By Tania Fitzgeorge-Balfour, science writer

Example of marine dinoflagellates (Dinophysis sp., not the species studied in the paper). Image credit: Rattiya Thongdumhyu/Shutterstock.com

Plastic pollution is not just a problem for larger marine animals. Dinoflagellates, which are single-celled marine predators, can also ingest microplastic which in turn limits their growth and consequently their overall abundance. This has implications for the larger animals that feed on these microscopic predators, in terms of available food and the transfer of energy up the food chain.

Microscopic marine predators can ingest microplastic, which in turn lowers their growth and overall abundance, finds a recent study published in Frontiers in Marine Science. This has implications for the larger marine animals that feed on these tiny predators, both in terms of available food and the transfer of energy up the food chain.

“The plastic pollution of our oceans isn’t just affecting whales and sea turtles, it also impacts the small, microscopic animals towards the bottom of the food chain,” says Susanne Menden-Deuer, coauthor of this research and a professor at the Graduate School of Oceanography, University of Rhode Island, US.

“Our study shows that some single-celled marine predators called dinoflagellates ingest microplastic particles, and when they do, they grow 30% more slowly than those that eat just their typical algal prey. Consequently, this can almost halve the abundance of dinoflagellates, which will have an impact on the larger marine animals that feed on these tiny creatures.”

Small but mighty plankton

Dinoflagellates may be amongst the smallest predators in the ocean, but they play a huge role in grazing on phytoplankton – the single-celled algae at the base of the marine food web – and moving this food energy up the food chain to larger animals.  

“We know that microplastics the size of phytoplankton exist in the ocean, so we wanted to understand if dinoflagellates ingest these particles thinking they are phytoplankton, and if doing so impacted their growth and ultimately the wider marine food web,” explains Victoria Fulfer, who completed this work as part of her doctoral studies at the University of Rhode Island.

The researchers separately incubated three different species of dinoflagellates with algal prey or high concentrations of microplastics in bottles of seawater to see if they ingested the plastic particles. They compared the growth of the dinoflagellates feeding on algae with those fed on plastic.

‘Spider-Man' dinoflagellate avoids plastic

“Two dinoflagellate species ingested the microplastics, although one did show a preference for their natural algal prey, suggesting not all of these tiny predators are affected equally. We then observed that the growth of the dinoflagellates that ate the plastic was considerably reduced,” explains Susanne Menden-Deuer. “Interestingly, one dinoflagellate did not eat the microplastic particles. It has a different feeding mechanism to the others. Like Spider-Man, it throws a web around its prey and digests it externally, which would not work for microplastics.”

Fulfer continues: “The reduced growth by the dinoflagellates that ingested microplastics means that the amount of energy transferred up the food chain is also reduced. It is clear the effects of microplastics at the base of the marine food web can have wide-spread consequences, but more research will be needed to understand how far the effects we have seen in this study will radiate through food webs and how soon we might be able to see those effects in the ocean.”

Menden-Deuer says the team also hope to examine how plastic aging may affect the predator’s plastic preference. “We know that bacteria grow on microplastics over time, changing the biology and chemistry on the plastic surface. Much of the signaling and communication in the plankton happens through chemicals. We expect that aged plastics would be eaten even more vigorously than the pristine ones we offered in our experiments.”

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