The use of bioindicators to assess airborne microplastic deposition in remote peatlands of La Réunion

Microplastic contamination spreads throughout all-natural environments, including the most remote. Although the microplastic cycle in nature is not fully understood, it is recognized that both terrestrial and marine environments contribute to microplastic emissions into the atmosphere. At the same time, atmospheric deposition is a significant source of distant microplastics to pristine environments. Still, our understanding of the origin, fate, and characteristics of airborne microplastics and the amount of these particles in natural compartments remains limited. Our study aims to quantify the atmospheric deposition of microplastics in remote locations of La Réunion island, a French overseas department situated in the Indian Ocean. La Réunion is one of the most affected regions by the beaching of (micro)plastics from the nearby garbage patch in the Indian Oceanic gyre. We hypothesize that the plastic-garbage patch of the Indian Oceanic gyre emits microplastics into the atmosphere that are transported to the terrestrial environments of La Réunion. Employing Raman spectroscopy, we have identified microplastics (>2 µm) deposited in bioindicators such as Sphagnum mosses and Usnea lichens. Sphagnum and Usnea were collected across seven sites selected according to different levels of human impact and atmospheric exposure. Our preliminary results indicate differences in microplastic concentrations in Sphagnum (5⋅106 -  1⋅107 MPs-1 dw) compared to Usnea (1⋅107 - 2⋅107 MPs g-1 dw), with polyethylene being the most abundant polymer in both bioindicators. The higher abundance of small microplastics (2-10 µm) suggests the ocean and long-range atmospheric transport are the main drivers of microplastic contamination to the remote areas of La Réunion. Our results will contribute to the comprehensive assessment of the global microplastic cycle by quantifying recent microplastic deposition in remote ombrotrophic peatlands.