Effects of low- or medium-pressure ultraviolet lamp irradiation on Microcystis aeruginosa and Anabaena variabilis

A pure culture of Microcystis aeruginosa or Anabaena variabilis, the representatives of water blooming algae, was exposed to low-pressure (LP) or medium-pressure (MP) UV lamps. Irradiated pure culture suspension was subsequently incubated for 7 d under white light fluorescent lamps. During incubation, profiles of the number of cells, DNA damage and photosynthetic activity were determined. When UV fluence was 600 mJ/cm2, M. aeruginosa cell numbers decreased throughout the 7-d incubation period, to produce 1.5 log reduction (LP) or 1.2 log reduction (MP) compared with control. The amount of DNA damage was 2.02×10−4 ESS/base (LP) and 3.42×10−4 ESS/base (MP) just after UV irradiation, which became 0.05×10−4 ESS/base and 0.23×10−4 ESS/base, respectively, after 3 d incubation. However, cell number kept decreasing, even after DNA repair. Photosynthetic activity decreased by 1.5 log within 1 d (LP) or 3 d (MP). Thus, reduction in photosynthetic activity could contribute to the reduction in M. aeruginosa cell numbers. A. variabilis cell numbers reduced by 2.3 log (LP) or 2.2 log (MP) during the 7-d incubation period; however, after DNA damage repair, cell number began to increase. The amount of DNA damage was 6.07×10−4 ESS/base (LP) and 4.48×10−4 ESS/base (MP) just after UV irradiation, which became 0.23×10−4 ESS/base and 0.40×10−4 ESS/base, respectively, after 3 d incubation. No reduction was observed in photosynthetic activity/cell. Therefore, DNA damage is the main contributor of the reduction in cell number of A. variabilis.