Biodegradation of different bioplastics by specialised microbial communities in a coastal brackish environment

Bioplastics are frequently promoted as a more sustainable alternative to conventional plastics. Microorganisms are metabolically versatile and central to marine ecosystems, yet the potential of marine microbial communities to degrade different bioplastics and the effect of environmental factors are poorly understood. Employing multi-seasonal in situ and in vitro experiments, we assessed the biodegradation of six bio-based bioplastic materials in the brackish Baltic Sea and characterised the associated microbial communities with metagenomics and metatranscriptomics. Cellulose acetate (CA), polybutylene succinate (PBS), and polyhydroxybutyrate (PHB) degraded at varying rates across materials, seasons, and experimental settings, with up to 28% weight attrition after 97 weeks in situ (CA) and 56% carbon loss to CO2 after four weeks in vitro (PBS). The three biodegradable plastics developed similar microbial communities with lower richness and different composition from those on the non-biodegradable materials (polyamide, polyethylene, and glass) and in the water column. The microbial populations on the biodegradable plastics included mostly aerobic and facultative anaerobic heterotrophs with a broad carbohydrate metabolism, and with a higher prevalence of denitrification and other nitrogen cycling traits compared to the non-biodegradable materials. Based on the metatranscriptomic signal of key genes involved in CA, PBS, and PHB breakdown, we identified diverse microbial populations that can potentially drive the biodegradation of these materials in the Baltic Sea, many of which encoded the potential to degrade multiple bioplastics. We propose the term bioplastisphere to denote the specialised microbial communities associated with biodegradable plastics.