Plastic physico-chemical properties shape the early marine plastisphere.

Oceanic plastic pollution provides surfaces for microbial biofilm development, which may potentiate ecosystem dysbiosis by releasing toxins, providing carbon for microbial growth, and transporting pathogens and invasive species. This study investigated how plastic type, physico-chemical characteristics, ultraviolet-ageing, water temperature, depth and submersion duration (up to twelve weeks) influence marine plastisphere communities across Aotearoa New Zealand. Initial colonisation was dominated by diatoms, with network analyses identifying keystone diatom taxa such as Bacillariaceae during the first week, followed by bacterial taxa such as Sulfurovum by week two. Early-stage community composition differed between seawater and hard surfaces (plastic and glass), and between plastics. Bacterial alpha-diversity was reduced on more hydrophobic plastics, while beta-diversity was influenced by manganese and zinc content (for bacteria), and crystallinity (for diatoms and bacteria). Random Block-Forest modelling revealed distinct microbial associations with polyamide and polyolefins (LLDPE and oxo-LLDPE), whereas PET exhibited lower taxonomic specificity and was frequently misclassified as PLA, its fellow polyester. Considering risk species, the pathogen Balamuthia was present on plastics but not from seawater, and the non-indigenous tunicates Ciona savigny and Ascidiella aspersa, and plant pathogen Puccinia graminis, were important plastic-specific indicator species. Our findings contribute to a holistic understanding of how plastic physico-chemical properties shape marine plastisphere dynamics and succession. They also highlight how plastics act as selective substrates for harmful and invasive species, posing risks to biodiversity, ecosystems, and marine industries.