Plastic litter changes the rhizosphere bacterial community of coastal dune plants

The presence of plastic litter in coastal environments like beach-dune systems has been well documented, and studies have shown that plastics made of non-biodegradable or biodegradable/compostable polymers buried in sand can alter substrate physical properties and adversely influence dune plant performance. However, whether and how plastic litter may also affect bacterial communities associated with dune plant rhizosphere, which play an important role in promoting plant growth, enhancing plant resistance to abiotic stress, and increasing sandy substrate stability, is unknown. Here, the impact of plastic litter, either non-biodegradable (NBP) or biodegradable/compostable (BP), on the structure and composition of rhizosphere bacterial communities of two dune plants, Thinopyrum junceum, a dune-building C3 species, and Sporobolus pumilus, a generalist C4 species, was explored by using a one-year field experiment combined with metabarcoding techniques. For T. junceum, both plastics did not affect neither the survival nor the biomass of plants, but they significantly increased the alpha-diversity, in terms of Shannon and Pielou index, of bacteria rhizosphere samples and changed the community composition. A greater abundance of Acidobacteria, Chlamydie, and Nitrospirae at phylum level and of Pirellulaceae at family level was observed in the presence of plastics. Conversely, plastics decreased the abundance of Rhizobiaceae compared to controls. For S. pumilus, NBP reduced drastically plant survival while BP increased root biomass compared to controls. BP did not affect alpha- and beta-diversity of rhizosphere bacterial communities, but a greater abundance of Patescibacteria was detected with BP than without. Our findings provide the first evidence that NBP and BP can shape rhizosphere bacterial communities associated with dune plants with potential consequences on soil biogeochemical cycles. Further studies should focus on long-term impacts of plastic pollution for sand microbiome and its interactions with dune plants and on how ecological functions related to specific bacteria taxa may be influenced by plastics.