UNLABELLED: Interspecies interactions are key factors affecting the stability of microbial communities. However, microbial interactions in marine biofilms, which constitute up to 80% of the microbial biomass in certain marine environments, are not well understood. We addressed this knowledge gap by coculturing four marine biofilm-derived Roseobacteraceae strains ( IMPORTANCE: Interspecies interactions are crucial for microbial community structure and function. Despite well-studied social behaviors in model microorganisms, species interactions in natural marine biofilms especially Roseobacteraceae with complex metabolic pathways are not well understood. Our findings suggest that positive microbial interactions, which can be mediated by branched-chain amino acid biosynthesis, are common among marine-biofilm Roseobacteraceae. This study provides new insights into microbial interactions and the ecology of marine biofilms.