Sorption of the common freshwater cyanotoxin microcystin to microplastics

Abstract

Interactions among multiple stressors in freshwater ecosystems remain poorly understood. In particular, microplastic and nutrient pollution are rarely studied in combination. However, microplastics are now commonly found in freshwater lakes with pervasive cyanobacterial harmful algal blooms, a common symptom of eutrophication. Cyanobacteria can produce various toxic secondary metabolites potentially lethal to humans, domestic and wild animals, and other aquatic life. Given the tendency of microplastics to accumulate waterborne contaminants, we assessed the sorption of the common freshwater cyanotoxin microcystin to four types of highly produced polymers: polyethylene terephthalate (PET), low-density polyethylene (LDPE), polystyrene (PS), and polyvinyl chloride (PVC). We performed laboratory batch sorption experiments with unaged and aged microplastics and an in-situ sorption experiment in a eutrophic lake. We found that waterborne microcystins sorbed poorly to microplastics under laboratory conditions irrespective of ageing; however, microplastics accumulated microcystins under natural conditions in a eutrophic lake. Further, we found that sorption capacity of microcystins differed among polymer types (LDPE ≈ PET > PS ≈ PVC), and affinity for plastic varied among microcystin congeners (MC-RR > MC-YR > MC-LR > MC-LA). Our findings led us to conclude that microplastics can directly interact with waterborne microcystins via sorption and also to hypothesize that microplastics may indirectly interact with cell-bound microcystins via cyanobacteria colonizing the microplastic’s surface. This work highlights a previously unexplored interaction between microplastics and a contaminant class prevalent in fresh water. Further, it contributes to a better understanding of the interaction between microplastic pollution and cyanobacterial harmful algal blooms, specifically the potential role of microplastics in influencing the environmental fate of toxins produced by cyanobacteria.