English abstract
Microplastics (MPs) have emerged as contaminants of concern as a consequence of
massive increases in plastic production over the past 50 years. MPs have been detected in
all ecological habitats, but aquatic habitats are the most affected. MPs can cause direct
toxicity to aquatic organisms and can also act as vectors that increase the bioavailability of
other pollutants. Cadmium (Cd2+) is a widespread and highly toxic metal with a high
chance of co-existing with MPs in aquatic environments. The aim of this study was to
evaluate the direct and indirect toxic effects of MPs by single and combined exposure of
MPs and Cd in zebrafish. High-density polyethylene pellets, either virgin or naturally aged,
were ground to micro-size particles. The first part of this study measured the Cd adsorption
capacity of virgin and aged MPs. The aim of the second experiment was to measure the
effects of the two types of MP, alone, pretreated with Cd, or in combination with ionic Cd,
on the bioaccumulation of Cd, levels of metallothionein, behavior, and histopathology of
adult Zebrafish (Danio rerio).
The adsorption capacity (qe) experiment was done by incubating MPs in Cd solutions and
measuring Cd decrease from solution at different time intervals. This experiment showed
that aged MPs had higher adsorption capacity than virgin MPs which is most likely to be
caused by the change in MPs physiochemical properties.
In the second experiment, zebrafish were exposed to an environmentally realistic
concentration of MPs (0.1% MPs enriched diets) and Cd (50 µg/L) and a combination of
the two for 21 days. Fish mortalities were recorded due to waterborne Cd2+ exposure while
MPs had no effect on survival. Fish growth and development were not affected. Compared
to a waterborne Cd alone, exposure to MPs and waterborne Cd resulted in higher Cd
bioaccumulation (specifically in males) while MT level was not affected. There was a
minimal transfer of Cd from pre-treated MPs. Anxiety-like behavior increased significantly
after co-exposure to MPs and waterborne Cd. The zebrafishes social tendency decreased
slightly in all treatments but only dietary exposure to aged MPs showed a statistically
significant effect. Slight impacts on fish boldness and shoaling behavior were observed.
Gills and liver histopathology showed slight alteration while most histopathological lesions
were in the intestine. Histopathological results indicated the capability of Cd to desorb out
MPs and cause toxicity.
Overall, the study supports the hypothesis that MPs can act as vectors for Cd. Interestingly,
MPs increased the uptake of waterborne Cd by zebrafish but did not appear to be direct
vectors, suggesting they acted by an indirect mechanism. Noteworthy, pre-treated MPs had
no distinct impacts on zebrafish mortality, Cd uptake, and behavior but at the
histopathological level, they had higher impacts. This study demonstrates that MPs can
enhance the bioaccumulation of Cd, but further study is needed to elucidate the mechanism.
