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Marine macroalgal biomass provides polysaccharides that could result in the valuable transformation of waste into an economic resource. Crude extracts, or polysaccharides, from Chaetomorpha aerea (Chlorophyta) and Agardhiella subulata and Hypnea cornuta (Rhodophyta), collected from the coastal brackish Lake Ganzirri (Italy), were evaluated for their effects on the adhesion and biofilm formation of Pseudomonas aeruginosa ATCC 27853 and Staphylococcus aureus ATCC 29213, as model organisms. Without exerting any antibacterial activity, each crude extract showed dose-dependent inhibitory effect on biofilm formation, differing each other and also on the strain tested. The highest concentration (400 μg mL−1) of C. aerea extract, a sulfated polysaccharide mainly composed by glucuronic acid, strongly acted on the initial attachment of P. aeruginosa (54% of biofilm reduction), whereas H. cornuta extract greatly acted on that of S. aureus (59%). The addition of A. subulata and C. aerea extracts reduced the biofilm growth of S. aureus , but did not its mature biofilm. Conversely, A. subulata extract, a κ-carrageenan was also able to reduce the developed biofilm of P. aeruginosa. As resulted by surface-coating and the bacterial affinity to hydrocarbons assays, extracts from C. aerea and H. cornuta were able to inhibit the adhesion to polystyrene of P. aeruginosa and S. aureus, and modify their cell-surface properties and hydrophobicity. Furthermore, all the extracts formed stable emulsions. No in vitro cytotoxicity effects were observed for H. cornuta extract, while they were moderate for C. aerea and high for A. subulata. However, all the extracts did not show any potential toxicity in vivo fish-embryo Danio rerio assay. These extracts could be used to prevent or remove the bacterial biofilms in several medical and non-medical applications. • Seaweed biomass provides polysaccharides able to hinder bacterial biofilm formation. • C. aerea and H. cornuta extracts strongly acted on the initial bacterial attachment. • A. subulata extract was able to reduce the developed biofilm of P. aeruginosa. • No in vivo toxicity effects of crude polysaccharides were observed in D. rerio assay. [ABSTRACT FROM AUTHOR]