The induction of UV-absorbing compounds known as mycosporine-like amino acids (MAAs) by red, green, blue, and white light (43% ambient radiation greater than390 nm) was examined in sublittoral Chondrus crispus Stackh. Fresh collections or long-term cultures of sublittoral thalli, collected from Helgoland, North Sea,Germany, and containing no measurable amounts of MAAs, were exposed to filtered natural radiation for up to 40 days. The MAA palythine (max 320 nm) wassynthesized in thalli in blue light to the same extent observed in control samples in white light. In contrast, thalli in green or red light contained only trace amounts ofMAAs. After the growth and synthesis period, the photosynthetic performance of thalli in each treatment, measured as pulse amplitude modulated chlorophyllfluorescence, was assessed after a defined UV dose in the laboratory. Thalli with MAAs were more resistant to UV than those without, and exposure to UV-A+B wasmore damaging than UV-A in that optimal (Fv/Fm) and effective (II) quantum yields were lower and a greater proportion of the primary electron acceptor of PSII,Q, became reduced at saturating irradiance. However, blue light-grown thalli were generally more sensitive than white light control samples to UV-A despite havingsimilar amounts of MAAs. The most sensitive thalli were those grown in red light, which had significantly greater reductions in Fv/Fm and II and greater Qreduction. Growth under UV radiation alone had been shown previously to lead to the synthesis of the MAA shinorine (max 334 nm) rather than palythine. In furtherexperiments, we found that preexposure to blue light followed by growth in natural UV-A led to a 7-fold increase in the synthesis of shinorine, compared with growthin UV-A or UV-A+B without blue light pretreatment. We hypothesize that there are two photoreceptors for MAA synthesis in C. crispus, one for blue light and onefor UV-A, which can act synergistically. This system would predispose C. crispus to efficiently synthesize UV protective compounds when radiation levels are rising,for example, on a seasonal basis. However, because the UV-B increase associated with artificial ozone reduction will not be accompanied by an increase in blue light,this triggering mechanism will have little additional adaptive value in the face of global change unless a global UV-B increase positively affects water column clarity.