**6. Conclusion**

The microplastics in the marine environment pose adverse effects on marine organisms, which eventually impact human health. Therefore, for the well-being of humans as well as the conservation of the environment, microplastic pollution is extensively investigated by researchers and scientists around the world. This study summarizes the sources of microplastics (primary and secondary), along with their characterization based on chemical composition, size, and shape. The abiotic and biotic degradation of these microplastics is discussed, showing how various macroplastics (i.e., plastic debris) break down into smaller fragments under the effects of various environmental factors (e.g., temperature, sunlight, and biological agents), chemical damage, and mechanical abrasion. Once formed, the marine habitat serves as the primary sink for the microplastics. The distribution and fate of the microplastics in the marine environment depend on the density, size, shape, and chemical composition of the microplastics, as well as the environmental factors (primarily wind and ocean current velocities). If the density of the microplastic is lower than that of the regional water, the microplastics remain suspended in the gyre (surface waves) and are prone to horizontal distribution because of wind and ocean current velocities. If the density of the microplastic is higher than that of the regional water or its density increases because of biofouling and aggregate formation, it would sink to the bottom of the marine habitat. Once sunk, microplastics can either accumulate in the marine sediment, or they can be redistributed because of bottom water current or bioturbation. Therefore, it is challenging to predict the fate of marine microplastics and requires the attention of researchers to fill the knowledge gap, specifically on the ecotoxicological impact of microplastic on the marine environment. An investigation is needed to study the mechanism of microplastic and chemical pollutant sorption by marine organisms as well as their mode of interaction, evaluate the route of transfer of these contaminants along the food web, and investigate the risk of microplastics on marine organisms as well as human.
