A marine Plankton-Plastic Predation Loop: selective grazing, excretion and reingestion of microplastics by the rotifer Brachionus plicatilis

Microplastics are pollutants of global concern, e.g. disturbing trophic interactions as an indigestible prey. While experimental studies show that some zooplankton ingest microplastics (MP), less is known about MP trophic interactions with rotifer zooplankton. This study therefore investigates whether rotifers can discriminate against ingesting microplastic when presented with alternative phytoplankton prey, and whether rotifers can reingest excreted microplastic particles. A series of grazing experiments were conducted using the rotifer Brachionus plicatilis subjected to a prey field of only the phytoplankter Isochrysis galbana, only similarly sized polyethylene (PE) beads, or a mixture of both. The phytoplankton prey abundance decreased more rapidly than the MP, indicating phytoplankton preference over plastics, i.e. a selective grazing by B. plicatilis against MP. Nevertheless, it was observed that the rotifers ingested MP, including those that had been previously ingested and subsequently excreted by the rotifers, suggesting that B. plicatilis may reingest MP as long as they are available in their environment. Thus, we propose a theoretical Plankton-Plastic Predation Loop, where the same MP particles are ingested several times over. This loop might imply that even small amounts of MP could impact marine food webs by continually disturbing and hindering the ingestion of algae by zooplankton and as well enable their transference to higher tropic levels.