Localization of Fe-containing superoxide dismutase in cyanobacteria from the Baltic Sea: depth and light dependency

The abundance and cellular location of Fe-containing superoxide dismutase (Fe-SOD) in trichomes of Nodularia , Aphanizomenon and Anabaena collected from various depths in the Baltic Sea, and in trichomes of a cultured Nodularia strain, BC Nod-9427, isolated from the Baltic Sea, was examined by immunogold labelling. For trichomes collected from natural populations the areal concentration of Fe-SOD labelling decreased with depth: trichomes collected from surface accumulations had between 8 and 11 gold particles μm^?2 whereas trichomes collected from a depth of 18 m were unlabelled. When trichomes collected from a depth of 10 m (mean areal labelling density 0·5 gold particles μm^?2) were exposed to the higher irradiances present at 1 m, the areal concentration of Fe-SOD increased to 3·5–4 gold particles μm^?2 within 4 h. When cultures of Nodularia strain BC Nod-9427, adapted to low light (10 μmol m^?2 s^?1), were transferred to an incident irradiance of 1350 μmol m^?2 s^?1, a doubling of the areal concentration of Fe-SOD gold label was observed within 1 h. Addition of 3-(3,4-dichlorophenyl)-1,1'-dimethylurea (DCMU) to cultures immediately before their transfer to increased illumination resulted in a decrease in areal Fe-SOD concentrations whereas addition of CdCl₂ caused an increase over and above that induced by the elevated irradiance. These results suggest that Baltic Sea cyanobacteria are able to modulate their Fe-SOD content but that this might be in response to oxidative stress rather than to light per se .