with microplastics) to agricultural soils, discarded waste weathering, and drift of particles from a source (i.e., tire wear particles). For the second scenario we consider whether there is a potential risk to smaller organisms exposed to microplastics generated by larger detritivores such as the isopods. While isopods can ingest microplastics ~<1,000 um in size, springtails (F. candida) need them to be much smaller to readily ingest.

These is also the potential for generational experiments with springtails to better understand the lasting effects exposure and consumption have on reproduction and juvenile development. It is hoped as well, though literature is slim on conducting generational and reproduction experiments for isopods, that these may also provide more conclusive evidence around the fate of microplastic contaminants.

Written by Christopher Musgrave