Impact de la pollution plastique sur les communautés microbiennes de rivière

Plastic pollution, resulting from massive production and use of plastic polymers since the 20th century, is now ubiquitous in the biosphere. Plastics durability makes them able to persist for decades to centuries, fragmenting into microplastics, that can accumulate everywhere from the deep ocean to the top of the mountains. In aquatic environments, plastics promote the formation of microbial communities similar to natural aquatic biofilms known as the 'plastisphere'. While the presence and impact of plastics on microbial communities has been extensively characterized for the marine ecosystems, freshwater ecosystems have been less explored. Furthermore, most of the studies focus on the effect of the microplastics size fraction rather than on macroplastics.The first part of this thesis compared the microbial colonization between two plastic types (non-biodegradable and biodegradable) and natural substrata (sediments, rocks, and leaves) in two sites of the same watershed with contrasting plastic-pollution levels. This comparison included an analysis of the structural and functional descriptors of microbial communities during substrata colonization. Overall, microbial densities and enzymatic activities involved in organic carbon, nitrogen, and phosphorous decomposition were higher on natural substrata (sediments, rocks, and leaves) than on plastics substrata. This could be due to the greater availability of organic carbon and nutrients in natural substrata. River microorganisms exhibited colonization differences between plastic types at the downstream site, with higher bacterial density and enzymatic activities values measured on the biodegradable compared to the non-biodegradable plastic. Consequently, the presence of biodegradable macroplastics would increase heterotrophy to the river ecosystem more than the presence of non-biodegradable macroplastics.The second part assessed the structural and functional impact of the same plastic types on sediment and rock microbial communities in a river microcosm experiment. Main findings revealed contrasting plastic effects depending on the approach used. At the substratum scale, the addition of non-biodegradable plastic increased β-glucosidase activity and the addition of biodegradable plastic reduced the fungal density in microbial communities from rocks. However, at the microcosm scale, the presence of both plastics reduced N-acetyl-glucosaminidase and phosphatase activities, as well as fungal and algal biomasses in the entire microcosm. At both substratum and microcosm scales, the addition of leaf litter tended to mitigate the plastic impact on microbial communities, especially those from rocks.The impact of plastics on benthic microbial communities depends on several factors, including the plastic type, water physicochemical characteristics, the presence of organic substrata (e.g. leaves), and the scale of the study. This research highlights the impact of macroplastics on the structure and function of rock microbial communities, and calls for a more balanced research between the study of microplastics and plastics form other size fractions.