Factors influencing the feeding responses of reef-building corals to microplastics

Plastic pollution has become a problem for many marine organisms. The omnipresent microplastics (MPs) are of particular concern due to their effect on a variety of species, including reef-building corals. Corals have been found to interact with MPs, ingest, and even incorporate them into their skeletons. However, knowledge on what affects the responses of coral polyps to these indigestible particles is still constrained. Here, we aimed to unravel factors influencing the feeding responses of coral polyps to MPs. Particularly, we (I) assessed coral feeding responses (i.e., reaction, ingestion, and incorporation) to different MP polymers and shapes in the presence vs. absence of a biofilm and food during feeding trials. We (II) characterized polymer-biofilm-associated compounds to infer potential mechanistic processes underlying the observed differences. We (III) evaluated effects of food availability on polyp activity under continuous MPs exposure. We found that the polymer type most strongly influenced the feeding response, and that MP shape additionally affected ingestion and incorporation. While we detected species-specific differences in responses, the presence of a biofilm and food had only minor effects. We identified 11 polymer-biofilm-associated compounds with potential stimulating and toxic influence. Polyp activity was not altered by the prevalence of natural food. These findings suggest that the particle recognition mechanisms of corals are, although generally well suited to differentiate between MPs and natural particles, rely primarily on chemoreception of substances leaching from MP polymers, with mechanoreception contributing to shape-specific responses. We conclude that future assessments of coral-MPs interactions should consider polymer- and shape-specific effects, as these appear to be the key driver of the coral feeding response to MPs.