2.1. Test organism and culture conditions
The homogenous, filamentous and hetrerocystous cyanobacteriumAnabaena sp. PCC 7120 was cultured in BG-11 medium (pH 7.5) in a temperature (25 ± 2 °C) controlled room under 75 μmol photons m−2 s−1 photosynthetically active radiation (PAR, 400–700 nm) provided by white fluorescent tubes (Osram L 40 W/25–1) with a 14:10 h regime of light:dark. The experiments were carried out in early log phase culture of Anabaena PCC 7120.