Immunocytochemical localization of Fe-SOD in different cells of Anabaena cylindrica Lemm. grown at two different photon irradiances.

Superoxide dismutase (SOD) converts superoxide radicals into hydrogen peroxide and molecular oxygen and therefore represents the primary defence against oxygen damage. Using antibodies against Anabaena cylindrica Lemm. iron-SOD, the isoenzyme was localized by immunogold labelling in vegetative cells, heterocysts and akinetes of the cyanobacterium Anabaena cylindrica grown at different light intensities. At a low photon irradiance (25 μmol photons m s ), the density of iron-SOD labelling was similar in vegetative cells and in heterocysts, whereas in akinetes only 60 % of the gold particles were present as compared with the other cell types. At a higher photon irradiance (140 μmol photons m S ), iron-SOD labelling increased by 100% in both vegetative cells and in heterocysts but did not change in akinetes. The cultures grown at the photon irradiance of 140 μmol photons m s showed a rate of photosynthetic oxygen evolution 40 % higher than that of cultures grown at the lower photon irradiance. The increase of SOD labelling in vegetative cells is in line with the increased photosynthetic oxygen evolution induced by light. The unchanged SOD labelling observed in the akinetes suggests that light intensity does not seem to affect the oxidative metabolism due to respiratory activity.