It can harm small organisms at the very least. Things like zooplankton can have their bodily processes interrupted by ingesting the materials. This indicates it can result in death: http://www.ices.dk/news-and-events/symposia/zp6/Pages/Effect...

Other studies indicating harm are available, while others have shown mixed results. For example, one where polystyrene microplastics depress and then appear to stimulate growth of algae: https://www.sciencedirect.com/science/article/pii/S004565351...