036 | Diversity and composition of microbial communities associated with deep-sea marine litter

Despite being one of the least explored environments on the planet, the deep ocean is increasingly recognised as a terminal reservoir for marine debris. Once introduced to the marine environment, some marine litter sinks to the seabed, accumulating in deep basins, submarine canyons and abyssal plains, where it remains for extended periods. The accumulation of debris in these areas is a cause for concern given the nature of deep-sea ecosystems, which are characterised by low energy availability, limited nutrient input, and slow biological turnover. The introduction of foreign materials creates novel hard surfaces and potential sources of organic matter, generating new ecological niches. Investigating the microbial communities associated with deep-sea litter is crucial for understanding how anthropogenic substrates influence community composition and ecological processes in deep marine ecosystems. As part of the “Operazione Mare Pulito”, 2022, this study analysed the bacterial communities associated with various items of seafloor litter collected during experimental trawl sampling campaigns. The bacterial communities were characterised using 16S rRNA gene amplicon sequencing at genus level. An experimental replica was performed for each sample, with each replica treated individually to obtain diversity indices. The replicas were then unified by calculating an average to obtain a representative dataset for community composition analyses. Combined alpha diversity and Bray-Curtis ordination analyses highlighted distinct microbial assemblages associated with different substrates, suggesting that litter material composition plays an important role in structuring bacterial communities. Across all substrates, the most abundant genera included Woesia, members of the Rhodopirellula lineage, Ruegeria, Roseovarius, Silicimonas, Erythrobacter and nitrifying-related taxa such as Nitrospira and Nitrosospira. However, their relative abundances varied markedly among different litter materials: smooth and transparent substrates, such as packaging materials and fishing lines, were characterized by a higher relative contribution of Alphaproteobacteria-associated genera, commonly linked to marine biofilm formation and surface colonization, including Ruegeria, Roseovarius and Erythrobacter. In contrast, more structured or complex materials, such as textile-derived substrates and plastic sheets, showed an increased relative abundance of genera such as Woesia and other taxa related to Verrucomicrobia and Planctomycetes, suggesting the development of more heterogeneous and potentially mature biofilm communities. In addition, genera associated with nitrogen cycling, including Nitrospira and Nitrosospira, displayed variable contributions across substrates, indicating material-specific differences in the functional potential of the associated microbial assemblages. Overall, genus-level composition analyses revealed clear substrate-specific patterns that were consistent with the separation observed in beta-diversity ordination analyses, highlighting the influence of litter material properties on the structuring of microbial communities in deep-sea–associated environments.