Microplastic contamination reduces productivity in a widespread freshwater photosymbiosis

Microplastic (plastic particles <5 mm in size) contamination is ubiquitous in nature and known to interact with organisms ranging from microbes to mammals. Notably, recent studies have shown that microplastics may interfere with photosymbiosis, an ecologically important association that has suffered pronounced recent declines in the face of contemporary climate change. However, limited findings thus far have largely focussed on select marine associations. Whether freshwater photosymbioses may also be affected remains poorly understood. Here, I aimed to help bridge this gap by asking whether microplastic contamination impacts several traits (growth rate, symbiont density, metabolic rate and feeding rate) in a common, widespread freshwater photosymbiosis, the Paramecium bursaria-Chlorella spp. association. To address how productivity, an important ecosystem service provided by photosymbiosis globally, could be affected, I also measured changes in photosymbiotic net productivity (net photosynthesis rate). To do so, I exposed the symbiosis to microplastics (microbeads extracted from commercial face wash) under laboratory conditions. My key result was that, compared with non-contaminated control cultures, the contaminated symbiosis demonstrated lower net productivity. This response raises concern for primary production rates in freshwater ecosystems contaminated with microplastics, adding to an established story of widespread degradation associated with microplastic pollution globally.