Microplastics limit energy production of small freshwater species

A new study shows that microplastic pollution reduces the energy production of a microscopic creature found in freshwater worldwide. Paramecium bursaria contains algae that live in their cells and provide energy through photosynthesis. A new study by the University of Exeter tested whether severe microplastic pollution in water could affect this symbiotic relationship.

The results showed a 50% reduction in net photosynthesis, which significantly affected the algae’s ability to generate energy and release oxygen.

“The relationship I’ve researched, known as photosymbiosis, is prevalent in both freshwater and oceans,” said lead author and Associate Research Fellow Dr Ben Makin at Exeter’s Institute for the Environment and Sustainability at Penrhyn Campus in Cornwall.

“We know that climate change can damage photo-symbiotic relationships, including corals (leading to “bleaching” events).

“Recent research has shown that microplastics (plastic particles smaller than 5 mm) can interfere with photosymbiosis, but more research is needed, particularly in freshwater habitats.”

“In this study, I chose a ‘classic’ example of photosymbiosis – Paramecium bursaria is found in freshwater around the world, and a similar symbiotic relationship exists in the oceans.”

The study placed Paramecium bursaria in water contaminated with microplastics in the lab. The contamination level was higher than normally seen in the natural world, but was detected in some places.

“At this stage, the question I wanted to answer was whether severe pollution could affect this important relationship and what those effects might be,” explained Dr Makin.

“This is a pilot experiment and more research is needed to test different concentrations of microplastics, different types of plastics, and their effects on different species.”

The findings do not show how microplastics cause the observed effects on photosymbiosis. It is possible for Paramecium bursaria to ingest plastic particles and/or chemicals from plastic that affect biological processes. The study followed the effects of microplastics (compared with control water samples without microplastics) on growth rate, symbiont density (how many algae were in each Paramecium bursaria), metabolic rate, feeding rate and net photosynthesis.

Only pure photosynthesis is significantly affected, but this can be important because it provides energy to the organism and promotes the “trade” of nutrients, which allows photosymbiosis to be sustained. It is noteworthy that this type of relationship is responsible for a significant part of photosynthesis worldwide – about half of all photosynthesis in the ocean is supported by photosymbiosis.

“These results may raise concerns about the significant contribution of photosymbiosis to primary production at the global level,” said Dr Makin. “Microplastics are a common pollutant and their effects on photosymbiosis, particularly in freshwater, are rather poorly characterized.”