How the environment shapes the plastisphere of microplastic in a coastal lagoon – a living lab test

Globally, the prevalence of microplastic particles in aquatic environments is increasing exponentially necessitating a need to understand the potential risks. Of particular concern is bacterial attachment to plastic surfaces (the plastisphere) and the risk that these novel surfaces might pose for the fate and transfer of environmental pathogens. To date, studies comparing geographically isolated samples have dominated over systematic approaches. Here, we utilised a temperate brackish lagoon with a natural environmental gradient (Fleet Lagoon, Dorset, U.K.) to explore the colonisation of bacterial communities on microplastics and natural controls (glass beads and the particle attached water fraction). We deployed polyethylene, polypropylene, polystyrene and glass-bead controls for four weeks across a salinity-temperature-pH gradient and characterised resulting biofilms and water-fraction communities using 16S rRNA amplicon sequencing. Analysing the plastisphere communities of post-exposure particles revealed that microplastic associated communities had a higher diversity than that observed for the planktonic bacterial community of the surrounding waters across all samples and sites. Beta-diversity analyses showed clear separation between particle-associated, particle-attached and planktonic communities, with polymer type explaining only a small proportion of variance compared with strong site-driven environmental effects. We also found that particle-associated communities exhibited some differences between the microplastic and the glass particles. Environmental parameters influenced bacterial communities attached to all particles, with Proteobacteria and Bacteroidetes dominating across all tested samples. Contrary to expectations, vibrios were not actively enriched on microplastic particles, with the highest relative abundance 4 times higher in the particle attached water fraction (13.07%) than on any microplastic particles.