Despite clear advances in characterizing marine biofilms, details on their formation and species succession remain scarce particularly during the early stage of development. We investigated the microbial community composition and succession in coastal marine biofilms on plastic. Samples were collected over 75 days of immersion with strengthened samplings during the early stages of biofilm establishment. Biofilm composition was estimated using Illumina Miseq and microbial community interactions were assessed through microbial association network analysis. In silico analyses showed that primers used in most of previous studies considerably underestimated marine biofilm diversity. Unintentionally ignored so far, we showed that Flavobacteriia might be key actors in the functioning of marine biofilms. Gamma-proteobacteria from the genus Oleibacter strongly dominated microbial communities during the first hours of biofilm formation. These pioneer communities were quickly replaced by alpha-proteobacteria and Flavobacteriia. Bacterial communities exhibited fast temporal structure dynamics with taxa displaying rapid increases and declines. A total of 90% of operational taxonomic units (OTUs) were intermittent or ephemeral reinforcing the conclusion that marine biofilms are highly dynamics. With 2/3 of positive significant connections between bacterial OTUs, microbial biofilm communities appear to be more inclined to develop inter-specific cooperation rather than competition and might thus form sets of functional guilds with mutual metabolic exchanges.