Algae and microplastics together in the American Great Lakes

Microplastics threaten aquatic habitats everywhere in the world. These particles smaller than one millimetre enter streams and rivers via runoff water and the effluents of wastewater treatment plants, as well as by the decomposition of macro-wastes abandoned on river banks. The lakes form both temporary and long-term storage area for plastic particles of different sizes during their journey to the ocean. Some 50 tonnes of plastic waste are discharged every year into Lake Geneva in Switzerland, of which only a sixth is carried away by the Rhone downstream. Water, sediments and aquatic organisms all bear the marks of this pollution. It is estimated that, every year, nearly 10,000 tonnes of plastic waste end in the American Great Lakes.

Microplastics hidden in green algae

Although more detailed knowledge must be obtained on the volumes and propagation of plastics in the ecosystem, a recent discovery by American researchers has made the issue even more complex. Published in May 2021 in the journal Environmental Pollution, the study establishes a correlation between the development of microplastics in the Great Lakes and the proliferation of a freshwater green alga, Cladophora. Introduced in the 1980s with the arrival of invasive mussels, this alga forms a submerged biomass covering a large surface area with a combined weight of 129,000 tonnes. The long, fibrous cell walls of this alga have a high capacity to sequester synthetic microfibres, a type of microplastic very common in water. Present in many fabrics, these microscopic fibres detach easily during washing, enter the wastewater and then aquatic habitats.

How do the microfibres interact with the algae?

To understand how these microfibres interact with aquatic vegetation and evaluate their volume withdrawn from water by the alga, scientists have taken samples of Cladophora on the coasts of Lakes Michigan and Erie, from stations located at depths of 3, 6, 10 and 18 metres in both lakes. The samples were taken at different times, one in spring, when Cladophora generally flowers, and the second in autumn, when it decomposes. The scientists have carried out two types of experiment: in the first, Cladophora was decomposed using hydrogen peroxide to isolate the microfibres trapped in the alga. In the second, samples of alga were combined with mixtures of microfibres (polyester, lycra, acrylic and cotton) in a laboratory. After their separation, the microfibres were counted in each sample to determine the speed at which they adhered to the alga. The researchers repeated the second experiment with alga that had been kept in a refrigerator for two months, to see whether there was a difference between the fresh alga and the alga that had begun to decompose.

These experiments showed that the alga contained large quantities of microfibres, whatever the type of plastic, and that the number of microfibres sequestered by the alga increased with time. However, the youngest and freshest alga appeared to be more efficient in sequestering the microfibres.

Research to be pursued

Could this alga eliminate microplastic pollution in the lakes? The solution is not so simple. Although Cladophora is temporarily capable of eliminating microfibres from water, it is probable that they are “released” and concentrate on the bottoms of, the lakes, when the alga decomposes. Furthermore, this alga provides a habitat for young fish and mussels. These hiding places made of microplastics contaminate these species and, at the end of the food chain, human beings. Lastly, in large quantities, these alga are harmful for other plant species.

Nonetheless, according to Julie Peller, one of the authors, the study could provide a glimpse of how the adherence of the algae could point the way to better technologies to eliminate microplastics, before they reach lakes and rivers:

I often think that when we’re looking for solutions to problems that we’ve created as human beings, we find a lot of information in the natural mechanisms used by Nature to clean itself”.