Can phytoplankton species impact microplastic behaviour within water column?

As plastic debris are becoming more and more ubiquitous in the ocean throughout the years, plastic particles and their degradations are becoming a growing issue. Modelling their transport and looking for hotspots of microplastics is becoming important to evaluate their impact on the environment. But only few data are available on their repartition due to their sampling difficulties. Most of the data on microplastics are limited to size larger than 333 μm. Scientists are trying to modelize microplastic repartition in the ocean using data on macro and microplastics, oceanic gyres and currents. The ocean being a complex environment with a lot of parameters that could influence a change in microplastic repartition. Plastic particle are becoming a nonnegligible part of the plankton with plastic which can represent six times the mass of plankton in some oceanic gyres (MOORE et al. 2001). Taking into account these high levels, it appears important to better identify and characterize the type of “interaction” occurring between plankton and microplastics. As a preliminary approach we focused on phytoplankton species. The aim of this study was thus to assess if microalgae could have an impact on microplastics distribution within the water column.
Four different species (Isochrysis galbana clone Tahitian (Prymnesiophycae), Heterocapsa triquetra (dinophyceae), Rhodomonas salina (cryptophyceae), Chaetoceros neogracile (diatom)) of microalgae were exposed to 2μm yellow-green fluorescent polystyrene microspheres. Two concentrations were tested (5.105 and 5.106 beads/ml). Microsphere repartition was quantified as free beads and as beads attached to microalgae. Microspheres stuck on glassware or on the bottom were also estimated as the difference between added beads to those free or attached to microalgae. Results showed different patterns of microsphere repartition depending on the species. Interaction with the diatom C. neogracile resulted in the highest concentrations of beads attached to microalgae while we obtained the highest levels of beads stuck to the glassware
or on the bottom with R. salina. These results highlight the importance of phytoplankton community in microplastics distribution in the water column. Depending on blooms and species present in the water we could observe a sink of microplastics towards the benthic compartment or a transport of microplastics towards the surface for those stuck on microalgae. These results
also suggest a potential impact of phytoplankton community on the distribution of microplastics within the food web as microplastics attached to microalgae are assumed to be more easily captured by filter feeders than free microplastics in the water column.