Microplastics colonised by toxic plankton in brackish ecosystems

Microplastic pollution is an increasingly widespread issue in both freshwater and marine ecosystems across the world. It is estimated that more than 8.3 billion tonnes of plastic has been created since the 1950s, with more than half of this figure produced in the last 13 years. Plastic pollution is an issue which spans terrestrial, freshwater and marine ecosystems: it is estimated that a fifth of the plastic entering the world’s oceans is transported by rivers.

Plastic fragments smaller than 5mm are known as microplastics. Research suggests that because of their potential uptake and transmission through food webs, microplastics prompt a significant, but poorly understood, threat to aquatic life.

A new study adds to this growing knowledge base, suggesting that microplastics are readily colonised by aquatic microorganisms, which could lead to the growth of toxin-producing plankton species.

Researchers from the Leibniz-Institute of Freshwater Ecology and Inland Fisheries (IGB) and the Leibniz Institute for Baltic Sea Research, Warnemünde (IOW) incubated polyethylene and polystyrene microplastics with natural microfauna at different stations in northern Germany: the Baltic Sea at Heiligendamm, the lower River Warnow and a wastewater treatment plant.

Their results, published in the Frontiers in Microbiology journal, show how microplastics act as valuable micro-habitats for microorganism colonisation, with over 500 different species observed from the 15-day experiment. The researchers suggest that a plastic item weighing one gram, floating in the sea, can support more living organisms than a thousand litres of surrounding seawater.

“Microplastics may represent a significant habitat and transport medium for microorganisms. Our experiments showed that microorganisms, such as Pfiesteria piscicida, enrich on plastic items, where they exhibit much higher densities than in the surrounding water or on driftwood,” said Dr. Maria Therese Kettner from IGB.

The dinoflagellate Pfiesteria piscicida highlighted by Dr. Kettner was the most common coloniser of microplastics in the study. It was observed at densities around fifty times as high as in the surrounding water, and around two to three times as high as on comparable wood particles.

Pfiesteria piscicida is a potentially toxic plankton species – its Latin name means ‘fish killer’ – which presents a significant threat to human and animal life at high concentrations. In addition, the dinoflagellate Heterocapsa – known for toxic ‘red tide’ blooms which can cause mass mortality of bivalves – was observed as another microplastic coloniser. In short, the study suggests that microplastic pollution may act as a catalyst for toxic plankton blooms.

IGB researcher Prof. Hans-Peter Grossart, who led the study, highlighted another key finding: “Unlike natural substances such as wood or colonies of algae, microplastic particles decay extremely slowly, and may therefore transport the organisms they host over long distances.” In other words, microplastics may provide a durable dispersal medium for harmful microorganisms, which could potentially be carried over long distances by rivers and ocean currents.

“However, communities on microplastic particles often change when they ‘travel’ and adapt to their new environment,” said marine microbiologist Dr. Matthias Labrenz. “Therefore, these aspects need further investigation,” suggested the IOW researcher. Research and policy on ‘invasive’ aquatic species often focuses on transport mechanisms such as shipping: this study suggests that microplastics may act as a vector for introducing potentially harmful microorganisms into new environments.

The researchers raises a third issue, suggesting that colonised microplastics have the potential to change carbon, nutrient and energy dynamics in aquatic environments. Recent research suggests that cyanobacteria which have colonised microplastics exhibit increased photosynthetic activity. Dr. Kettner and colleagues suggest that since numerous algae species were detected in this study, an increase in photosynthetic activity in areas of microplastic pollution could be expected.

The research team highlight that colonised microplastic particles add to the overall load of organic aggregates in marine environments. Some of these particles are likely to be transported vertically between the sea surface and floor by ocean currents. As a result, the researchers suggest that microplastic colonisation observed in the study has the potential to affect the oceanic carbon pump and vertical fluxes of nutrients in marine environments.

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Marie Therese Kettner; Sonja Oberbeckmann; Matthias Labrenz; Hans-Peter Grossart, (2019) “The eukaryotic life on microplastics in brackish ecosystems” Frontiers in Microbiology, 10 art. 538 (open-access)