Influence of Microplastics on the Nutritional and Locomotive Activity of Dinoflagellate Oxyrrhis marina under Experimental Conditions

The incorporation of microplastic particles (MPs) into the microbial food web and their impact on physiology of consuming organisms remains largely underexplored. The heterotrophic dinoflagellate Oxyrrhis marina is an excellent model for understanding these processes. In this study, flow cytometry was used to analyze the dynamics of consumption by this predator of its natural prey, the microalga Isochrysis galbana (ISO), and plastic microspheres (MS) of the same size. Furthermore, the effects of the diets containing these components on the swimming speed and movement patterns of Oxyrrhis marina cells were assessed. This was achieved using a computational method for analyzing video recordings of O. marina movement. It was observed from the first minutes of the experiment that the dinoflagellates actively consumed both types of prey, but by the end of the experiment, the number of MS in the medium decreased to a lesser extent, from 4.4 to 2.2 × 105 cells/mL, while Isochrysis galbana cells were almost completely grazed, with their abundance dropping by more than two orders of magnitude, from 4.9 × 105 cells/mL to 2.3 × 103 cells/mL. Such dynamics were associated with a compensatory increase in the number of microspheres in the medium due to their excretion and repeated phagocytosis by Oxyrrhis marina. Interestingly, the ingestion of plastic microspheres, which led to an increase in dinoflagellate cell size, did not significantly reduce their mobility or impair their locomotion. The ‘unproductive’ consumption of microplastics, which provided no nutritional benefit, resulted in a statistically significant reduction in dinoflagellate abundance compared to controls and the microalgae-fed experiment. This decline was attributed to the high energy expenditure of the population in constantly searching for, phagocytosing, and excreting microspheres. Contrary to expectations, the predators did not abandon this unproductive feeding strategy. Instead, their cell mobility increased over time, exacerbating the situation. These processes could have far-reaching negative implications for the entire food web. Specifically, microplastics 'packaged' by unicellular organisms could be transferred to higher trophic levels, potentially accumulating in mollusks, fish, and larger predators.