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Bacteria associated withFucus vesiculosusandDelesseria sanguinea, two macroalgae from the Kiel Fjord were investigated seasonally over two years by cultivation-based methods. A total of 166 bacterial strains were isolated from the macroalgae, affiliated to seven classes of bacteria (Actinobacteria,Bacilli,Alphaproteobacteria,Betaproteobacteria,Gammaproteobacteria,CytophagiaandFlavobacteria). According to 16S rRNA gene sequence similarities they were arranged in 82 phylotypes of > 99.0% sequence identity. Assuming that chemical factors rule the bacteria–macroalga and bacteria–bacteria interactions on algal surfaces, we tested the antibiotic activity of the bacterial isolates not only against a panel of four standard test organisms (Bacillus subtilis,Candida glabrata,Escherichia coliandStaphylococcus lentus) but also four macroalga-associated microorganisms:Algicola bacteriolyticaandPseudoalteromonas elyakovii(macroalgal pathogens), andBacillus algicolaandFormosa algae(strains associated with algal surfaces). Organic extracts of more than 51% of the isolates from the two macroalgae inhibited the growth of at least one of the tested microorganisms. As much as 46% and 45% of the isolates derived fromF. vesiculosusandD. sanguinea, respectively, showed antimicrobial activity against the set of macroalga-associated bacteria, compared with 13 and 19% against a standard set of microorganisms. High antibacterial activity against macroalgal pathogens and bacterial competitors support the assumption that complex chemical interactions shape the relationships of bacteria associated with macroalgae and suggest that these bacteria are a rich source of antimicrobial metabolites. [ABSTRACT FROM PUBLISHER]