In coastal areas, and especially in estuaries, organisms are constantly exposed to a variety of suspended particles. The particles are mostly of natural origin, i.e. fine sand grains, diatoms shells, cellulose fibres or chitinous fragments. Since several decades, however, man-made synthetic plastic particles are accumulating in the environment. The later are of great interest in environmental and toxicological sciences as they are suspected to induce cellular modifications and stress in organisms upon ingestion. This situation raises a number of questions whether natural and synthetic microparticles induce similar effects and damage in cells and organisms. In order to investigate this issue, marine organisms with different feeding modes will be subjected to natural and synthetic particles. Blue mussels (Mytilus edulis) and brown shrimps (Crangon crangon) will be collected from the field, acclimatized to laboratory conditions, and exposed to natural and synthetic particles for 6, 24, and 48 hours. Microscopic analysis will be done to localize the particles in digestive organs. Histochemical and biochemical analyses will be used to detect stress markers in cells and tissues. The goal of this doctoral project is to understand the hazardous effects of microparticles and to differentiate between anthropogenic and natural items. The findings will be beneficial in estimating the actual hazard potential of microplastics and in defining actual threat boundaries for marine invertebrates.