Influence of Microplastics on the Nutritional and Locomotive Activity of Dinoflagellate Oxyrrhis marina in the Experiment

The incorporation of microplastic particles (MPs) into the microbial food chain and their impact on physiology of consuming organisms has been largely underexplored. The heterotrophic dinoflagellate Oxyrrhis marina serves as a good model for understanding these processes. In this work, flow cytometry methods were 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. In addition, the effect of the diets containing these components on the swimming speed and movement patterns of Oxyrrhis marina cells was evaluated using a computer method for analyzing video recordings of O. marina movement. It was shown that from the first minutes of the experiment, the dinoflagellates actively consumed both the preys, 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/mL, while Isochrysis galbana cells were almost completely grazed, and their abundance decreased by more than two orders of magnitude, from 4.9 · 105 cells/mL to 2.3 · 103 cells/mL. Such dynamics were associated with compensation for the number of microspheres in the medium due to their excretion and repeated phagocytosis by Oxyrrhis marina. The increase in the size of dinoflagellate cells, which was a consequence of the consumption of plastic microspheres, did not lead to a noticeable decrease in their mobility and impaired locomotion. ‘Unproductive’ feeding of the dinoflagellates on microplastics did not supply them with nutrients and was the reason for a statistically significant decrease in their abundance (compared to the control and experiment with microalgae). This seemed to be due to the unreasonably high energy consumption of their population for constant search, phagocytosis, and excretion of microspheres. There were no signs of the predator’s rejection of such an unproductive nutrition strategy; on the contrary, cell mobility increased over time, which only worsened the situation. Such processes can have far-reaching negative consequences for the entire food chain. In particular, microplastics “packaged” by unicellular organisms can be transported to higher trophic levels and accumulate in mollusks, fish, and larger predators.