The terrestrial plastisphere

Plastic is everywhere. Its ubiquity in soils, air and waters has emerged as a persistent environmental issue - impacting ecosystems worldwide. While early studies on plastic pollution primarily focused on marine environments, research has only recently shifted its attention to terrestrial environments, where exposure to plastic pollution is now thought to significantly exceed that of marine environments. As of 2024, in the midst of global change, it has become evident that our soils have become major sinks for plastic debris. In soil, microbial communities secure soil nutrient cycling and the presence of plastic debris may alter those processes by influencing both activity, composition and metabolism of the environmental soil microbiome. Given this state of the art in soil microbial ecology, my thesis revolves around the question “how does plastic debris in soil affect the microbial soil community?” In my thesis chapters, I focused on three research questions in particular: 1) Is plastic debris in soil a distinct microbial habitat? 2) What are the molecular mechanisms behind plastic colonization and possible degradation? 3) Does plastic debris enrich specific microbial taxa (and genes) capable of plastic biodegradation? To answer these questions, I characterized microbial communities in two highly polluted soil ecosystems and compared them to plastic-free soils using 16S rRNA amplicon sequencing. I assessed the plastic-colonizing potential of microorganisms from these sites by examining their adhesion to UV-weathered polyethylene (PE) with FESEM and SEM analysis. Additionally, I conducted an incubation experiment to analyse gene expression profiles of microbial biofilm communities attached to soil-buried PE using metagenomic mRNA analysis. I also isolated bacterial strains from plastic debris as potential candidates for PE degradation. My findings indicate that plastic debris in soil creates a unique microbial habitat with reduced species diversity compared to less polluted soils. This distinctiveness is marked by the prevalence of plastic-degrading microbial taxa from the rare biosphere in early-stage PE-attached biofilms. I detected genes with potential to degrade plastic compounds in isolated bacterial strains and both studied soil types, suggesting that PE-specific enzymes are present even in natural soil ecosystems. Furthermore, my results highlight the importance of Cyanobacteria and nitrogen-fixing microorganisms in plastic-polluted soils, securing crucial secondary functions alongside the advantages of plastic-degrading taxa. Overall, my research elucidates the complex microbial dynamics in plastic-polluted soils, emphasizing the novel properties and challenges of this anthropogenic microhabitat. These findings enhance our understanding of the impacts of plastic debris on soil ecosystems and contribute to strategies for mitigating plastic pollution.