**1. Introduction**

Plastic pollution was one of the biggest environmental challenges until the discovery of microplastics (MPs) in the early 21st century. While plastics are easily visible and their environmental impacts are well documented, MPs are not visible and their ecological impacts are less understood [1].

MPs are exceptional pollutants with a broad range of individual properties. For instance, they are made of different polymers with different densities and chemical compositions (there are currently more than 5,300 types of synthetic polymers); they exist in variable shapes (fibres, fragments, foams, films, spheres, flakes, foils, sheets and granules) and are found in a wide range of sizes. These heterogeneous properties result in heterogeneous behaviour, fates and effects that are far more complex compared to other environmental pollutants. To add to this complexity, their properties and behaviour can also change over time, thus their ecological effects [1–3].

According to their physio-chemical properties, MPs are distributed differently in aquatic environments, which makes them available for uptake by a wide range of aquatic biota including plants. MPs are reported to interact with aquatic plants and accumulate into plants' tissues. This enables them to penetrate aquatic food webs at multiple trophic levels and ecological niches. Yet, the degree and type of effects that they cause when consumed by organisms depend on their properties including polymer type, size, shape and colour, as well as their constituent chemicals [4–8].

Due to their greater surface area, MPs have a propensity to adsorb other pollutants such as metals, pharmaceuticals and persistent organic pollutants (POPs). They also host pathogens, such as bacteria and viruses, thus, providing an additional pathway of exposure of aquatic species to contaminants. In other words, MPs can serve as a micro-vector for a mix of toxic chemicals and pathogens [9–14].
