Original paper

Effects of UV-B irradiation on survival, spectral characteristics and nitrogenase activity of cyanobacteria from different habitats

Pattanaik, Bagmi; Adhikary, Siba P.

Abstract

Growth response of three terrestrial cyanobacteria Nostoc microscopicum strain UU591207, Tolypothrix arenophila strain UU523203 and Aulosira fertilissima strain UU25118 to UV-B irradiation was studied. Of these the former two were the dominant component in the crust occurring on the exposed rocks of Ajanta caves and on the brick surfaces of excavated sites at Lalitgiri respectively and the later occur as crust on the rice field soils after harvest of crop. Methanolic extract (90%, v/v) of the freshly collected crusts showed prominent absorption in the UV region of the spectrum due to mycosporine amino acid-like substances (MAAs) and scytonemin pigments. Quantity of these pigments was progressively decreased with duration of wetting of the crusts. N. microscopicum and T. arenophila retained their absorption at the UV-A and UV-B region of the spectrum due to MAAs and scytonemin in laboratory culture under fluorescent light. To the contrary, A. fertilissima did not show absorption in the UV-region of the spectrum in culture. N. microscopicum and T. arenophila survived ever after 60 h of UV-B irradiation; the former was more tolerant than the later species. However, A. fertilissima did not survive even after 6 h of UV-B dose. Upon irradiation with UV-B, the absorption of MAAs was decreased in T. arenophila and in N. microscopicum, however, absorption at 384 nm due to scytonemin was remained almost unaffected up to 24 h of irradiation followed by a decrease. The ratio of absorption of chl a: carotenoid: scytonemin of the irradiated cells of N. microscopicum and T. arenophila showed that the quantity of scytonemin was much higher than the other pigments up to 60 h of UV irradiation. Similar pigment ratio showing a higher amount of scytonemin was also observed even after cultivating the irradiated cells up to 20 days. Acetylene reduction activity (ARA) of all these organisms was ceased when they were exposed to > 30 min of UV-B. ARA was regained after culturing the UV irradiated cells only to a limited extent but was dependent on the duration of pre-UV exposure and tolerance level of the respective organisms.

Keywords

cyanobacteriacrustuv-b irradiationmycosporine amino acid-like substances (maas)scytoneminnitrogenase activity