Environmental Factors Support the Formation of Specific Bacterial Assemblages on Microplastics

TLDR

Microplastics are widely distributed in marine environments, yet their potential to act as vectors for distinct or pathogenic bacterial assemblages remains poorly understood. The study aimed to determine how varying environmental conditions influence the composition and specificity of bacterial communities associated with microplastics compared to natural particles. Polystyrene, polyethylene, and wooden pellets were incubated for two weeks across a marine‑to‑freshwater gradient and their bacterial assemblages, along with surrounding water communities, were profiled using high‑throughput 16S rRNA gene sequencing. The results revealed that plastic‑specific bacterial assemblages arise only under particular conditions (low nutrients, high salinity), that the PAH‑utilizing genus *Erythrobacter* consistently colonizes microplastics across the gradient, and that while no pathogenic enrichment occurred on PE or PS, microplastics in wastewater treatment plants harbored antibiotic‑resistance bacteria, indicating a potential hotspot for horizontal gene transfer and underscoring the ecological importance of microplastic‑promoted bacterial populations.

Abstract

While the global distribution of microplastics (MP) in the marine environment is currently being critically evaluated, the potential role of MP as a vector for distinct microbial assemblages or even pathogenic bacteria is hardly understood. To gain a deeper understanding, we investigated how different in situ conditions contribute to the composition and specificity of MP-associated bacterial communities in relation to communities on natural particles. Polystyrene (PS), polyethylene (PE), and wooden pellets were incubated for 2 weeks along an environmental gradient, ranging from marine (coastal Baltic Sea) to freshwater (waste water treatment plant, WWTP) conditions. The associated assemblages as well as the water communities were investigated applying high-throughput 16S rRNA gene sequencing. Our setup allowed for the first time to determine MP-dependent and -independent assemblage factors as subject to different environmental conditions in one system. Most importantly, plastic-specific assemblages were found to develop solely under certain conditions, such as lower nutrient concentration and higher salinity, while the bacterial genus Erythrobacter, known for the ability to utilize polycyclic aromatic hydrocarbons (PAH), was found specifically on MP across a broader section of the gradient. We discovered no enrichment of potential pathogens on PE or PS; however, the abundant colonization of MP in a WWTP by certain bacteria commonly associated with antibiotic resistance suggests MP as a possible hotspot for horizontal gene transfer. Taken together, our study clarifies that the surrounding environment prevailingly shapes the biofilm communities, but that MP-specific assemblage factors exist. These findings point to the ecological significance of specific MP-promoted bacterial populations in aquatic environments and particularly in plastic accumulation zones.