Disintegration of ultrastructure is another hallmark of PCD and includes vacuolization and degradation of internal cellular components, although plasma membranes remain intact in prokaryotic cells (Berman-Frank et al. 2004). Typical inclu- sion bodies of cyanobacteria are the thylakoids, polyhedral bodies, glycogen granules, lipid inclusions, cyanophycin granules, polyphosphate bodies, ribosomes, and a visible nu- clear zone with clumps of DNA (Jensen 1993). After co- culturing with M. spicatum, it was observed that external con- figuration of M. aeruginosa cells remained intact while the interior structures changed, in which inclusions collapsed, DNA dispersed, and the nuclear zone boundary became less distinct or even disappeared. Similar ultrastructure alterations were previously reported for M. aeruginosa exposed to the alleochemical gramine (0.5–8.0 mg L−1) (Hong et al. 2010), M. denticulata treated with H2O2 (Darehshouri et al. 2008), and D. tertiolecta deprived with light (Segovia et al. 2003). In this study, these morphological changes, DNA fragmentation, and activation of caspase-3-like activity detected are in accor- dance with PCD hallmarks observed under adverse conditions by others (Moharikar et al. 2006; Berman-Frank et al.2004; Zuppini et al. 2007).