Deciphering plant-microplastic interactions in freshwaters through Spirodela polyrhiza: towards phytoremediation applications

Marco Dainelli ¹, Sara Falsini ¹, Stéphanie Boutet ², Ilaria Colzi ¹, Andrea Coppi ¹, Massimiliano Corso ², Giovanni Stefano ¹, Sandra Ristori ³, Cristina Gonnelli ¹, Alessio Papini ¹

Department of Biology, University of Florence, Via Micheli 1, 50121 Firenze, Italy ¹, Université Paris-Saclay, INRAE, AgroParisTech, Institut Jean-Pierre Bourgin (IJPB), 78000 Versailles, France ², Department of Chemistry, University of Florence, Via della Lastruccia 3, Sesto Fiorentino, 50019 Firenze, Italy ³

Microplastics (MPs, 1 µm – 5 mm) pose a growing threat to ecosystems, as evidenced by the substantial efforts made by the scientific community in recent years to investigate their presence in the environment and their toxic effects on living organisms. Nonetheless, resolving this issue must be a priority, with the search for new technologies to address this goal at the forefront. This research aimed to investigate how MPs interact with the floating freshwater plant Spirodela polyrhiza with a focus on evaluating its potential, along with that of its exudates, for mitigating these emerging environmental pollutants. Manufactured blue-fluorescent spherical MPs with diameter between 1 and 5 µm were used to prepare N-medium with a particle concentration of 0.05 g L-1, which was then administered to S. polyrhiza colonies for one week under controlled conditions (MP+S). The experimental setup also included two control conditions: N-medium without plants but containing MPs at 0.05 g L-1 (MP), and growth medium with S. polyrhiza but without MPs (S). The presence of particles did not affect plant growth in terms of frond number and total frond area, and no differences were observed between the treatment and control group (S) in fresh and dry weight at the end of the week of exposure. In addition, tissue accumulation of macro- and micronutrients, measured using X-ray fluorescence (XRF), was not affected by the presence of MPs. The number of items in solution and their area were analyzed at the end of the experiment using optical light microscopy. 400 particles were measured, and their sizes were categorized into classes. In the control condition (MP), smaller particles predominated, with a significant number in the 0-10 µm² class. In contrast, particles in the treatment condition (MP+S) were less abundant in the smaller classes and more prevalent in larger ones, indicating aggregation in the presence of S. polyrhiza. The treatment condition showed a significantly lower number of items, suggesting particle adsorption by the roots and underside of the fronds, confirmed by confocal microscopy analysis. The binding force of the MPs on S. polyrhiza was strong, as a large quantity of them remained on the surfaces even after washing and incubation for 4 days in an MP-free medium. The Zeta potential of particles was also measured at the start and end of the test: control MPs remained stable at -17 mV, while those incubated with plants showed a less negative value of -11 mV. These effects on the behaviour of particles in solution in the presence of S. polyrhiza were thought to result from the exudation of specific compounds by the plant-root system. To investigate this, compounds were analyzed using LC-MS/MS untargeted metabolomics, yielding preliminary data that identified specific metabolic features exuded only in the presence of MPs. Validating their role in particle aggregation and surface adsorption could have potential biotechnological applications in the phytoremediation of freshwater systems.

Main author career stage: PhD student

Contribution type: Talk

First choice session: 4. Structure, physiology, and development

Second choice session: 3. Biodiversity and global change