• The presence of superoxide dismutase (SOD) enzymes and the response of SOD after in vitro induction and decay of a surface bloom are shown in cultures of the cyanobacterium Microcystis aeruginosa. • The SOD enzymes of surface blooms, early degenerate and completely degenerate cultures were assayed by staining for SOD activity, immunoblotting and immunogold labelling. • One band of Mn- and three bands of Fe-SOD were detected in cell extracts. During surface bloom formation, Fe-SOD activity increased fivefold compared with that in control cells; no variation was detected in Mn-SOD activity. However, in early degenerate cultures, Fe-SOD activity decreased to that seen in control cultures, while activity disappeared in completely degenerate cultures. Immunogold labelling showed that Fe-SOD was localized in the cytoplasmic and thylakoid membranes of Microcystis. The extent of labelling paralleled the course of Fe-SOD activity with an increase in particles in surface blooming cells. • The results suggest Fe-SOD increased due to photooxidative stress. However, under prolonged photooxidative stress, high concentrations of active oxygen species could directly, or indirectly, inactivate and degrade Fe-SOD.
Keywords: Microcystis aeruginosa; immunogold labelling; photooxidation; photosynthesis; superoxide dismutase (SOD).