Forgot to say, if it wasn't obvious: I'm not a microbiologist.

There are a few recent invivo studies on marine life showing several negative effects. I've seen also a few reviews from the past two years, for example: https://www.sciencedirect.com/science/article/pii/S258900422...

There is a lot of interest in microplastics lately so I guess we will soon know how harmful they really are.