How could microplastics accumulate in the bodies of marine mammals?

Question

I have read several literature reviews and studies on the effects of microplastic particles on fish and invertebrates (one example includes the review by Franzellitti et al. (2019)) and there are multiple instances where these particles where found in the nervous system, lymph, and other organs following controlled exposures.

There are also multiple studies that looked at microplastics in marine mammals, but usually they focus on the contents of the gastro-intestinal tract. My question is: how these particles are likely to accumulate in the bodies marine mammals compared to smaller organisms - could the size of the blood vessels of larger whales influence the uptake of microplastics? Some researchers recommend studying this by using marine mammal cell cultures, but I am wondering how well the results of such studies can reproduce what might be happening at a full-body scale. (I study mainly ecology and the behaviour of marine mammals, so my knowledge of differences between various organisms at a cellular level is limited).

Thank you!

Answer 1

Microplastics are bioavailable to marine organisms, through direct ingestion and/or via trophic transfer. Most studies of microplastics in marine mammals only analysed microplastics in the digestive tract and scats (see Zantis et al. 2021 for a review) and thus, at this stage, there is no evidence that microplastics can cross the intestine barrier and/or enter the blood stream and accumulate in different organs in marine mammals. However, in some marine vertebrates (fish), microplastics have been found in other organs (e.g., brain, liver).The small size of microplastic might allow them to pass through the intestinal epithelium barrier through cell internalization and possible subsequent translocation. However, information concerning the extent of ingestion, accumulation, translocation into organs and possible pathways of transition into cells is still unclear (Wright et al. 2013).

Interestingly, it has been very recently demonstrated that microplastics are found in human bloodstream (Leslie et al. 2022). It is scientifically plausible that plastic particles may be transported to organs via the bloodstream. Regarding your question, "how these particles are likely to accumulate in the bodies marine mammals compared to smaller organisms", it's a difficult one. You need to consider the exposure route (direct ingestion and/or via trophic transfer) and the particle size (and the bioaccumulation and biomagnification processes).