Degradation of cyanobacteria and microalgae using ultraviolet radiation at 222 nm and its application in the bleaching of lampenflora on cave rock surfaces

Cyanobacterial and microalgae blooms negatively affect ecological environments. One of the emerging problems of these blooms occurs in calcareous caves, where visible lighting systems in tourist caves induce the formation of photosynthetic biofilms known as lampenflora on rock surfaces. In this study, we evaluated the inactivation effect of ultraviolet (UV) light irradiation at 222 nm (rare gas halide lamp) and 254 nm (mercury lamp) on cyanobacteria and microalgae in a laboratory and on lampenflora in Ryusenshindo Cave in Japan. Based on the laboratory experiments, irradiation at 222-nm was more effective than that at 254-nm for degrading plants. The photosensitivity of chlorophyll-a was higher at 222 nm than at 254 nm. The results using pulse amplitude modulation fluorometry indicated that the photosystem Ⅱinhibition rate was higher than the Chl-a bleaching rate and the cell degradation rate at both 222 and 254 nm. Because the narrow spectral-band light of 222 nm is considered safer for mammalian skin and eyes than the conventional 254 nm, UV irradiation at 222 nm can be used for tourist passages in caves during visitor time to prevent lampenflora formation.