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¹ Ankara University, Department of Biology, Faculty of Science, Ankara, Turkey DOI : 10.4194/1303-2712-v19_11_04 Viewed : 3771 - Downloaded : 4903 In the present work, production of bioactive compounds by four different Dunaliella species (Dunaliella sp. 1-4) and antimicrobial activities of them was investigated against different bacteria. The effects of different nitrogen concentrations (0.5 g/L-2.0 g/L), illumination intensities (1200lx-4800lx), NaCI concentrations [10-25%(w/v)], and incubation periods (7-28 d) on the content of the bioactive compounds and their antimicrobial activities were studied. The highest antimicrobial activity of bioactive compounds was observed in Dunaliella sp. 2; optimum conditions for the highest antimicrobial activity were found extracts obtained from biomasses grown in media with 1.0 g/L nitrogen and 20% (w/v) NaCI, under 4800 lx light intensity, after 14 d of incubation period. Different solvent types [ethanol, methanol, hexane, chloroform, Tris-HCI (pH:8; 0.5 M), and water] were also investigated to find more efficient antimicrobial activity. Chloroform-extracts had the highest bioactive character within the solvents tested. The chloroform-extracts were chemically characterized by GC–MS and HPLC-DAD. Stearic acid having the highest peak area as fatty acids found in Dunaliella sp. 2 was detected. However, pigments such as lutein followed by α- carotene and zeaxanthin with antimicrobial activity were identified; simple phenols like ferulic acid responsible for the antimicrobial activity was analysed. Keywords : Bioactive compounds, Antimicrobial activity, Dunaliella sp., Microalgae, Stress condition