In cyanobacteria both photosynthesis and nitrogen fixation can be affected by UV radiation. Two of the most abundant heterocystous cyanobacteria isolates from a temperate ricefield in Uruguay belonging to Anabaena and Calothrix genus were exposed for 1 or 3 hours to UV-B dosis similar to those to which they are exposed in summer. Anabaena survival after 1 h of UV-B exposure was 10% whereas in Calothrix's was 30%. Both the quantum yields of photosybtem II fluorescence and O 2 photoevolution decreased with time of UV-B exposure for Calothrix and only till 1 h for Anabaena. Only the Calothrix strain presented phycoerithryn as antenna pigment and constitutive UV-B screening mycosporine like aminoacids. In the Anabaena strain, nitrogenase activity was drastically reduced with UV-B irradiation but in Calothrix was not affected. Proline content and lipid peroxidation increased after 3 hours of UV-B exposure only in Anabaena sp. The antioxidant enzyme activities evaluated followed different trends for both isolates, with an increase in superoxide dismutase activity in the Calothrix isolate. These results show that the two nitrogen-fixing cyanobacteria studied have different responses to UV-B radiation and that cyanobacteria diversity may be considered when selecting strains to be used as biofertilizers.

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