Superoxide dismutase activity in the cyanobacterium Microcystis aeruginosa after surface bloom formation

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.