Plastic pollution in marine environments is a growing ecological issue, yet the role of biofouling (biological colonisation of plastic surfaces) remains insufficiently understood, particularly with regard to its effects on plastic degradation and transport. This study examined how biofouling alters expanded PVC debris over time, both biologically and physically, through an in situ experiment conducted at the Marina of Oeiras, Portugal. Circular PVC slabs were submerged and retrieved after 7, 30, 60, and 180 days. Colonisation was assessed via microscopy DNA-based microbial identification using Sanger sequencing and dry-weight measurements.

Over the exposure period, clear ecological succession was observed: early colonisers such as Skeletonema pseudocostatum and Minidiscus spinulatus (diatoms) gave way to more diverse communities, including Nannochloropsis oculata and Melosira tropica and invertebrates like Watersipora subtorquata and Balanus trigonus. This biological progression reflected a maturing biofilm that likely influenced both the microbial composition and the material properties of the plastic.

Microbial analyses identified several plastic-associated species with known biodegradative capabilities, including Pseudomonas hunanensis, Comamonas testosteroni, and Fusarium spp., indicating potential early-stage microbial degradation. The presence of faecal indicator bacteria such as Escherichia coli and Enterobacter sp. indicated anthropogenic influence, suggesting that human-derived nutrients may shape plastisphere communities.

These biological changes were paralleled by physical alterations:; disc weight increased steadily with time and depth, consistent with biomass accumulation.

Overall, marine biofouling drives significant biological and physical changes in plastic debris, with early evidence of microbial degradation. These interactions could influence the long-term fate, transport, and persistence of plastics in the ocean and should be integrated into marine plastic pollution models.

Acknowledgments:
This research was supported by the European Regional Development Fund (ERDF) through the Interreg Atlantic Area Programme, under project Interreg FreeliterAT.