In the present study, six cyanobacteria isolates were evaluated for the PAL enzyme activity, and their methanol extracts were assessed for the total phenolic amount and other antioxidant parameters. Synechocystis sp. BASO444 and Synechocystis sp. BASO673 isolates with high levels of total phenols (66.0 +/- 1.2 mu g/mg, 78.1 +/- 1.8 mu g/mg, respectively) also showed high levels of PAL activities (20.5 +/- 3.1 U/mg protein, 17.2 +/- 2.3 U/mg protein, respectively) and strong antioxidant activities. To understand the effect of L-phenylalanine (L-phe) on the PAL activity, total phenolic amount, and phenolic constituents, isolates were evaluated with 100 mg/l L-phe. While PAL activities exhibited no significant change with L-phe addition, total phenolic amount of the isolates significantly increased. HPLC analysis revealed gallic acid, trans-cinnamic acid, p-coumaric acid, and ferulic acid as the main compounds. Results suggested that the two isolate mights be an important source for the L-phe inducible phenolic compounds. (C) 2012 Elsevier Ltd. All rights reserved.