Biofilms on plastic, metabolically fantastic: functional dynamics of the microbial plastisphere

Plastic pollution in marine ecosystems has become an environmental issue of global concern. Plastic items ending up in the ocean serve as a new habitat, named “plastisphere”, harbouring complex microbial communities. While much of the plastisphere-related research has focused on the environmental drivers shaping its structure, its biogeochemical role has been generally overlooked. To assess the microbial plastisphere’s metabolic potential, 1×0.5cm plastic pieces made of five polymers (HDPE, LDPE, PET, PP, PS) were incubated under in situ conditions in two Mediterranean sites (Gulf of Trieste and Gulf of Naples), in winter and summer. The 60-day-long experiments were sampled at four time points to investigate prokaryotic biofilm formation and its metabolic potential through confocal imagery and omic techniques, employing specific stains to detect metabolic activity. Biofilm-specific exoenzymatic rates were assessed through fluorogenic substrate assays. Biofilm developing on HDPE, LDPE and PP showed faster exoenzymatic rates and higher prokaryotic abundances than PET and PS. Generally, glycolytic activity peaked after 15-30 days, whereas protein degradation increased only at the last time points. After long-term observations, biofilm forming on 1cm2 of plastic displayed organic matter degradation rates comparable to those measured in 1L of seawater, highlighting the biogeochemical role of the microbial plastisphere.