Ultrastructural variations in Microcystis aeruginosa (Chroococcales, Cyanophyta) during a surface bloom induced by high incident light irradiance

Abstract

ABSTRACT We evaluated the ultrastructural variations occurring during a surface bloom in Microcystis aeruginosa Kützing PCC 7005, by comparing cultures exposed to high incident light irradiance (90 µmol photons m-2 s-1) and to low irradiance (9 µmol photons m-2 s-1). At 12h, and 15 and 30 days in culture, the cells were fixed in glutaraldehyde and subjected to cytochemical assays. Exposure to high irradiance induced structural changes in the cell wall, and differences in the photosynthetic apparatus and granule reserves. After 15 days of high irradiance, the following features were observed: a reduced number of thylakoids and changes in their arrangement; high accumulation of glycogen, poly-beta-hydroxybutyrate, lipid bodies and cyanophycin; conspicuous structural changes in the cell wall. At day 30 of high irradiance, the viable cells had few thylakoids, arranged parallel to each other and perpendicular to the plasma membrane, and abundant glycogen. Nitrogen, calcium and phosphorus, as detected by electron spectroscopic imaging, were present in different sites of cells grown at low and high irradiance. The results of this study, together with previous data on variation in Fe-superoxide dismutase (Canini et al., 2001a), suggest that defense mechanisms against high and prolonged irradiance only operate for a short time.