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Microorganisms are able to colonise abiotic surfaces in marine waters, supporting ecological and biogeochemical functions. In turn, environmental factors may determine the accrual and activity of microbial biofilms. The environment is subject to global climate change and pollution by plastic, and therefore we focused on the response of natural marine biofilm on common plastic items (bottles) to seasonality, increases in temperature, and light regime in experimental systems. Chlorophyll-a, prokaryotic abundance and replication frequency, organic matter (OM), and enzymatic activity were measured. Statistical analysis indicated that different environmental conditions modified the biofilms. Summer conditions favoured photoautotrophic organisms. The increase of photoautotrophic biomass could have caused the prokaryotic microorganisms' lowest abundances. Temperature rise affected chlorophyll-a and increased hydrolytic activities, responsible for OM degradation, as also recorded in the absence of light. In winter, temperature variation led to a delayed increase of enzymatic activity, suggesting the need for a time lag to potentiate OM recycling. The correlations between prokaryotic abundance and the other variables highlighted tighter links in cases of temperature alteration. Our results indicated that a potential temperature increase, and light limitation due to plastic sinking in the water column, could modify the biofilm community, increasing the role of prokaryotic organisms.