Effects of Different Microplastics on Nematodes in the Soil Environment: Tracking the Extractable Additives Using an Ecotoxicological Approach

With
increasing interest in the effects of microplastics on the soil environment,
there is a need to thoroughly evaluate the potential adverse effects
of these particles as a function of their characteristics (size, shape,
and composition). In addition, extractable chemical additives from
microplastics have been identified as an important toxicity pathway
in the aquatic environment. However, currently, little is known about
the effects of such additives on the soil environment. In this study
on nematodes (Caenorhabditis elegans), we adopted an ecotoxicological approach to assess the potential
effects of 13 different microplastics (0.001–1% of soil dry
weight) with different characteristics and extractable additives.
We found that poly­(ethylene terephthalate) (PET) fragments and polyacrylicnitrile
(PAN) fibers show the highest toxicity, while high-density polyethylene
(HDPE), polypropylene (PP), and polystyrene (PS) fragments induced
relatively less adverse effects on nematodes. In addition, low-density
polyethylene (LDPE) induced no toxicity within our test concentration
range for the acute period. Acute toxicity was mainly attributed to
the extractable additives: when the additives were extracted, the
toxic effects of each microplastic disappeared in the acute soil toxicity
test. The harmful effects of the LDPE films and PAN fibers increased
when the microplastics were maintained in the soil for a long-term
period with frequent wet–dry cycles. We here provide clear
evidence that microplastic toxicity in the soil is highly related
to extractable additives. Our results suggest that future experiments
consider extractable additives as key explanatory variables.