The shape and smell of microplastics may influence whether they are eaten by tiny but vital marine zooplankton, a new study suggests.
Marine zooplankton (drifting, mostly microscopically small, creatures) are an important part of the marine food web and one of the groups of organisms that are likely to be affected by the increasing amount of microplastics in the ocean. Despite this, the underlying mechanisms that influence how zooplankton can end up eating microplastics are still poorly understood.
The team behind the new study, from Plymouth Marine Laboratory, University of Essex, and University of Plymouth, investigated how the microplastic ingestion rate was affected by different shapes of plastic and whether it was affected by the presence of chemicals derived from marine algae that can colonise the surface of microplastics.
Three species of plankton were used in the study: copepod species Calanus helgolandicus and Acartia tonsa; and the larvae of the European lobster Homarus gammarus. Their feeding habits were studied using bead, fibre and fragment-shaped microplastics to understand whether there was a preference towards certain shapes.
Additionally, some of the microplastics were infused with chemical compounds (including dimethyl sulfide, or DMS) that are produced by marine algal species. Such small algal species are able to live on the microplastics as part of the biofilm that covers the surface over time. The algae are a food source for zooplankton, so the researchers suspected that these chemicals, which may mimic the smell of natural prey, could attract the zooplankton to consume the microplastics.
The results showed that each of the three zooplankton species had a preference for a different microplastic shape. Copepods C. helgolandicus and A. tonsa went for fragments and fibres, respectively, whilst the larval lobsters preferred the beads. This indicates that the different feeding strategies of the zooplankton may well be influencing which microplastics they end up consuming.
Importantly, the presence of the algal-derived chemicals had a clear effect on the plankton's feeding. The H. gammarus larvae and A. tonsa copepod both ate more fibres and fragments that were infused with DMS than not, whilst the C. helgolandicus’s microplastic ingestion increased across all shapes when infused with DMS.
The study shows how important the feeding strategies of different zooplankton are when it comes to their likelihood of eating marine microplastics and, in particular, suggests that species using chemosensory clues to find their food are at an increased risk of ingesting microplastics.
Zara Botterell, lead author and a PhD student with PML and the University of Essex, said: "When we're looking at the impacts of marine microplastics, it's important to remember the different ways that marine organisms can interact with them. As microplastics age in the sea, they build up biofilms which, as our study suggests, could be playing a significant part in the likelihood of zooplankton eating the microplastic.
"We need to understand the interaction between plastic and marine animals to predict the effects plastics are having on the marine environment, to support both efforts to reduce marine plastic pollution, and the wider work going on around conserving our fragile ocean ecosystems."