Near-UV radiation promotes growth of Chlorella

The wavelength of natural sunlight reaching the Earth's surface was detected to be above 300 nm using a multichannel spectrodetector and the ratio of UV to photosynthetically active radiation (PAR) of sunlight in Japan (137 degrees E/35 degrees 11' N, altitude; 50 m) in early summer was estimated to be 0.07:1 (i.e., 7%). On the basis of this UV/PAR ratio, Chlorella ellipsoidea (IAM-27) cells were cultured in flasks under various conditions of UV irradiation in growth chambers. The growth (cell division) of these cells without near-UV radiation was inferior to that with near-UV radiation. Growth at a UV/PAR ratio of 7% (natural conditions), determined tentatively using our detector in the present study, was maximal similar to those at 14% and 28%; whereas that at 0.7% was somewhat less and that at 70% was considerably less. Growth was linked with the activities of stress enzymes. NAD(P)H-dependent oxidase [NAD(P)-DH] and xanthine oxidase (XOD), extracted from Chlorella exposed to near-UV radiation demonstrated lower activities than those from Chlorella not exposed to near-UV radiation. On the other hand, superoxide dismutase (SOD), catalase (CAT) and ascorbate peroxidase (APOD) extracted from Chlorella exposed to near-UV radiation have higher activities than those from unexposed Chlorella. Near-UV radiation clearly acted as an important factor for growth (cell division), at least at UV/PAR ratios of up to 0.28:1.